[PATCH printk v5 00/30] wire up write_atomic() printing

[PATCH printk v5 00/30] wire up write_atomic() printing

[John Ogness]

Hi,

This is v5 of a series to wire up the nbcon consoles so that
they actually perform printing using their write_atomic()
callback. v4 is here [0]. For information about the motivation
of the atomic consoles, please read the cover letter of v1 [1].

The main focus of this series:

- For nbcon consoles, always call write_atomic() directly from
  printk() caller context for the panic CPU.

- For nbcon consoles, call write_atomic() when unlocking the
  console lock.

- Only perform the console lock/unlock dance if legacy or boot
  consoles are registered.

- For legacy consoles, if nbcon consoles are registered, do not
  attempt to print from printk() caller context for the panic
  CPU until nbcon consoles have had a chance to print the most
  significant messages.

- Mark emergency sections. In these sections printk() calls
  will only store the messages. Upon exiting the emergency
  section, nbcon consoles are flushed directly and legacy
  console flushing is triggered via irq_work.

This series does _not_ include threaded printing or nbcon
drivers. Those features will be added in separate follow-up
series.

Note: With this series, a system with _only_ nbcon consoles
      registered will not perform console printing unless the
      console lock or nbcon port lock are used or on panic.
      This is on purpose. When nbcon kthreads are introduced,
      they will fill the gaps.

The changes since v4:

- In serial_core_add_one_port(), initialize the port lock
  before setting @cons (since uart_port_set_cons() uses the
  port lock).

- For unregister_console_locked(), take the con->device_lock
  when removing the console from the console list.

- Remove the struct nbcon_drvdata and instead rely on the port
  lock being taken when adding/removing uart nbcon console list
  items.

- Move the nbcon context for drivers into the struct console
  (formery in struct nbcon_drvdata).

- Simplify the port lock wrapper implementation since we can
  rely on the registration state not changing while the port
  lock is held.

- Change nbcon_driver_acquire() to nbcon_driver_try_acquire()
  in order to support try-semantics for the port lock wrappers.
  Also update its kerneldoc to clarify its usage.

- Implement true try-lock semantics for the try-variants of the
  port lock wrappers.

- Remove the retry-loop in __nbcon_atomic_flush_pending() since
  there is never a need to retry. If a context takes over
  ownership it also takes over responsibility to print the
  records.

- Invert the return value of nbcon_atomic_emit_one() and
  add kerneldoc and comments about the meaning of the return
  value.

- For nbcon_legacy_emit_next_record() use the same return
  value and kerneldoc explanation as nbcon_atomic_emit_one().

- Invert the meaning of the return value of
  __nbcon_atomic_flush_pending_con() and use various errno
  values to report the reason for failure.

- Add nbcon_atomic_flush_pending_con() to flush all records if
  records were added while flushing. (Once printer threads are
  available, we can rely on them to print the remaining
  records.)

- For nbcon_driver_release(), flush all records if records were
  added while holding the port lock. (Once printer threads are
  available, we can rely on them to print the remaining
  records.)

- Add nbcon_cpu_emergency_flush() to allow periodically
  flushing if there has been many records stored in emergency
  context. It also attempts legacy flushing when safe.

- Change lockdep_print_held_locks() and debug_show_all_locks()
  to rely on their callers marking emergency sections because
  these functions can be called in non-emergency situations.
  Note that debug_show_all_locks() still calls
  nbcon_cpu_emergency_flush() is case it used in emergency.

- Rename console_init_seq() to get_init_console_seq() and have
  it return the new seq rather than setting @newcon->seq.

- Change nbcon_init() to take the initial sequence number as
  an argument.

- For __pr_flush(), move the barrier() to ensure no
  intermediate use of the printing_via_unlock() macro.

- For nbcon_cpu_emergency_exit(), update the comments and
  WARN_ON_ONCE position as suggested.

- Move the printing_via_unlock() macro into internal.h so that
  it can be used by nbcon.c as well (in
  nbcon_cpu_emergency_flush()).

- Update the kerneldoc for nbcon callbacks write_atomic() and
  device_lock().

- Add clarification in console_srcu_read_flags() kerneldoc.

- Change kerneldoc nbcon_context_try_acquire() context to
  mention device_lock() or local_irq_save() requirement.

John Ogness

[0] https://lore.kernel.org/lkml/20240402221129.2613843-1-john.ogness@linutronix.de
[1] https://lore.kernel.org/lkml/20230302195618.156940-1-john.ogness@linutronix.de

John Ogness (25):
  printk: Add notation to console_srcu locking
  printk: nbcon: Remove return value for write_atomic()
  printk: nbcon: Add detailed doc for write_atomic()
  printk: nbcon: Add callbacks to synchronize with driver
  printk: nbcon: Use driver synchronization while (un)registering
  serial: core: Provide low-level functions to lock port
  serial: core: Introduce wrapper to set @uart_port->cons
  console: Improve console_srcu_read_flags() comments
  nbcon: Provide functions for drivers to acquire console for
    non-printing
  serial: core: Implement processing in port->lock wrapper
  printk: nbcon: Do not rely on proxy headers
  printk: nbcon: Fix kerneldoc for enums
  printk: Make console_is_usable() available to nbcon
  printk: Let console_is_usable() handle nbcon
  printk: Add @flags argument for console_is_usable()
  printk: nbcon: Add helper to assign priority based on CPU state
  printk: Track registered boot consoles
  printk: nbcon: Use nbcon consoles in console_flush_all()
  printk: nbcon: Add unsafe flushing on panic
  printk: Avoid console_lock dance if no legacy or boot consoles
  printk: Track nbcon consoles
  printk: Coordinate direct printing in panic
  panic: Mark emergency section in oops
  rcu: Mark emergency sections in rcu stalls
  lockdep: Mark emergency sections in lockdep splats

Petr Mladek (1):
  printk: Properly deal with nbcon consoles on seq init

Sebastian Andrzej Siewior (1):
  printk: Check printk_deferred_enter()/_exit() usage

Thomas Gleixner (3):
  printk: nbcon: Provide function to flush using write_atomic()
  printk: nbcon: Implement emergency sections
  panic: Mark emergency section in warn

 drivers/tty/serial/8250/8250_core.c |   6 +-
 drivers/tty/serial/amba-pl011.c     |   2 +-
 drivers/tty/serial/serial_core.c    |  16 +-
 include/linux/console.h             | 116 ++++++-
 include/linux/printk.h              |  33 +-
 include/linux/serial_core.h         | 117 ++++++-
 kernel/locking/lockdep.c            |  84 ++++-
 kernel/panic.c                      |   9 +
 kernel/printk/internal.h            |  71 +++-
 kernel/printk/nbcon.c               | 488 +++++++++++++++++++++++++++-
 kernel/printk/printk.c              | 303 +++++++++++++----
 kernel/printk/printk_ringbuffer.h   |   2 +
 kernel/printk/printk_safe.c         |  12 +
 kernel/rcu/tree_exp.h               |   9 +
 kernel/rcu/tree_stall.h             |  11 +
 15 files changed, 1148 insertions(+), 131 deletions(-)


base-commit: a2b4cab9da7746c42f87c13721d305baf0085a20
-- 
2.39.2

[PATCH printk v5 01/30] printk: Add notation to console_srcu locking

[John Ogness]

kernel/printk/printk.c:284:5: sparse: sparse: context imbalance in
'console_srcu_read_lock' - wrong count at exit
include/linux/srcu.h:301:9: sparse: sparse: context imbalance in
'console_srcu_read_unlock' - unexpected unlock

Fixes: 6c4afa79147e ("printk: Prepare for SRCU console list protection")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/printk.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 612c73333848..c7c0ee2b47eb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -282,6 +282,7 @@ EXPORT_SYMBOL(console_list_unlock);
  * Return: A cookie to pass to console_srcu_read_unlock().
  */
 int console_srcu_read_lock(void)
+	__acquires(&console_srcu)
 {
 	return srcu_read_lock_nmisafe(&console_srcu);
 }
@@ -295,6 +296,7 @@ EXPORT_SYMBOL(console_srcu_read_lock);
  * Counterpart to console_srcu_read_lock()
  */
 void console_srcu_read_unlock(int cookie)
+	__releases(&console_srcu)
 {
 	srcu_read_unlock_nmisafe(&console_srcu, cookie);
 }
-- 
2.39.2

Re: [PATCH printk v5 01/30] printk: Add notation to console_srcu locking

[Petr Mladek]

On Thu 2024-05-02 23:44:10, John Ogness wrote:
> kernel/printk/printk.c:284:5: sparse: sparse: context imbalance in
> 'console_srcu_read_lock' - wrong count at exit
> include/linux/srcu.h:301:9: sparse: sparse: context imbalance in
> 'console_srcu_read_unlock' - unexpected unlock
> 
> Fixes: 6c4afa79147e ("printk: Prepare for SRCU console list protection")
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

[PATCH printk v5 02/30] printk: Properly deal with nbcon consoles on seq init

[John Ogness]

From: Petr Mladek <pmladek@suse.com>

If a non-boot console is registering and boot consoles exist,
the consoles are flushed before being unregistered. This allows
the non-boot console to continue where the boot console left
off.

If for whatever reason flushing fails, the lowest seq found from
any of the enabled boot consoles is used. Until now con->seq was
checked. However, if it is an nbcon boot console, the function
nbcon_seq_read() must be used to read seq because con->seq is
not updated for nbcon consoles.

Check if it is an nbcon boot console and if so call
nbcon_seq_read() to read seq.

Also, avoid usage of con->seq as temporary storage of the
starting record. Instead, rename console_init_seq() to
get_init_console_seq() and just return the value. For nbcon
consoles set the sequence via nbcon_init(), for legacy consoles
set con->seq.

The cleaned design should make sure that the value stays and is
set before the printing kthread is created.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/internal.h |  4 ++--
 kernel/printk/nbcon.c    | 10 +++-------
 kernel/printk/printk.c   | 41 ++++++++++++++++++++++++++++------------
 3 files changed, 34 insertions(+), 21 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 6c2afee5ef62..739b82adc383 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -75,7 +75,7 @@ u16 printk_parse_prefix(const char *text, int *level,
 u64 nbcon_seq_read(struct console *con);
 void nbcon_seq_force(struct console *con, u64 seq);
 bool nbcon_alloc(struct console *con);
-void nbcon_init(struct console *con);
+void nbcon_init(struct console *con, u64 init_seq);
 void nbcon_free(struct console *con);
 
 #else
@@ -96,7 +96,7 @@ static inline bool printk_percpu_data_ready(void) { return false; }
 static inline u64 nbcon_seq_read(struct console *con) { return 0; }
 static inline void nbcon_seq_force(struct console *con, u64 seq) { }
 static inline bool nbcon_alloc(struct console *con) { return false; }
-static inline void nbcon_init(struct console *con) { }
+static inline void nbcon_init(struct console *con, u64 init_seq) { }
 static inline void nbcon_free(struct console *con) { }
 
 #endif /* CONFIG_PRINTK */
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index c8093bcc01fe..de260531b0da 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -172,9 +172,6 @@ void nbcon_seq_force(struct console *con, u64 seq)
 	u64 valid_seq = max_t(u64, seq, prb_first_valid_seq(prb));
 
 	atomic_long_set(&ACCESS_PRIVATE(con, nbcon_seq), __u64seq_to_ulseq(valid_seq));
-
-	/* Clear con->seq since nbcon consoles use con->nbcon_seq instead. */
-	con->seq = 0;
 }
 
 /**
@@ -961,20 +958,19 @@ bool nbcon_alloc(struct console *con)
 /**
  * nbcon_init - Initialize the nbcon console specific data
  * @con:	Console to initialize
+ * @init_seq:	Sequence number of the first record to be emitted
  *
  * nbcon_alloc() *must* be called and succeed before this function
  * is called.
- *
- * This function expects that the legacy @con->seq has been set.
  */
-void nbcon_init(struct console *con)
+void nbcon_init(struct console *con, u64 init_seq)
 {
 	struct nbcon_state state = { };
 
 	/* nbcon_alloc() must have been called and successful! */
 	BUG_ON(!con->pbufs);
 
-	nbcon_seq_force(con, con->seq);
+	nbcon_seq_force(con, init_seq);
 	nbcon_state_set(con, &state);
 }
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c7c0ee2b47eb..e6b91401895f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3348,19 +3348,21 @@ static void try_enable_default_console(struct console *newcon)
 		newcon->flags |= CON_CONSDEV;
 }
 
-static void console_init_seq(struct console *newcon, bool bootcon_registered)
+/* Return the starting sequence number for a newly registered console. */
+static u64 get_init_console_seq(struct console *newcon, bool bootcon_registered)
 {
 	struct console *con;
 	bool handover;
+	u64 init_seq;
 
 	if (newcon->flags & (CON_PRINTBUFFER | CON_BOOT)) {
 		/* Get a consistent copy of @syslog_seq. */
 		mutex_lock(&syslog_lock);
-		newcon->seq = syslog_seq;
+		init_seq = syslog_seq;
 		mutex_unlock(&syslog_lock);
 	} else {
 		/* Begin with next message added to ringbuffer. */
-		newcon->seq = prb_next_seq(prb);
+		init_seq = prb_next_seq(prb);
 
 		/*
 		 * If any enabled boot consoles are due to be unregistered
@@ -3381,7 +3383,7 @@ static void console_init_seq(struct console *newcon, bool bootcon_registered)
 			 * Flush all consoles and set the console to start at
 			 * the next unprinted sequence number.
 			 */
-			if (!console_flush_all(true, &newcon->seq, &handover)) {
+			if (!console_flush_all(true, &init_seq, &handover)) {
 				/*
 				 * Flushing failed. Just choose the lowest
 				 * sequence of the enabled boot consoles.
@@ -3394,19 +3396,30 @@ static void console_init_seq(struct console *newcon, bool bootcon_registered)
 				if (handover)
 					console_lock();
 
-				newcon->seq = prb_next_seq(prb);
+				init_seq = prb_next_seq(prb);
 				for_each_console(con) {
-					if ((con->flags & CON_BOOT) &&
-					    (con->flags & CON_ENABLED) &&
-					    con->seq < newcon->seq) {
-						newcon->seq = con->seq;
+					u64 seq;
+
+					if (!(con->flags & CON_BOOT) ||
+					    !(con->flags & CON_ENABLED)) {
+						continue;
 					}
+
+					if (con->flags & CON_NBCON)
+						seq = nbcon_seq_read(con);
+					else
+						seq = con->seq;
+
+					if (seq < init_seq)
+						init_seq = seq;
 				}
 			}
 
 			console_unlock();
 		}
 	}
+
+	return init_seq;
 }
 
 #define console_first()				\
@@ -3438,6 +3451,7 @@ void register_console(struct console *newcon)
 	struct console *con;
 	bool bootcon_registered = false;
 	bool realcon_registered = false;
+	u64 init_seq;
 	int err;
 
 	console_list_lock();
@@ -3515,10 +3529,13 @@ void register_console(struct console *newcon)
 	}
 
 	newcon->dropped = 0;
-	console_init_seq(newcon, bootcon_registered);
+	init_seq = get_init_console_seq(newcon, bootcon_registered);
 
-	if (newcon->flags & CON_NBCON)
-		nbcon_init(newcon);
+	if (newcon->flags & CON_NBCON) {
+		nbcon_init(newcon, init_seq);
+	} else {
+		newcon->seq = init_seq;
+	}
 
 	/*
 	 * Put this console in the list - keep the
-- 
2.39.2

[PATCH printk v5 03/30] printk: nbcon: Remove return value for write_atomic()

[John Ogness]

The return value of write_atomic() does not provide any useful
information. On the contrary, it makes things more complicated
for the caller to appropriately deal with the information.

Change write_atomic() to not have a return value. If the
message did not get printed due to loss of ownership, the
caller will notice this on its own. If ownership was not lost,
it will be assumed that the driver successfully printed the
message and the sequence number for that console will be
incremented.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 include/linux/console.h |  2 +-
 kernel/printk/nbcon.c   | 15 +++++++--------
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 779d388af8a0..54b98e4f0544 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -327,7 +327,7 @@ struct console {
 	struct hlist_node	node;
 
 	/* nbcon console specific members */
-	bool			(*write_atomic)(struct console *con,
+	void			(*write_atomic)(struct console *con,
 						struct nbcon_write_context *wctxt);
 	atomic_t		__private nbcon_state;
 	atomic_long_t		__private nbcon_seq;
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index de260531b0da..dd7c3180b335 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -849,7 +849,6 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 	unsigned long con_dropped;
 	struct nbcon_state cur;
 	unsigned long dropped;
-	bool done;
 
 	/*
 	 * The printk buffers are filled within an unsafe section. This
@@ -889,16 +888,16 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 	wctxt->unsafe_takeover = cur.unsafe_takeover;
 
 	if (con->write_atomic) {
-		done = con->write_atomic(con, wctxt);
+		con->write_atomic(con, wctxt);
 	} else {
-		nbcon_context_release(ctxt);
+		/*
+		 * This function should never be called for legacy consoles.
+		 * Handle it as if ownership was lost and try to continue.
+		 */
 		WARN_ON_ONCE(1);
-		done = false;
-	}
-
-	/* If not done, the emit was aborted. */
-	if (!done)
+		nbcon_context_release(ctxt);
 		return false;
+	}
 
 	/*
 	 * Since any dropped message was successfully output, reset the
-- 
2.39.2

[PATCH printk v5 04/30] printk: Check printk_deferred_enter()/_exit() usage

[John Ogness]

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Add validation that printk_deferred_enter()/_exit() are called in
non-migration contexts.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 include/linux/printk.h      |  9 +++++----
 kernel/printk/internal.h    |  3 +++
 kernel/printk/printk_safe.c | 12 ++++++++++++
 3 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 2fde40cc9677..d8b3f51d9e98 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -157,15 +157,16 @@ int _printk(const char *fmt, ...);
  */
 __printf(1, 2) __cold int _printk_deferred(const char *fmt, ...);
 
-extern void __printk_safe_enter(void);
-extern void __printk_safe_exit(void);
+extern void __printk_deferred_enter(void);
+extern void __printk_deferred_exit(void);
+
 /*
  * The printk_deferred_enter/exit macros are available only as a hack for
  * some code paths that need to defer all printk console printing. Interrupts
  * must be disabled for the deferred duration.
  */
-#define printk_deferred_enter __printk_safe_enter
-#define printk_deferred_exit __printk_safe_exit
+#define printk_deferred_enter() __printk_deferred_enter()
+#define printk_deferred_exit() __printk_deferred_exit()
 
 /*
  * Please don't use printk_ratelimit(), because it shares ratelimiting state
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 739b82adc383..831f95f26d48 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -53,6 +53,9 @@ int vprintk_store(int facility, int level,
 __printf(1, 0) int vprintk_default(const char *fmt, va_list args);
 __printf(1, 0) int vprintk_deferred(const char *fmt, va_list args);
 
+void __printk_safe_enter(void);
+void __printk_safe_exit(void);
+
 bool printk_percpu_data_ready(void);
 
 #define printk_safe_enter_irqsave(flags)	\
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 6d10927a07d8..4421ccac3113 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -26,6 +26,18 @@ void __printk_safe_exit(void)
 	this_cpu_dec(printk_context);
 }
 
+void __printk_deferred_enter(void)
+{
+	cant_migrate();
+	__printk_safe_enter();
+}
+
+void __printk_deferred_exit(void)
+{
+	cant_migrate();
+	__printk_safe_exit();
+}
+
 asmlinkage int vprintk(const char *fmt, va_list args)
 {
 #ifdef CONFIG_KGDB_KDB
-- 
2.39.2

[PATCH printk v5 05/30] printk: nbcon: Add detailed doc for write_atomic()

[John Ogness]

The write_atomic() callback has special requirements and is
allowed to use special helper functions. Provide detailed
documentation of the callback so that a developer has a
chance of implementing it correctly.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 include/linux/console.h | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 54b98e4f0544..3291cc340f1a 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -285,7 +285,7 @@ struct nbcon_write_context {
 /**
  * struct console - The console descriptor structure
  * @name:		The name of the console driver
- * @write:		Write callback to output messages (Optional)
+ * @write:		Legacy write callback to output messages (Optional)
  * @read:		Read callback for console input (Optional)
  * @device:		The underlying TTY device driver (Optional)
  * @unblank:		Callback to unblank the console (Optional)
@@ -302,7 +302,6 @@ struct nbcon_write_context {
  * @data:		Driver private data
  * @node:		hlist node for the console list
  *
- * @write_atomic:	Write callback for atomic context
  * @nbcon_state:	State for nbcon consoles
  * @nbcon_seq:		Sequence number of the next record for nbcon to print
  * @pbufs:		Pointer to nbcon private buffer
@@ -327,8 +326,34 @@ struct console {
 	struct hlist_node	node;
 
 	/* nbcon console specific members */
-	void			(*write_atomic)(struct console *con,
-						struct nbcon_write_context *wctxt);
+
+	/**
+	 * @write_atomic:
+	 *
+	 * NBCON callback to write out text in any context.
+	 *
+	 * This callback is called with the console already acquired. However,
+	 * a higher priority context is allowed to take it over by default.
+	 *
+	 * The callback must call nbcon_enter_unsafe() and nbcon_exit_unsafe()
+	 * around any code where the takeover is not safe, for example, when
+	 * manipulating the serial port registers.
+	 *
+	 * nbcon_enter_unsafe() will fail if the context has lost the console
+	 * ownership in the meantime. In this case, the callback is no longer
+	 * allowed to go forward. It must back out immediately and carefully.
+	 * The buffer content is also no longer trusted since it no longer
+	 * belongs to the context.
+	 *
+	 * The callback should allow the takeover whenever it is safe. It
+	 * increases the chance to see messages when the system is in trouble.
+	 *
+	 * The callback can be called from any context (including NMI).
+	 * Therefore it must avoid usage of any locking and instead rely
+	 * on the console ownership for synchronization.
+	 */
+	void (*write_atomic)(struct console *con, struct nbcon_write_context *wctxt);
+
 	atomic_t		__private nbcon_state;
 	atomic_long_t		__private nbcon_seq;
 	struct printk_buffers	*pbufs;
-- 
2.39.2

Re: [PATCH printk v5 05/30] printk: nbcon: Add detailed doc for write_atomic()

[Petr Mladek]

On Thu 2024-05-02 23:44:14, John Ogness wrote:
> The write_atomic() callback has special requirements and is
> allowed to use special helper functions. Provide detailed
> documentation of the callback so that a developer has a
> chance of implementing it correctly.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

[PATCH printk v5 06/30] printk: nbcon: Add callbacks to synchronize with driver

[John Ogness]

Console drivers typically must deal with access to the hardware
via user input/output (such as an interactive login shell) and
output of kernel messages via printk() calls. To provide the
necessary synchronization, usually some driver-specific locking
mechanism is used (for example, the port spinlock for uart
serial consoles).

Until now, usage of this driver-specific locking has been hidden
from the printk-subsystem and implemented within the various
console callbacks. However, for nbcon consoles, it is necessary
that the printk-subsystem uses the driver-specific locking so
that nbcon console ownership can be acquired _after_ the
driver-specific locking has succeeded. This allows for lock
contention to exist on the more context-friendly driver-specific
locking rather than nbcon console ownership (for non-emergency
and non-panic cases).

Require nbcon consoles to implement two new callbacks
(device_lock(), device_unlock()) that will use whatever
synchronization mechanism the driver is using for itself.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 include/linux/console.h | 43 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index 3291cc340f1a..33a029d976c3 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -354,6 +354,49 @@ struct console {
 	 */
 	void (*write_atomic)(struct console *con, struct nbcon_write_context *wctxt);
 
+	/**
+	 * @device_lock:
+	 *
+	 * NBCON callback to begin synchronization with driver code.
+	 *
+	 * Console drivers typically must deal with access to the hardware
+	 * via user input/output (such as an interactive login shell) and
+	 * output of kernel messages via printk() calls. This callback is
+	 * called by the printk-subsystem whenever it needs to synchronize
+	 * with hardware access by the driver. It should be implemented to
+	 * use whatever synchronization mechanism the driver is using for
+	 * itself (for example, the port lock for uart serial consoles).
+	 *
+	 * The callback is always called from task context. It may use any
+	 * synchronization method required by the driver.
+	 *
+	 * IMPORTANT: The callback MUST disable migration. The console driver
+	 *	may be using a synchronization mechanism that already takes
+	 *	care of this (such as spinlocks). Otherwise this function must
+	 *	explicitly call migrate_disable().
+	 *
+	 * The flags argument is provided as a convenience to the driver. It
+	 * will be passed again to device_unlock(). It can be ignored if the
+	 * driver does not need it.
+	 */
+	void (*device_lock)(struct console *con, unsigned long *flags);
+
+	/**
+	 * @device_unlock:
+	 *
+	 * NBCON callback to finish synchronization with driver code.
+	 *
+	 * It is the counterpart to device_lock().
+	 *
+	 * This callback is always called from task context. It must
+	 * appropriately re-enable migration (depending on how device_lock()
+	 * disabled migration).
+	 *
+	 * The flags argument is the value of the same variable that was
+	 * passed to device_lock().
+	 */
+	void (*device_unlock)(struct console *con, unsigned long flags);
+
 	atomic_t		__private nbcon_state;
 	atomic_long_t		__private nbcon_seq;
 	struct printk_buffers	*pbufs;
-- 
2.39.2

Re: [PATCH printk v5 06/30] printk: nbcon: Add callbacks to synchronize with driver

[Petr Mladek]

Hi,

On Thu 2024-05-02 23:44:15, John Ogness wrote:
> Console drivers typically must deal with access to the hardware
> via user input/output (such as an interactive login shell) and
> output of kernel messages via printk() calls. To provide the
> necessary synchronization, usually some driver-specific locking
> mechanism is used (for example, the port spinlock for uart
> serial consoles).
>
> Until now, usage of this driver-specific locking has been hidden
> from the printk-subsystem and implemented within the various
> console callbacks. However, for nbcon consoles, it is necessary
> that the printk-subsystem uses the driver-specific locking so
> that nbcon console ownership can be acquired _after_ the
> driver-specific locking has succeeded. This allows for lock
> contention to exist on the more context-friendly driver-specific
> locking rather than nbcon console ownership (for non-emergency
> and non-panic cases).

Honestly, the above paragraph is kind of a puzzle. My first
understanding was that the driver-specific lock will always
need to be taken before acquiring the nbcon console ownership.

I believe that it is actually correct. But the right meaning
is hidden in the wording. I had to read it many times to get
it correctly and I knew what I was looking for.

I have to admit that providing a good explanation is probably
very hard. A proof is the long discussion in v4, see
https://lore.kernel.org/r/20240402221129.2613843-7-john.ogness@linutronix.de

BTW: I wonder if you use AI for generating the commit message.
     My experience is that AI produces longer fancy sentences
     which might be good for a novel but they sometimes hide
     the important details.



> Require nbcon consoles to implement two new callbacks
> (device_lock(), device_unlock()) that will use whatever
> synchronization mechanism the driver is using for itself.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

My attempt of a more strightforwward explanation:

<proposal>
Console drivers typically must deal with access to the hardware
via user input/output (such as an interactive login shell) and
output of kernel messages via printk() calls. To provide the
necessary synchronization, usually some driver-specific locking
mechanism is used (for example, the port spinlock for uart
serial consoles).

Until now, usage of this driver-specific locking has been hidden
from the printk-subsystem and implemented within the various
console callbacks. However, nbcon consoles would need to use it
even in the generic code.

Add device_lock() and device_unlock() callback which will need
to get implemented by nbcon consoles.

The callbacks will use whatever synchronization mechanism the driver
is using for itself. The minimum requirement is to prevent CPU
migration. It would allow a context friendly acquiring of nbcon
console ownership in non-emergency and non-panic context.
</proposal>

That said, I could live even with the original commit message.
Eitherway:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

Re: [PATCH printk v5 06/30] printk: nbcon: Add callbacks to synchronize with driver

[John Ogness]

On 2024-05-17, Petr Mladek <pmladek@suse.com> wrote:
> BTW: I wonder if you use AI for generating the commit message.
>      My experience is that AI produces longer fancy sentences
>      which might be good for a novel but they sometimes hide
>      the important details.

I do not know if that is a compliment or an insult.

For the record, I do not use AI. The "long fancy sentences hiding
important details" are coming from a sober brain... mine.

> My attempt of a more strightforwward explanation:

[...]

Your version does not mention why the generic code now needs to use the
driver-specific locking, but I suppose that does not matter (and only
adds confusion instead of explanation).

I will use your version.

> Reviewed-by: Petr Mladek <pmladek@suse.com>

Thanks.

John

Re: [PATCH printk v5 06/30] printk: nbcon: Add callbacks to synchronize with driver

[Petr Mladek]

On Fri 2024-05-17 16:06:30, John Ogness wrote:
> On 2024-05-17, Petr Mladek <pmladek@suse.com> wrote:
> > BTW: I wonder if you use AI for generating the commit message.
> >      My experience is that AI produces longer fancy sentences
> >      which might be good for a novel but they sometimes hide
> >      the important details.
> 
> I do not know if that is a compliment or an insult.
> 
> For the record, I do not use AI. The "long fancy sentences hiding
> important details" are coming from a sober brain... mine.

Ah, take it as a complaint then ;-) Complicated things are just hard
to explain.

> > My attempt of a more strightforwward explanation:
> 
> [...]
> 
> Your version does not mention why the generic code now needs to use the
> driver-specific locking, but I suppose that does not matter (and only
> adds confusion instead of explanation).

I removed these details on purpose. I think that they will be
easier to understand with the code.

Best Regards,
Petr

[PATCH printk v5 07/30] printk: nbcon: Use driver synchronization while (un)registering

[John Ogness]

Console drivers typically have to deal with access to the
hardware via user input/output (such as an interactive login
shell) and output of kernel messages via printk() calls.

They use some classic driver-specific locking mechanism in most
situations. But console->write_atomic() callbacks, used by nbcon
consoles, are synchronized only by acquiring the console
context.

The synchronization via the console context ownership is possible
only when the console driver is registered. It is when a
particular device driver is connected with a particular console
driver.

The two synchronization mechanisms must be synchronized between
each other. It is tricky because the console context ownership
is quite special. It might be taken over by a higher priority
context. Also CPU migration must be disabled. The most tricky
part is to (dis)connect these two mechanisms during the console
(un)registration.

Use the driver-specific locking callbacks: device_lock(),
device_unlock(). They allow taking the device-specific lock
while the device is being (un)registered by the related console
driver.

For example, these callbacks lock/unlock the port lock for
serial port drivers.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/printk.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e6b91401895f..15d19d8461ed 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3451,6 +3451,7 @@ void register_console(struct console *newcon)
 	struct console *con;
 	bool bootcon_registered = false;
 	bool realcon_registered = false;
+	unsigned long flags;
 	u64 init_seq;
 	int err;
 
@@ -3537,6 +3538,19 @@ void register_console(struct console *newcon)
 		newcon->seq = init_seq;
 	}
 
+	/*
+	 * If another context is actively using the hardware of this new
+	 * console, it will not be aware of the nbcon synchronization. This
+	 * is a risk that two contexts could access the hardware
+	 * simultaneously if this new console is used for atomic printing
+	 * and the other context is still using the hardware.
+	 *
+	 * Use the driver synchronization to ensure that the hardware is not
+	 * in use while this new console transitions to being registered.
+	 */
+	if ((newcon->flags & CON_NBCON) && newcon->write_atomic)
+		newcon->device_lock(newcon, &flags);
+
 	/*
 	 * Put this console in the list - keep the
 	 * preferred driver at the head of the list.
@@ -3561,6 +3575,10 @@ void register_console(struct console *newcon)
 	 * register_console() completes.
 	 */
 
+	/* This new console is now registered. */
+	if ((newcon->flags & CON_NBCON) && newcon->write_atomic)
+		newcon->device_unlock(newcon, flags);
+
 	console_sysfs_notify();
 
 	/*
@@ -3589,6 +3607,7 @@ EXPORT_SYMBOL(register_console);
 /* Must be called under console_list_lock(). */
 static int unregister_console_locked(struct console *console)
 {
+	unsigned long flags;
 	int res;
 
 	lockdep_assert_console_list_lock_held();
@@ -3607,8 +3626,18 @@ static int unregister_console_locked(struct console *console)
 	if (!console_is_registered_locked(console))
 		return -ENODEV;
 
+	/*
+	 * Use the driver synchronization to ensure that the hardware is not
+	 * in use while this console transitions to being unregistered.
+	 */
+	if ((console->flags & CON_NBCON) && console->write_atomic)
+		console->device_lock(console, &flags);
+
 	hlist_del_init_rcu(&console->node);
 
+	if ((console->flags & CON_NBCON) && console->write_atomic)
+		console->device_unlock(console, flags);
+
 	/*
 	 * <HISTORICAL>
 	 * If this isn't the last console and it has CON_CONSDEV set, we
-- 
2.39.2

Re: [PATCH printk v5 07/30] printk: nbcon: Use driver synchronization while (un)registering

[John Ogness]

On 2024-05-02, John Ogness <john.ogness@linutronix.de> wrote:
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e6b91401895f..15d19d8461ed 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3537,6 +3538,19 @@ void register_console(struct console *newcon)
>  		newcon->seq = init_seq;
>  	}
>  
> +	/*
> +	 * If another context is actively using the hardware of this new
> +	 * console, it will not be aware of the nbcon synchronization. This
> +	 * is a risk that two contexts could access the hardware
> +	 * simultaneously if this new console is used for atomic printing
> +	 * and the other context is still using the hardware.
> +	 *
> +	 * Use the driver synchronization to ensure that the hardware is not
> +	 * in use while this new console transitions to being registered.
> +	 */
> +	if ((newcon->flags & CON_NBCON) && newcon->write_atomic)
> +		newcon->device_lock(newcon, &flags);
> +
>  	/*
>  	 * Put this console in the list - keep the
>  	 * preferred driver at the head of the list.
> @@ -3561,6 +3575,10 @@ void register_console(struct console *newcon)
>  	 * register_console() completes.
>  	 */
>  
> +	/* This new console is now registered. */
> +	if ((newcon->flags & CON_NBCON) && newcon->write_atomic)
> +		newcon->device_unlock(newcon, flags);
> +

Rather than writing the conditions twice, I will add a local variable
and check that instead.

bool use_device_lock = (newcon->flags & CON_NBCON) && newcon->write_atomic;

...

if (use_device_lock)
	newcon->device_lock(newcon, &flags);

...

if (use_device_lock)
	newcon->device_unlock(newcon, flags);

> @@ -3607,8 +3626,18 @@ static int unregister_console_locked(struct console *console)
>  	if (!console_is_registered_locked(console))
>  		return -ENODEV;
>  
> +	/*
> +	 * Use the driver synchronization to ensure that the hardware is not
> +	 * in use while this console transitions to being unregistered.
> +	 */
> +	if ((console->flags & CON_NBCON) && console->write_atomic)
> +		console->device_lock(console, &flags);
> +
>  	hlist_del_init_rcu(&console->node);
>  
> +	if ((console->flags & CON_NBCON) && console->write_atomic)
> +		console->device_unlock(console, flags);
> +

Ditto for unregister_console_locked().

John

Re: [PATCH printk v5 07/30] printk: nbcon: Use driver synchronization while (un)registering

[Petr Mladek]

On Mon 2024-05-06 12:33:28, John Ogness wrote:
> On 2024-05-02, John Ogness <john.ogness@linutronix.de> wrote:
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index e6b91401895f..15d19d8461ed 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -3537,6 +3538,19 @@ void register_console(struct console *newcon)
> >  		newcon->seq = init_seq;
> >  	}
> >  
> > +	/*
> > +	 * If another context is actively using the hardware of this new
> > +	 * console, it will not be aware of the nbcon synchronization. This
> > +	 * is a risk that two contexts could access the hardware
> > +	 * simultaneously if this new console is used for atomic printing
> > +	 * and the other context is still using the hardware.
> > +	 *
> > +	 * Use the driver synchronization to ensure that the hardware is not
> > +	 * in use while this new console transitions to being registered.
> > +	 */
> > +	if ((newcon->flags & CON_NBCON) && newcon->write_atomic)
> > +		newcon->device_lock(newcon, &flags);
> > +
> >  	/*
> >  	 * Put this console in the list - keep the
> >  	 * preferred driver at the head of the list.
> > @@ -3561,6 +3575,10 @@ void register_console(struct console *newcon)
> >  	 * register_console() completes.
> >  	 */
> >  
> > +	/* This new console is now registered. */
> > +	if ((newcon->flags & CON_NBCON) && newcon->write_atomic)
> > +		newcon->device_unlock(newcon, flags);
> > +
> 
> Rather than writing the conditions twice, I will add a local variable
> and check that instead.
> 
> bool use_device_lock = (newcon->flags & CON_NBCON) && newcon->write_atomic;
> 
> ...
> 
> if (use_device_lock)
> 	newcon->device_lock(newcon, &flags);
> 
> ...
> 
> if (use_device_lock)
> 	newcon->device_unlock(newcon, flags);
> 

Makes sense. With this change:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

[PATCH printk v5 08/30] serial: core: Provide low-level functions to lock port

[John Ogness]

It will be necessary at times for the uart nbcon console
drivers to acquire the port lock directly (without the
additional nbcon functionality of the port lock wrappers).
These are special cases such as the implementation of the
device_lock()/device_unlock() callbacks or for internal
port lock wrapper synchronization.

Provide low-level variants __uart_port_lock_irqsave() and
__uart_port_unlock_irqrestore() for this purpose.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 include/linux/serial_core.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 55b1f3ba48ac..bb3324d49453 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -588,6 +588,24 @@ struct uart_port {
 	void			*private_data;		/* generic platform data pointer */
 };
 
+/*
+ * Only for console->device_lock()/_unlock() callbacks and internal
+ * port lock wrapper synchronization.
+ */
+static inline void __uart_port_lock_irqsave(struct uart_port *up, unsigned long *flags)
+{
+	spin_lock_irqsave(&up->lock, *flags);
+}
+
+/*
+ * Only for console->device_lock()/_unlock() callbacks and internal
+ * port lock wrapper synchronization.
+ */
+static inline void __uart_port_unlock_irqrestore(struct uart_port *up, unsigned long flags)
+{
+	spin_unlock_irqrestore(&up->lock, flags);
+}
+
 /**
  * uart_port_lock - Lock the UART port
  * @up:		Pointer to UART port structure
-- 
2.39.2

[PATCH printk v5 09/30] serial: core: Introduce wrapper to set @uart_port->cons

[John Ogness]

Introduce uart_port_set_cons() as a wrapper to set @cons of a
uart_port. The wrapper sets @cons under the port lock in order
to prevent @cons from disappearing while another context is
holding the port lock. This is necessary for a follow-up
commit relating to the port lock wrappers, which rely on @cons
not changing between lock and unlock.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 drivers/tty/serial/8250/8250_core.c |  6 +++---
 drivers/tty/serial/amba-pl011.c     |  2 +-
 drivers/tty/serial/serial_core.c    | 16 ++++++++--------
 include/linux/serial_core.h         | 17 +++++++++++++++++
 4 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index b62ad9006780..41d74ee3d95a 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -627,11 +627,11 @@ static int univ8250_console_setup(struct console *co, char *options)
 
 	port = &serial8250_ports[co->index].port;
 	/* link port to console */
-	port->cons = co;
+	uart_port_set_cons(port, co);
 
 	retval = serial8250_console_setup(port, options, false);
 	if (retval != 0)
-		port->cons = NULL;
+		uart_port_set_cons(port, NULL);
 	return retval;
 }
 
@@ -689,7 +689,7 @@ static int univ8250_console_match(struct console *co, char *name, int idx,
 			continue;
 
 		co->index = i;
-		port->cons = co;
+		uart_port_set_cons(port, co);
 		return serial8250_console_setup(port, options, true);
 	}
 
diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index cf2c890a560f..347aacf8400f 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -2496,7 +2496,7 @@ static int pl011_console_match(struct console *co, char *name, int idx,
 			continue;
 
 		co->index = i;
-		port->cons = co;
+		uart_port_set_cons(port, co);
 		return pl011_console_setup(co, options);
 	}
 
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index d6a58a9e072a..0c13ea6a3afa 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -3145,8 +3145,15 @@ static int serial_core_add_one_port(struct uart_driver *drv, struct uart_port *u
 	state->uart_port = uport;
 	uport->state = state;
 
+	/*
+	 * If this port is in use as a console then the spinlock is already
+	 * initialised.
+	 */
+	if (!uart_console_registered(uport))
+		uart_port_spin_lock_init(uport);
+
 	state->pm_state = UART_PM_STATE_UNDEFINED;
-	uport->cons = drv->cons;
+	uart_port_set_cons(uport, drv->cons);
 	uport->minor = drv->tty_driver->minor_start + uport->line;
 	uport->name = kasprintf(GFP_KERNEL, "%s%d", drv->dev_name,
 				drv->tty_driver->name_base + uport->line);
@@ -3155,13 +3162,6 @@ static int serial_core_add_one_port(struct uart_driver *drv, struct uart_port *u
 		goto out;
 	}
 
-	/*
-	 * If this port is in use as a console then the spinlock is already
-	 * initialised.
-	 */
-	if (!uart_console_registered(uport))
-		uart_port_spin_lock_init(uport);
-
 	if (uport->cons && uport->dev)
 		of_console_check(uport->dev->of_node, uport->cons->name, uport->line);
 
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index bb3324d49453..038693774f21 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -606,6 +606,23 @@ static inline void __uart_port_unlock_irqrestore(struct uart_port *up, unsigned
 	spin_unlock_irqrestore(&up->lock, flags);
 }
 
+/**
+ * uart_port_set_cons - Safely set the @cons field for a uart
+ * @up:		The uart port to set
+ * @con:	The new console to set to
+ *
+ * This function must be used to set @up->cons. It uses the port lock to
+ * synchronize with the port lock wrappers in order to ensure that the console
+ * cannot change or disappear while another context is holding the port lock.
+ */
+static inline void uart_port_set_cons(struct uart_port *up, struct console *con)
+{
+	unsigned long flags;
+
+	__uart_port_lock_irqsave(up, &flags);
+	up->cons = con;
+	__uart_port_unlock_irqrestore(up, flags);
+}
 /**
  * uart_port_lock - Lock the UART port
  * @up:		Pointer to UART port structure
-- 
2.39.2

Re: [PATCH printk v5 09/30] serial: core: Introduce wrapper to set @uart_port->cons

[Théo Lebrun]

Hello,

On Thu May 2, 2024 at 11:38 PM CEST, John Ogness wrote:
> Introduce uart_port_set_cons() as a wrapper to set @cons of a
> uart_port. The wrapper sets @cons under the port lock in order
> to prevent @cons from disappearing while another context is
> holding the port lock. This is necessary for a follow-up
> commit relating to the port lock wrappers, which rely on @cons
> not changing between lock and unlock.
>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Tested series on Mobileye EyeQ5, a MIPS64 platform that uses PL011.

Tested-by: Théo Lebrun <theo.lebrun@bootlin.com> # EyeQ5, AMBA-PL011

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

Re: [PATCH printk v5 09/30] serial: core: Introduce wrapper to set @uart_port->cons

[Greg Kroah-Hartman]

On Thu, May 02, 2024 at 11:44:18PM +0206, John Ogness wrote:
> Introduce uart_port_set_cons() as a wrapper to set @cons of a
> uart_port. The wrapper sets @cons under the port lock in order
> to prevent @cons from disappearing while another context is
> holding the port lock. This is necessary for a follow-up
> commit relating to the port lock wrappers, which rely on @cons
> not changing between lock and unlock.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Re: [PATCH printk v5 09/30] serial: core: Introduce wrapper to set @uart_port->cons

[Petr Mladek]

On Thu 2024-05-02 23:44:18, John Ogness wrote:
> Introduce uart_port_set_cons() as a wrapper to set @cons of a
> uart_port. The wrapper sets @cons under the port lock in order
> to prevent @cons from disappearing while another context is
> holding the port lock. This is necessary for a follow-up
> commit relating to the port lock wrappers, which rely on @cons
> not changing between lock and unlock.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

[PATCH printk v5 10/30] console: Improve console_srcu_read_flags() comments

[John Ogness]

It was not clear when exactly console_srcu_read_flags() must be
used vs. directly reading @console->flags.

Refactor and clarify that console_srcu_read_flags() is only
needed if the console is registered or the caller is in a
context where the registration status of the console may change
(due to another context).

The function requires the caller holds @console_srcu, which will
ensure that the caller sees an appropriate @flags value for the
registered console and that exit/cleanup routines will not run
if the console is in the process of unregistration.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 include/linux/console.h | 28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 33a029d976c3..aea4f5aa4a45 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -428,28 +428,34 @@ extern void console_list_unlock(void) __releases(console_mutex);
 extern struct hlist_head console_list;
 
 /**
- * console_srcu_read_flags - Locklessly read the console flags
+ * console_srcu_read_flags - Locklessly read flags of a possibly registered
+ *				console
  * @con:	struct console pointer of console to read flags from
  *
- * This function provides the necessary READ_ONCE() and data_race()
- * notation for locklessly reading the console flags. The READ_ONCE()
- * in this function matches the WRITE_ONCE() when @flags are modified
- * for registered consoles with console_srcu_write_flags().
+ * Locklessly reading @con->flags provides a consistent read value because
+ * there is at most one CPU modifying @con->flags and that CPU is using only
+ * read-modify-write operations to do so.
  *
- * Only use this function to read console flags when locklessly
- * iterating the console list via srcu.
+ * Requires console_srcu_read_lock to be held, which implies that @con might
+ * be a registered console. The purpose of holding console_srcu_read_lock is
+ * to guarantee that the console state is valid (CON_SUSPENDED/CON_ENABLED)
+ * and that no exit/cleanup routines will run if the console is currently
+ * undergoing unregistration.
+ *
+ * If the caller is holding the console_list_lock or it is _certain_ that
+ * @con is not and will not become registered, the caller may read
+ * @con->flags directly instead.
  *
  * Context: Any context.
+ * Return: The current value of the @con->flags field.
  */
 static inline short console_srcu_read_flags(const struct console *con)
 {
 	WARN_ON_ONCE(!console_srcu_read_lock_is_held());
 
 	/*
-	 * Locklessly reading console->flags provides a consistent
-	 * read value because there is at most one CPU modifying
-	 * console->flags and that CPU is using only read-modify-write
-	 * operations to do so.
+	 * The READ_ONCE() matches the WRITE_ONCE() when @flags are modified
+	 * for registered consoles with console_srcu_write_flags().
 	 */
 	return data_race(READ_ONCE(con->flags));
 }
-- 
2.39.2

Re: [PATCH printk v5 10/30] console: Improve console_srcu_read_flags() comments

[Petr Mladek]

On Thu 2024-05-02 23:44:19, John Ogness wrote:
> It was not clear when exactly console_srcu_read_flags() must be
> used vs. directly reading @console->flags.
> 
> Refactor and clarify that console_srcu_read_flags() is only
> needed if the console is registered or the caller is in a
> context where the registration status of the console may change
> (due to another context).
> 
> The function requires the caller holds @console_srcu, which will
> ensure that the caller sees an appropriate @flags value for the
> registered console and that exit/cleanup routines will not run
> if the console is in the process of unregistration.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Sounds good:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

[PATCH printk v5 11/30] nbcon: Provide functions for drivers to acquire console for non-printing

[John Ogness]

Provide functions nbcon_driver_try_acquire() and
nbcon_driver_release() to allow drivers to acquire the nbcon
console and mark it unsafe for handover/takeover.

These functions are to be used by nbcon drivers when performing
non-printing activities that should be synchronized with their
atomic_write() callback.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 include/linux/console.h |  2 ++
 include/linux/printk.h  | 14 +++++++++++
 kernel/printk/nbcon.c   | 55 ++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index aea4f5aa4a45..3c20938f3676 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -304,6 +304,7 @@ struct nbcon_write_context {
  *
  * @nbcon_state:	State for nbcon consoles
  * @nbcon_seq:		Sequence number of the next record for nbcon to print
+ * @nbcon_driver_ctxt:	Context available for driver non-printing operations
  * @pbufs:		Pointer to nbcon private buffer
  */
 struct console {
@@ -399,6 +400,7 @@ struct console {
 
 	atomic_t		__private nbcon_state;
 	atomic_long_t		__private nbcon_seq;
+	struct nbcon_context	__private nbcon_driver_ctxt;
 	struct printk_buffers	*pbufs;
 };
 
diff --git a/include/linux/printk.h b/include/linux/printk.h
index d8b3f51d9e98..d0a1106388d1 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -9,6 +9,8 @@
 #include <linux/ratelimit_types.h>
 #include <linux/once_lite.h>
 
+struct console;
+
 extern const char linux_banner[];
 extern const char linux_proc_banner[];
 
@@ -193,6 +195,8 @@ void show_regs_print_info(const char *log_lvl);
 extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold;
 extern asmlinkage void dump_stack(void) __cold;
 void printk_trigger_flush(void);
+extern bool nbcon_driver_try_acquire(struct console *con);
+extern void nbcon_driver_release(struct console *con);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)
@@ -272,6 +276,16 @@ static inline void dump_stack(void)
 static inline void printk_trigger_flush(void)
 {
 }
+
+static inline bool nbcon_driver_try_acquire(struct console *con)
+{
+	return false;
+}
+
+static inline void nbcon_driver_release(struct console *con)
+{
+}
+
 #endif
 
 bool this_cpu_in_panic(void);
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index dd7c3180b335..b8a7e3e136d3 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -5,7 +5,9 @@
 #include <linux/kernel.h>
 #include <linux/console.h>
 #include <linux/delay.h>
+#include <linux/export.h>
 #include <linux/slab.h>
+#include <linux/string.h>
 #include "internal.h"
 /*
  * Printk console printing implementation for consoles which does not depend
@@ -528,6 +530,7 @@ static struct printk_buffers panic_nbcon_pbufs;
  * nbcon_context_try_acquire - Try to acquire nbcon console
  * @ctxt:	The context of the caller
  *
+ * Context:	Under @ctxt->con->device_lock() or local_irq_save().
  * Return:	True if the console was acquired. False otherwise.
  *
  * If the caller allowed an unsafe hostile takeover, on success the
@@ -535,7 +538,6 @@ static struct printk_buffers panic_nbcon_pbufs;
  * in an unsafe state. Otherwise, on success the caller may assume
  * the console is not in an unsafe state.
  */
-__maybe_unused
 static bool nbcon_context_try_acquire(struct nbcon_context *ctxt)
 {
 	unsigned int cpu = smp_processor_id();
@@ -989,3 +991,54 @@ void nbcon_free(struct console *con)
 
 	con->pbufs = NULL;
 }
+
+/**
+ * nbcon_driver_try_acquire - Try to acquire nbcon console and enter unsafe
+ *				section
+ * @con:	The nbcon console to acquire
+ *
+ * Context:	Under the locking mechanism implemented in
+ *		@con->device_lock() including disabling migration.
+ *
+ * Console drivers will usually use their own internal synchronization
+ * mechasism to synchronize between console printing and non-printing
+ * activities (such as setting baud rates). However, nbcon console drivers
+ * supporting atomic consoles may also want to mark unsafe sections when
+ * performing non-printing activities in order to synchronize against their
+ * atomic_write() callback.
+ *
+ * This function acquires the nbcon console using priority NBCON_PRIO_NORMAL
+ * and marks it unsafe for handover/takeover.
+ */
+bool nbcon_driver_try_acquire(struct console *con)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(con, nbcon_driver_ctxt);
+
+	cant_migrate();
+
+	memset(ctxt, 0, sizeof(*ctxt));
+	ctxt->console	= con;
+	ctxt->prio	= NBCON_PRIO_NORMAL;
+
+	if (!nbcon_context_try_acquire(ctxt))
+		return false;
+
+	if (!nbcon_context_enter_unsafe(ctxt))
+		return false;
+
+	return true;
+}
+EXPORT_SYMBOL_GPL(nbcon_driver_try_acquire);
+
+/**
+ * nbcon_driver_release - Exit unsafe section and release the nbcon console
+ * @con:	The nbcon console acquired in nbcon_driver_try_acquire()
+ */
+void nbcon_driver_release(struct console *con)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(con, nbcon_driver_ctxt);
+
+	if (nbcon_context_exit_unsafe(ctxt))
+		nbcon_context_release(ctxt);
+}
+EXPORT_SYMBOL_GPL(nbcon_driver_release);
-- 
2.39.2

Re: [PATCH printk v5 11/30] nbcon: Provide functions for drivers to acquire console for non-printing

[John Ogness]

On 2024-05-02, John Ogness <john.ogness@linutronix.de> wrote:
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index dd7c3180b335..b8a7e3e136d3 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -989,3 +991,54 @@ void nbcon_free(struct console *con)
>  
>  	con->pbufs = NULL;
>  }
> +
> +/**
> + * nbcon_driver_try_acquire - Try to acquire nbcon console and enter unsafe
> + *				section
> + * @con:	The nbcon console to acquire
> + *
> + * Context:	Under the locking mechanism implemented in
> + *		@con->device_lock() including disabling migration.

 * Return:	True if the console was acquired. False otherwise.

John

Re: [PATCH printk v5 11/30] nbcon: Provide functions for drivers to acquire console for non-printing

[Petr Mladek]

On Thu 2024-05-02 23:44:20, John Ogness wrote:
> Provide functions nbcon_driver_try_acquire() and
> nbcon_driver_release() to allow drivers to acquire the nbcon
> console and mark it unsafe for handover/takeover.

I would prefer to rename 'driver' to 'device' to keep the existing
naming scheme in struct console. The API will be used to
synchronize the 'device' element.

> These functions are to be used by nbcon drivers when performing
> non-printing activities that should be synchronized with their
> atomic_write() callback.

I am still trying to create a mental model around the synchronization
between the device-specific locks and nbcon context. Also it took
me some time to realize that just disambling the migration is
perfectly fine here.

I propose a bit extended commit message:

<proposal>
Subject: nbcon: Add API to acquire context for non-printing operations

Provide functions nbcon_device_try_acquire() and
nbcon_device_release() which will try to acquire the nbcon console
ownership with NBCON_PRIO_NORMAL and mark it unsafe for handover/takeover.

These functions are to be used together with the device-specific
locking when performing non-printing activities on the console device.
They will allow synchronization against atomic_write() callback which
will be serialized, for higher priority contexts, only by acquiring
the console context ownership.

Pitfalls:

The API requires to be called in a context with a disabled migration
because it uses per-CPU variables internally.

The context is set unsafe for a takeover all the time. It guarantees
full serialization against any atomic_write() caller except for
the final flush in panic() which might try an unsafe takeover.
</proposal>

> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -304,6 +304,7 @@ struct nbcon_write_context {
>   *
>   * @nbcon_state:	State for nbcon consoles
>   * @nbcon_seq:		Sequence number of the next record for nbcon to print
> + * @nbcon_driver_ctxt:	Context available for driver non-printing operations

This provides a context for "@device".

We should call this "@nbcon_device_ctxt" to avoid confusion.


>   * @pbufs:		Pointer to nbcon private buffer
>   */
>  struct console {
> @@ -399,6 +400,7 @@ struct console {
>  
>  	atomic_t		__private nbcon_state;
>  	atomic_long_t		__private nbcon_seq;
> +	struct nbcon_context	__private nbcon_driver_ctxt;
>  	struct printk_buffers	*pbufs;
>  };
>  
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index d8b3f51d9e98..d0a1106388d1 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -9,6 +9,8 @@
>  #include <linux/ratelimit_types.h>
>  #include <linux/once_lite.h>
>  
> +struct console;
> +
>  extern const char linux_banner[];
>  extern const char linux_proc_banner[];
>  
> @@ -193,6 +195,8 @@ void show_regs_print_info(const char *log_lvl);
>  extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold;
>  extern asmlinkage void dump_stack(void) __cold;
>  void printk_trigger_flush(void);
> +extern bool nbcon_driver_try_acquire(struct console *con);
> +extern void nbcon_driver_release(struct console *con);

Similar here:

s/driver/device/

>  #else
>  static inline __printf(1, 0)
>  int vprintk(const char *s, va_list args)


With all the renames and updated commit message:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

[PATCH printk v5 12/30] serial: core: Implement processing in port->lock wrapper

[John Ogness]

Currently the port->lock wrappers uart_port_lock(),
uart_port_unlock() (and their variants) only lock/unlock
the spin_lock.

If the port is an nbcon console, the wrappers must also
acquire/release the console and mark the region as unsafe. This
allows general port->lock synchronization to be synchronized
with the nbcon console ownership.

Note that __uart_port_using_nbcon() relies on the port->lock
being held while a console is added and removed from the
console list (i.e. all uart nbcon drivers *must* take the
port->lock in their device_lock() callbacks).

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 include/linux/serial_core.h | 82 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 80 insertions(+), 2 deletions(-)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 038693774f21..a2f307dce221 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -12,6 +12,8 @@
 #include <linux/console.h>
 #include <linux/interrupt.h>
 #include <linux/circ_buf.h>
+#include <linux/lockdep.h>
+#include <linux/printk.h>
 #include <linux/spinlock.h>
 #include <linux/sched.h>
 #include <linux/tty.h>
@@ -623,6 +625,60 @@ static inline void uart_port_set_cons(struct uart_port *up, struct console *con)
 	up->cons = con;
 	__uart_port_unlock_irqrestore(up, flags);
 }
+
+/* Only for internal port lock wrapper usage. */
+static inline bool __uart_port_using_nbcon(struct uart_port *up)
+{
+	lockdep_assert_held_once(&up->lock);
+
+	if (likely(!uart_console(up)))
+		return false;
+
+	/*
+	 * @up->cons is only modified under the port lock. Therefore it is
+	 * certain that it cannot disappear here.
+	 *
+	 * @up->cons->node is added/removed from the console list under the
+	 * port lock. Therefore it is certain that the registration status
+	 * cannot change here, thus @up->cons->flags can be read directly.
+	 */
+	if (hlist_unhashed_lockless(&up->cons->node) ||
+	    !(up->cons->flags & CON_NBCON) ||
+	    !up->cons->write_atomic) {
+		return false;
+	}
+
+	return true;
+}
+
+/* Only for internal port lock wrapper usage. */
+static inline bool __uart_port_nbcon_try_acquire(struct uart_port *up)
+{
+	if (!__uart_port_using_nbcon(up))
+		return true;
+
+	return nbcon_driver_try_acquire(up->cons);
+}
+
+/* Only for internal port lock wrapper usage. */
+static inline void __uart_port_nbcon_acquire(struct uart_port *up)
+{
+	if (!__uart_port_using_nbcon(up))
+		return;
+
+	while (!nbcon_driver_try_acquire(up->cons))
+		cpu_relax();
+}
+
+/* Only for internal port lock wrapper usage. */
+static inline void __uart_port_nbcon_release(struct uart_port *up)
+{
+	if (!__uart_port_using_nbcon(up))
+		return;
+
+	nbcon_driver_release(up->cons);
+}
+
 /**
  * uart_port_lock - Lock the UART port
  * @up:		Pointer to UART port structure
@@ -630,6 +686,7 @@ static inline void uart_port_set_cons(struct uart_port *up, struct console *con)
 static inline void uart_port_lock(struct uart_port *up)
 {
 	spin_lock(&up->lock);
+	__uart_port_nbcon_acquire(up);
 }
 
 /**
@@ -639,6 +696,7 @@ static inline void uart_port_lock(struct uart_port *up)
 static inline void uart_port_lock_irq(struct uart_port *up)
 {
 	spin_lock_irq(&up->lock);
+	__uart_port_nbcon_acquire(up);
 }
 
 /**
@@ -649,6 +707,7 @@ static inline void uart_port_lock_irq(struct uart_port *up)
 static inline void uart_port_lock_irqsave(struct uart_port *up, unsigned long *flags)
 {
 	spin_lock_irqsave(&up->lock, *flags);
+	__uart_port_nbcon_acquire(up);
 }
 
 /**
@@ -659,7 +718,15 @@ static inline void uart_port_lock_irqsave(struct uart_port *up, unsigned long *f
  */
 static inline bool uart_port_trylock(struct uart_port *up)
 {
-	return spin_trylock(&up->lock);
+	if (!spin_trylock(&up->lock))
+		return false;
+
+	if (!__uart_port_nbcon_try_acquire(up)) {
+		spin_unlock(&up->lock);
+		return false;
+	}
+
+	return true;
 }
 
 /**
@@ -671,7 +738,15 @@ static inline bool uart_port_trylock(struct uart_port *up)
  */
 static inline bool uart_port_trylock_irqsave(struct uart_port *up, unsigned long *flags)
 {
-	return spin_trylock_irqsave(&up->lock, *flags);
+	if (!spin_trylock_irqsave(&up->lock, *flags))
+		return false;
+
+	if (!__uart_port_nbcon_try_acquire(up)) {
+		spin_unlock_irqrestore(&up->lock, *flags);
+		return false;
+	}
+
+	return true;
 }
 
 /**
@@ -680,6 +755,7 @@ static inline bool uart_port_trylock_irqsave(struct uart_port *up, unsigned long
  */
 static inline void uart_port_unlock(struct uart_port *up)
 {
+	__uart_port_nbcon_release(up);
 	spin_unlock(&up->lock);
 }
 
@@ -689,6 +765,7 @@ static inline void uart_port_unlock(struct uart_port *up)
  */
 static inline void uart_port_unlock_irq(struct uart_port *up)
 {
+	__uart_port_nbcon_release(up);
 	spin_unlock_irq(&up->lock);
 }
 
@@ -699,6 +776,7 @@ static inline void uart_port_unlock_irq(struct uart_port *up)
  */
 static inline void uart_port_unlock_irqrestore(struct uart_port *up, unsigned long flags)
 {
+	__uart_port_nbcon_release(up);
 	spin_unlock_irqrestore(&up->lock, flags);
 }
 
-- 
2.39.2

Re: [PATCH printk v5 12/30] serial: core: Implement processing in port->lock wrapper

[Greg Kroah-Hartman]

On Thu, May 02, 2024 at 11:44:21PM +0206, John Ogness wrote:
> Currently the port->lock wrappers uart_port_lock(),
> uart_port_unlock() (and their variants) only lock/unlock
> the spin_lock.
> 
> If the port is an nbcon console, the wrappers must also
> acquire/release the console and mark the region as unsafe. This
> allows general port->lock synchronization to be synchronized
> with the nbcon console ownership.
> 
> Note that __uart_port_using_nbcon() relies on the port->lock
> being held while a console is added and removed from the
> console list (i.e. all uart nbcon drivers *must* take the
> port->lock in their device_lock() callbacks).
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> ---
>  include/linux/serial_core.h | 82 ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 80 insertions(+), 2 deletions(-)

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Re: [PATCH printk v5 12/30] serial: core: Implement processing in port->lock wrapper

[Petr Mladek]

On Thu 2024-05-02 23:44:21, John Ogness wrote:
> Currently the port->lock wrappers uart_port_lock(),
> uart_port_unlock() (and their variants) only lock/unlock
> the spin_lock.
> 
> If the port is an nbcon console, the wrappers must also
> acquire/release the console and mark the region as unsafe. This
> allows general port->lock synchronization to be synchronized
> with the nbcon console ownership.
> 
> Note that __uart_port_using_nbcon() relies on the port->lock
> being held while a console is added and removed from the
> console list (i.e. all uart nbcon drivers *must* take the
> port->lock in their device_lock() callbacks).
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

[PATCH printk v5 13/30] printk: nbcon: Do not rely on proxy headers

[John Ogness]

The headers kernel.h, serial_core.h, and console.h allow for the
definitions of many types and functions from other headers.
Rather than relying on these as proxy headers, explicitly
include all headers providing needed definitions. Also sort the
list alphabetically to be able to easily detect duplicates.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/internal.h          |  8 ++++++--
 kernel/printk/nbcon.c             | 13 ++++++++++++-
 kernel/printk/printk_ringbuffer.h |  2 ++
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 831f95f26d48..6c1d01eabf3b 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -2,11 +2,12 @@
 /*
  * internal.h - printk internal definitions
  */
-#include <linux/percpu.h>
 #include <linux/console.h>
-#include "printk_ringbuffer.h"
+#include <linux/percpu.h>
+#include <linux/types.h>
 
 #if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL)
+struct ctl_table;
 void __init printk_sysctl_init(void);
 int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 			      void *buffer, size_t *lenp, loff_t *ppos);
@@ -43,6 +44,9 @@ enum printk_info_flags {
 	LOG_CONT	= 8,	/* text is a fragment of a continuation line */
 };
 
+struct printk_ringbuffer;
+struct dev_printk_info;
+
 extern struct printk_ringbuffer *prb;
 
 __printf(4, 0)
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index b8a7e3e136d3..c429f4cfbb2d 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -2,13 +2,24 @@
 // Copyright (C) 2022 Linutronix GmbH, John Ogness
 // Copyright (C) 2022 Intel, Thomas Gleixner
 
-#include <linux/kernel.h>
+#include <linux/atomic.h>
+#include <linux/bug.h>
 #include <linux/console.h>
 #include <linux/delay.h>
+#include <linux/errno.h>
 #include <linux/export.h>
+#include <linux/init.h>
+#include <linux/irqflags.h>
+#include <linux/minmax.h>
+#include <linux/percpu.h>
+#include <linux/preempt.h>
 #include <linux/slab.h>
+#include <linux/smp.h>
+#include <linux/stddef.h>
 #include <linux/string.h>
+#include <linux/types.h>
 #include "internal.h"
+#include "printk_ringbuffer.h"
 /*
  * Printk console printing implementation for consoles which does not depend
  * on the legacy style console_lock mechanism.
diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
index 52626d0f1fa3..bd2a892deac1 100644
--- a/kernel/printk/printk_ringbuffer.h
+++ b/kernel/printk/printk_ringbuffer.h
@@ -5,6 +5,8 @@
 
 #include <linux/atomic.h>
 #include <linux/dev_printk.h>
+#include <linux/stddef.h>
+#include <linux/types.h>
 
 /*
  * Meta information about each stored message.
-- 
2.39.2

[PATCH printk v5 14/30] printk: nbcon: Fix kerneldoc for enums

[John Ogness]

Kerneldoc requires enums to be specified as such. Otherwise it is
interpreted as function documentation.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 include/linux/console.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 3c20938f3676..71d0638aa818 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -137,7 +137,7 @@ static inline int con_debug_leave(void)
  */
 
 /**
- * cons_flags - General console flags
+ * enum cons_flags - General console flags
  * @CON_PRINTBUFFER:	Used by newly registered consoles to avoid duplicate
  *			output of messages that were already shown by boot
  *			consoles or read by userspace via syslog() syscall.
@@ -218,7 +218,7 @@ struct nbcon_state {
 static_assert(sizeof(struct nbcon_state) <= sizeof(int));
 
 /**
- * nbcon_prio - console owner priority for nbcon consoles
+ * enum nbcon_prio - console owner priority for nbcon consoles
  * @NBCON_PRIO_NONE:		Unused
  * @NBCON_PRIO_NORMAL:		Normal (non-emergency) usage
  * @NBCON_PRIO_EMERGENCY:	Emergency output (WARN/OOPS...)
-- 
2.39.2

[PATCH printk v5 15/30] printk: Make console_is_usable() available to nbcon

[John Ogness]

Move console_is_usable() as-is into internal.h so that it can
be used by nbcon printing functions as well.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/internal.h | 32 ++++++++++++++++++++++++++++++++
 kernel/printk/printk.c   | 30 ------------------------------
 2 files changed, 32 insertions(+), 30 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 6c1d01eabf3b..339563dd60bb 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -85,6 +85,36 @@ bool nbcon_alloc(struct console *con);
 void nbcon_init(struct console *con, u64 init_seq);
 void nbcon_free(struct console *con);
 
+/*
+ * Check if the given console is currently capable and allowed to print
+ * records.
+ *
+ * Requires the console_srcu_read_lock.
+ */
+static inline bool console_is_usable(struct console *con)
+{
+	short flags = console_srcu_read_flags(con);
+
+	if (!(flags & CON_ENABLED))
+		return false;
+
+	if ((flags & CON_SUSPENDED))
+		return false;
+
+	if (!con->write)
+		return false;
+
+	/*
+	 * Console drivers may assume that per-cpu resources have been
+	 * allocated. So unless they're explicitly marked as being able to
+	 * cope (CON_ANYTIME) don't call them until this CPU is officially up.
+	 */
+	if (!cpu_online(raw_smp_processor_id()) && !(flags & CON_ANYTIME))
+		return false;
+
+	return true;
+}
+
 #else
 
 #define PRINTK_PREFIX_MAX	0
@@ -106,6 +136,8 @@ static inline bool nbcon_alloc(struct console *con) { return false; }
 static inline void nbcon_init(struct console *con, u64 init_seq) { }
 static inline void nbcon_free(struct console *con) { }
 
+static inline bool console_is_usable(struct console *con) { return false; }
+
 #endif /* CONFIG_PRINTK */
 
 extern struct printk_buffers printk_shared_pbufs;
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 15d19d8461ed..b0e99ee472e1 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2697,36 +2697,6 @@ int is_console_locked(void)
 }
 EXPORT_SYMBOL(is_console_locked);
 
-/*
- * Check if the given console is currently capable and allowed to print
- * records.
- *
- * Requires the console_srcu_read_lock.
- */
-static inline bool console_is_usable(struct console *con)
-{
-	short flags = console_srcu_read_flags(con);
-
-	if (!(flags & CON_ENABLED))
-		return false;
-
-	if ((flags & CON_SUSPENDED))
-		return false;
-
-	if (!con->write)
-		return false;
-
-	/*
-	 * Console drivers may assume that per-cpu resources have been
-	 * allocated. So unless they're explicitly marked as being able to
-	 * cope (CON_ANYTIME) don't call them until this CPU is officially up.
-	 */
-	if (!cpu_online(raw_smp_processor_id()) && !(flags & CON_ANYTIME))
-		return false;
-
-	return true;
-}
-
 static void __console_unlock(void)
 {
 	console_locked = 0;
-- 
2.39.2

[PATCH printk v5 16/30] printk: Let console_is_usable() handle nbcon

[John Ogness]

The nbcon consoles use a different printing callback. For nbcon
consoles, check for the write_atomic() callback instead of
write().

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/internal.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 339563dd60bb..943e8f083609 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -87,6 +87,8 @@ void nbcon_free(struct console *con);
 
 /*
  * Check if the given console is currently capable and allowed to print
+ * records. Note that this function does not consider the current context,
+ * which can also play a role in deciding if @con can be used to print
  * records.
  *
  * Requires the console_srcu_read_lock.
@@ -101,8 +103,13 @@ static inline bool console_is_usable(struct console *con)
 	if ((flags & CON_SUSPENDED))
 		return false;
 
-	if (!con->write)
-		return false;
+	if (flags & CON_NBCON) {
+		if (!con->write_atomic)
+			return false;
+	} else {
+		if (!con->write)
+			return false;
+	}
 
 	/*
 	 * Console drivers may assume that per-cpu resources have been
-- 
2.39.2

[PATCH printk v5 17/30] printk: Add @flags argument for console_is_usable()

[John Ogness]

The caller of console_is_usable() usually needs @console->flags
for its own checks. Rather than having console_is_usable() read
its own copy, make the caller pass in the @flags. This also
ensures that the caller saw the same @flags value.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/internal.h | 8 ++------
 kernel/printk/printk.c   | 5 +++--
 2 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 943e8f083609..e2de2d262db4 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -90,13 +90,9 @@ void nbcon_free(struct console *con);
  * records. Note that this function does not consider the current context,
  * which can also play a role in deciding if @con can be used to print
  * records.
- *
- * Requires the console_srcu_read_lock.
  */
-static inline bool console_is_usable(struct console *con)
+static inline bool console_is_usable(struct console *con, short flags)
 {
-	short flags = console_srcu_read_flags(con);
-
 	if (!(flags & CON_ENABLED))
 		return false;
 
@@ -143,7 +139,7 @@ static inline bool nbcon_alloc(struct console *con) { return false; }
 static inline void nbcon_init(struct console *con, u64 init_seq) { }
 static inline void nbcon_free(struct console *con) { }
 
-static inline bool console_is_usable(struct console *con) { return false; }
+static inline bool console_is_usable(struct console *con, short flags) { return false; }
 
 #endif /* CONFIG_PRINTK */
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b0e99ee472e1..9cdc110a46a2 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2942,9 +2942,10 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 
 		cookie = console_srcu_read_lock();
 		for_each_console_srcu(con) {
+			short flags = console_srcu_read_flags(con);
 			bool progress;
 
-			if (!console_is_usable(con))
+			if (!console_is_usable(con, flags))
 				continue;
 			any_usable = true;
 
@@ -3823,7 +3824,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 			 * that they make forward progress, so only increment
 			 * @diff for usable consoles.
 			 */
-			if (!console_is_usable(c))
+			if (!console_is_usable(c, flags))
 				continue;
 
 			if (flags & CON_NBCON) {
-- 
2.39.2

[PATCH printk v5 18/30] printk: nbcon: Add helper to assign priority based on CPU state

[John Ogness]

Add a helper function to use the current state of the CPU to
determine which priority to assign to the printing context.

The EMERGENCY priority handling is added in a follow-up commit.
It will use a per-CPU variable.

Note: nbcon_driver_try_acquire(), which is used by console
      drivers to acquire the nbcon console for non-printing
      activities, will always use NORMAL priority.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/internal.h |  2 ++
 kernel/printk/nbcon.c    | 16 ++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index e2de2d262db4..5826cd4eed58 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -84,6 +84,7 @@ void nbcon_seq_force(struct console *con, u64 seq);
 bool nbcon_alloc(struct console *con);
 void nbcon_init(struct console *con, u64 init_seq);
 void nbcon_free(struct console *con);
+enum nbcon_prio nbcon_get_default_prio(void);
 
 /*
  * Check if the given console is currently capable and allowed to print
@@ -138,6 +139,7 @@ static inline void nbcon_seq_force(struct console *con, u64 seq) { }
 static inline bool nbcon_alloc(struct console *con) { return false; }
 static inline void nbcon_init(struct console *con, u64 init_seq) { }
 static inline void nbcon_free(struct console *con) { }
+static inline enum nbcon_prio nbcon_get_default_prio(void) { return NBCON_PRIO_NONE; }
 
 static inline bool console_is_usable(struct console *con, short flags) { return false; }
 
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index c429f4cfbb2d..e37af5c72d38 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -937,6 +937,22 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 	return nbcon_context_exit_unsafe(ctxt);
 }
 
+/**
+ * nbcon_get_default_prio - The appropriate nbcon priority to use for nbcon
+ *				printing on the current CPU
+ *
+ * Context:	Any context which could not be migrated to another CPU.
+ * Return:	The nbcon_prio to use for acquiring an nbcon console in this
+ *		context for printing.
+ */
+enum nbcon_prio nbcon_get_default_prio(void)
+{
+	if (this_cpu_in_panic())
+		return NBCON_PRIO_PANIC;
+
+	return NBCON_PRIO_NORMAL;
+}
+
 /**
  * nbcon_alloc - Allocate buffers needed by the nbcon console
  * @con:	Console to allocate buffers for
-- 
2.39.2

[PATCH printk v5 19/30] printk: nbcon: Provide function to flush using write_atomic()

[John Ogness]

From: Thomas Gleixner <tglx@linutronix.de>

Provide nbcon_atomic_flush_pending() to perform flushing of all
registered nbcon consoles using their write_atomic() callback.

Unlike console_flush_all(), nbcon_atomic_flush_pending() will
only flush up through the newest record at the time of the
call. This prevents a CPU from printing unbounded when other
CPUs are adding records. If new records are added while
flushing, it is expected that the dedicated printer threads
will print those records. If the printer thread is not
available (which is always the case at this point in the
rework), nbcon_atomic_flush_pending() _will_ flush all records
in the ringbuffer.

Unlike console_flush_all(), nbcon_atomic_flush_pending() will
fully flush one console before flushing the next. This helps to
guarantee that a block of pending records (such as a stack
trace in an emergency situation) can be printed atomically at
once before releasing console ownership.

nbcon_atomic_flush_pending() is safe in any context because it
uses write_atomic() and acquires with unsafe_takeover disabled.

Use it in console_flush_on_panic() before flushing legacy
consoles. The legacy write() callbacks are not fully safe when
oops_in_progress is set.

Also use it in nbcon_driver_release() to flush records added
while the driver had the console locked to perform non-printing
operations.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
---
 kernel/printk/internal.h |   2 +
 kernel/printk/nbcon.c    | 162 ++++++++++++++++++++++++++++++++++++++-
 kernel/printk/printk.c   |   2 +
 3 files changed, 163 insertions(+), 3 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 5826cd4eed58..5a93825871de 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -85,6 +85,7 @@ bool nbcon_alloc(struct console *con);
 void nbcon_init(struct console *con, u64 init_seq);
 void nbcon_free(struct console *con);
 enum nbcon_prio nbcon_get_default_prio(void);
+void nbcon_atomic_flush_pending(void);
 
 /*
  * Check if the given console is currently capable and allowed to print
@@ -140,6 +141,7 @@ static inline bool nbcon_alloc(struct console *con) { return false; }
 static inline void nbcon_init(struct console *con, u64 init_seq) { }
 static inline void nbcon_free(struct console *con) { }
 static inline enum nbcon_prio nbcon_get_default_prio(void) { return NBCON_PRIO_NONE; }
+static inline void nbcon_atomic_flush_pending(void) { }
 
 static inline bool console_is_usable(struct console *con, short flags) { return false; }
 
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index e37af5c72d38..7532eba55d15 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -850,7 +850,6 @@ EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
  * When true is returned, @wctxt->ctxt.backlog indicates whether there are
  * still records pending in the ringbuffer,
  */
-__maybe_unused
 static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 {
 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
@@ -953,6 +952,148 @@ enum nbcon_prio nbcon_get_default_prio(void)
 	return NBCON_PRIO_NORMAL;
 }
 
+/*
+ * __nbcon_atomic_flush_pending_con - Flush specified nbcon console using its
+ *					write_atomic() callback
+ * @con:			The nbcon console to flush
+ * @stop_seq:			Flush up until this record
+ *
+ * Return:	0 if @con was flushed up to @stop_seq Otherwise, error code on
+ *		failure.
+ *
+ * Errors:
+ *
+ *	-EPERM:		Unable to acquire console ownership.
+ *
+ *	-EAGAIN:	Another context took over ownership while printing.
+ *
+ *	-ENOENT:	A record before @stop_seq is not available.
+ *
+ * If flushing up to @stop_seq was not successful, it only makes sense for the
+ * caller to try again when -EAGAIN was returned. When -EPERM is returned,
+ * this context is not allowed to acquire the console. When -ENOENT is
+ * returned, it cannot be expected that the unfinalized record will become
+ * available.
+ */
+static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
+{
+	struct nbcon_write_context wctxt = { };
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
+	int err = 0;
+
+	ctxt->console			= con;
+	ctxt->spinwait_max_us		= 2000;
+	ctxt->prio			= nbcon_get_default_prio();
+
+	if (!nbcon_context_try_acquire(ctxt))
+		return -EPERM;
+
+	while (nbcon_seq_read(con) < stop_seq) {
+		/*
+		 * nbcon_emit_next_record() returns false when the console was
+		 * handed over or taken over. In both cases the context is no
+		 * longer valid.
+		 */
+		if (!nbcon_emit_next_record(&wctxt))
+			return -EAGAIN;
+
+		if (!ctxt->backlog) {
+			if (nbcon_seq_read(con) < stop_seq)
+				err = -ENOENT;
+			break;
+		}
+	}
+
+	nbcon_context_release(ctxt);
+	return err;
+}
+
+/**
+ * nbcon_atomic_flush_pending_con - Flush specified nbcon console using its
+ *					write_atomic() callback
+ * @con:			The nbcon console to flush
+ * @stop_seq:			Flush up until this record
+ *
+ * This will stop flushing before @stop_seq if another context has ownership.
+ * That context is then responsible for the flushing. Likewise, if new records
+ * are added while this context was flushing and there is no other context
+ * to handle the printing, this context must also flush those records.
+ */
+static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
+{
+	unsigned long flags;
+	int err;
+
+again:
+	/*
+	 * Atomic flushing does not use console driver synchronization (i.e.
+	 * it does not hold the port lock for uart consoles). Therefore IRQs
+	 * must be disabled to avoid being interrupted and then calling into
+	 * a driver that will deadlock trying to acquire console ownership.
+	 */
+	local_irq_save(flags);
+
+	err = __nbcon_atomic_flush_pending_con(con, stop_seq);
+
+	local_irq_restore(flags);
+
+	/*
+	 * If flushing was successful but more records are available, this
+	 * context must flush those remaining records because there is no
+	 * other context that will do it.
+	 */
+	if (!err && prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
+		stop_seq = prb_next_reserve_seq(prb);
+		goto again;
+	}
+
+	/*
+	 * If there was a new owner, that context is responsible for
+	 * completing the flush.
+	 */
+}
+
+/**
+ * __nbcon_atomic_flush_pending - Flush all nbcon consoles using their
+ *					write_atomic() callback
+ * @stop_seq:			Flush up until this record
+ */
+static void __nbcon_atomic_flush_pending(u64 stop_seq)
+{
+	struct console *con;
+	int cookie;
+
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(con) {
+		short flags = console_srcu_read_flags(con);
+
+		if (!(flags & CON_NBCON))
+			continue;
+
+		if (!console_is_usable(con, flags))
+			continue;
+
+		if (nbcon_seq_read(con) >= stop_seq)
+			continue;
+
+		nbcon_atomic_flush_pending_con(con, stop_seq);
+	}
+	console_srcu_read_unlock(cookie);
+}
+
+/**
+ * nbcon_atomic_flush_pending - Flush all nbcon consoles using their
+ *				write_atomic() callback
+ *
+ * Flush the backlog up through the currently newest record. Any new
+ * records added while flushing will not be flushed. This is to avoid
+ * one CPU printing unbounded because other CPUs continue to add records.
+ */
+void nbcon_atomic_flush_pending(void)
+{
+	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb));
+}
+
 /**
  * nbcon_alloc - Allocate buffers needed by the nbcon console
  * @con:	Console to allocate buffers for
@@ -1064,8 +1205,23 @@ EXPORT_SYMBOL_GPL(nbcon_driver_try_acquire);
 void nbcon_driver_release(struct console *con)
 {
 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(con, nbcon_driver_ctxt);
+	int cookie;
 
-	if (nbcon_context_exit_unsafe(ctxt))
-		nbcon_context_release(ctxt);
+	if (!nbcon_context_exit_unsafe(ctxt))
+		return;
+
+	nbcon_context_release(ctxt);
+
+	/*
+	 * This context must flush any new records added while the console
+	 * was locked. The console_srcu_read_lock must be taken to ensure
+	 * the console is usable throughout flushing.
+	 */
+	cookie = console_srcu_read_lock();
+	if (console_is_usable(con, console_srcu_read_flags(con)) &&
+	    prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
+		__nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb));
+	}
+	console_srcu_read_unlock(cookie);
 }
 EXPORT_SYMBOL_GPL(nbcon_driver_release);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9cdc110a46a2..8c93ef6292ac 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3172,6 +3172,8 @@ void console_flush_on_panic(enum con_flush_mode mode)
 		console_srcu_read_unlock(cookie);
 	}
 
+	nbcon_atomic_flush_pending();
+
 	console_flush_all(false, &next_seq, &handover);
 }
 
-- 
2.39.2

Re: [PATCH printk v5 19/30] printk: nbcon: Provide function to flush using write_atomic()

[Petr Mladek]

On Thu 2024-05-02 23:44:28, John Ogness wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> Provide nbcon_atomic_flush_pending() to perform flushing of all
> registered nbcon consoles using their write_atomic() callback.
> 
> Unlike console_flush_all(), nbcon_atomic_flush_pending() will
> only flush up through the newest record at the time of the
> call. This prevents a CPU from printing unbounded when other
> CPUs are adding records. If new records are added while
> flushing, it is expected that the dedicated printer threads
> will print those records. If the printer thread is not
> available (which is always the case at this point in the
> rework), nbcon_atomic_flush_pending() _will_ flush all records
> in the ringbuffer.
> 
> Unlike console_flush_all(), nbcon_atomic_flush_pending() will
> fully flush one console before flushing the next. This helps to
> guarantee that a block of pending records (such as a stack
> trace in an emergency situation) can be printed atomically at
> once before releasing console ownership.
> 
> nbcon_atomic_flush_pending() is safe in any context because it
> uses write_atomic() and acquires with unsafe_takeover disabled.
> 
> Use it in console_flush_on_panic() before flushing legacy
> consoles. The legacy write() callbacks are not fully safe when
> oops_in_progress is set.
> 
> Also use it in nbcon_driver_release() to flush records added
> while the driver had the console locked to perform non-printing
> operations.
> 
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -953,6 +952,148 @@ enum nbcon_prio nbcon_get_default_prio(void)
>  	return NBCON_PRIO_NORMAL;
>  }
>  
> +/*
> + * __nbcon_atomic_flush_pending_con - Flush specified nbcon console using its
> + *					write_atomic() callback
> + * @con:			The nbcon console to flush
> + * @stop_seq:			Flush up until this record
> + *
> + * Return:	0 if @con was flushed up to @stop_seq Otherwise, error code on
> + *		failure.
> + *
> + * Errors:
> + *
> + *	-EPERM:		Unable to acquire console ownership.
> + *
> + *	-EAGAIN:	Another context took over ownership while printing.
> + *
> + *	-ENOENT:	A record before @stop_seq is not available.
> + *
> + * If flushing up to @stop_seq was not successful, it only makes sense for the
> + * caller to try again when -EAGAIN was returned. When -EPERM is returned,
> + * this context is not allowed to acquire the console. When -ENOENT is
> + * returned, it cannot be expected that the unfinalized record will become
> + * available.
> + */
> +static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
> +{
> +	struct nbcon_write_context wctxt = { };
> +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
> +	int err = 0;
> +
> +	ctxt->console			= con;
> +	ctxt->spinwait_max_us		= 2000;
> +	ctxt->prio			= nbcon_get_default_prio();
> +
> +	if (!nbcon_context_try_acquire(ctxt))
> +		return -EPERM;
> +
> +	while (nbcon_seq_read(con) < stop_seq) {
> +		/*
> +		 * nbcon_emit_next_record() returns false when the console was
> +		 * handed over or taken over. In both cases the context is no
> +		 * longer valid.
> +		 */
> +		if (!nbcon_emit_next_record(&wctxt))
> +			return -EAGAIN;
> +
> +		if (!ctxt->backlog) {

This would deserve a comment:

			/* Are there reserved and no-yet finalized records? */
> +			if (nbcon_seq_read(con) < stop_seq)
> +				err = -ENOENT;
> +			break;
> +		}
> +	}
> +
> +	nbcon_context_release(ctxt);
> +	return err;
> +}
> +
> +/**
> + * nbcon_atomic_flush_pending_con - Flush specified nbcon console using its
> + *					write_atomic() callback
> + * @con:			The nbcon console to flush
> + * @stop_seq:			Flush up until this record
> + *
> + * This will stop flushing before @stop_seq if another context has ownership.
> + * That context is then responsible for the flushing. Likewise, if new records
> + * are added while this context was flushing and there is no other context
> + * to handle the printing, this context must also flush those records.
> + */
> +static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
> +{
> +	unsigned long flags;
> +	int err;
> +
> +again:
> +	/*
> +	 * Atomic flushing does not use console driver synchronization (i.e.
> +	 * it does not hold the port lock for uart consoles). Therefore IRQs
> +	 * must be disabled to avoid being interrupted and then calling into
> +	 * a driver that will deadlock trying to acquire console ownership.
> +	 */
> +	local_irq_save(flags);
> +
> +	err = __nbcon_atomic_flush_pending_con(con, stop_seq);
> +
> +	local_irq_restore(flags);
> +
> +	/*
> +	 * If flushing was successful but more records are available, this
> +	 * context must flush those remaining records because there is no
> +	 * other context that will do it.
> +	 */
> +	if (!err && prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
> +		stop_seq = prb_next_reserve_seq(prb);
> +		goto again;
> +	}
> +
> +	/*
> +	 * If there was a new owner, that context is responsible for
> +	 * completing the flush.
> +	 */

This is a bit weird code layout. I wonder if it will get extended
in some future patchset but...

Anyway, there are three possible errors and the above paragraph
talks about one situation. Let's get through them:

  -EPERM: OK, can't do much

  -EAGAIN: OK, the other context is responsible for flush

  -ENOENT: ??? It took me quite some time to scratch my head
	around this. IMHO, it makes sense after all but it
	would deserve a comment.

What about reshufling the code a bit?

<proposal>
	/*
	 * If there was a new owner (-EPERM, -EAGAIN), that context is
	 * responsible for completing.
	 *
	 * Do not wait for not-yet finalized records (-ENOENT) to a possible
	 * deadlock. They will either get flushed by the writer or eventually
	 * skipped on panic CPU.
	 */
	 if (err)
		return;

	/*
	 * If flushing was successful but more records are available, this
	 * context must flush those remaining records because there is no
	 * other context that will do it.
	 */
	if (prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
		stop_seq = prb_next_reserve_seq(prb);
		goto again;
	}
</proposal>


> +}
> +
> @@ -1064,8 +1205,23 @@ EXPORT_SYMBOL_GPL(nbcon_driver_try_acquire);
>  void (struct console *con)
>  {
>  	struct nbcon_context *ctxt = &ACCESS_PRIVATE(con, nbcon_driver_ctxt);
> +	int cookie;
>  
> -	if (nbcon_context_exit_unsafe(ctxt))
> -		nbcon_context_release(ctxt);
> +	if (!nbcon_context_exit_unsafe(ctxt))
> +		return;
> +
> +	nbcon_context_release(ctxt);
> +
> +	/*
> +	 * This context must flush any new records added while the console
> +	 * was locked. The console_srcu_read_lock must be taken to ensure
> +	 * the console is usable throughout flushing.
> +	 */
> +	cookie = console_srcu_read_lock();

In principle, this should not be needed because the console is
added/removed under con->device_lock() in register_console()/unregister_console().
And this function nbcon_driver_release() should be called under the
same lock.

If only we could add an lockdep_assert() here. But I can't think of
any simple solution.



> +	if (console_is_usable(con, console_srcu_read_flags(con)) &&
> +	    prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
> +		__nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb));
> +	}
> +	console_srcu_read_unlock(cookie);
>  }
>  EXPORT_SYMBOL_GPL(nbcon_driver_release);

Best Regards,
Petr

Re: [PATCH printk v5 19/30] printk: nbcon: Provide function to flush using write_atomic()

[John Ogness]

On 2024-05-20, Petr Mladek <pmladek@suse.com> wrote:
> What about reshufling the code a bit?
>
> <proposal>
  [...]
> </proposal>

Your proposal is fine and is easier to follow. Thanks.

>> +	/*
>> +	 * This context must flush any new records added while the console
>> +	 * was locked. The console_srcu_read_lock must be taken to ensure
>> +	 * the console is usable throughout flushing.
>> +	 */
>> +	cookie = console_srcu_read_lock();
>
> In principle, this should not be needed because the console is
> added/removed under con->device_lock() in
> register_console()/unregister_console().  And this function
> nbcon_driver_release() should be called under the same lock.

I added a comment here to make it clear why it is needed. It is about
making sure the console is _usable_ during the flushing, i.e. it
prevents console_suspend() and console_stop() from exiting while the
flush is in progress.

I will leave this as is for v6.

John

[PATCH printk v5 20/30] printk: Track registered boot consoles

[John Ogness]

Unfortunately it is not known if a boot console and a regular
(legacy or nbcon) console use the same hardware. For this reason
they must not be allowed to print simultaneously.

For legacy consoles this is not an issue because they are
already synchronized with the boot consoles using the console
lock. However nbcon consoles can be triggered separately.

Add a global flag @have_boot_console to identify if any boot
consoles are registered. This will be used in follow-up commits
to ensure that boot consoles and nbcon consoles cannot print
simultaneously.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8c93ef6292ac..e1cf0f7d6ff2 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -463,6 +463,14 @@ static int console_msg_format = MSG_FORMAT_DEFAULT;
 /* syslog_lock protects syslog_* variables and write access to clear_seq. */
 static DEFINE_MUTEX(syslog_lock);
 
+/*
+ * Specifies if a boot console is registered. If boot consoles are present,
+ * nbcon consoles cannot print simultaneously and must be synchronized by
+ * the console lock. This is because boot consoles and nbcon consoles may
+ * have mapped the same hardware.
+ */
+static bool have_boot_console;
+
 #ifdef CONFIG_PRINTK
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
 /* All 3 protected by @syslog_lock. */
@@ -3511,6 +3519,9 @@ void register_console(struct console *newcon)
 		newcon->seq = init_seq;
 	}
 
+	if (newcon->flags & CON_BOOT)
+		have_boot_console = true;
+
 	/*
 	 * If another context is actively using the hardware of this new
 	 * console, it will not be aware of the nbcon synchronization. This
@@ -3580,7 +3591,9 @@ EXPORT_SYMBOL(register_console);
 /* Must be called under console_list_lock(). */
 static int unregister_console_locked(struct console *console)
 {
+	bool found_boot_con = false;
 	unsigned long flags;
+	struct console *c;
 	int res;
 
 	lockdep_assert_console_list_lock_held();
@@ -3638,6 +3651,17 @@ static int unregister_console_locked(struct console *console)
 	if (console->exit)
 		res = console->exit(console);
 
+	/*
+	 * With this console gone, the global flags tracking registered
+	 * console types may have changed. Update them.
+	 */
+	for_each_console(c) {
+		if (c->flags & CON_BOOT)
+			found_boot_con = true;
+	}
+	if (!found_boot_con)
+		have_boot_console = found_boot_con;
+
 	return res;
 }
 
-- 
2.39.2

[PATCH printk v5 21/30] printk: nbcon: Use nbcon consoles in console_flush_all()

[John Ogness]

Allow nbcon consoles to print messages in the legacy printk()
caller context (printing via unlock) by integrating them into
console_flush_all(). The write_atomic() callback is used for
printing.

Provide nbcon_legacy_emit_next_record(), which acts as the
nbcon variant of console_emit_next_record(). Call this variant
within console_flush_all() for nbcon consoles. Since nbcon
consoles use their own @nbcon_seq variable to track the next
record to print, this also must be appropriately handled in
console_flush_all().

Note that the legacy printing logic uses @handover to detect
handovers for printing all consoles. For nbcon consoles,
handovers/takeovers occur on a per-console basis and thus do
not cause the console_flush_all() loop to abort.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/internal.h |  6 +++
 kernel/printk/nbcon.c    | 87 ++++++++++++++++++++++++++++++++++++++++
 kernel/printk/printk.c   | 17 +++++---
 3 files changed, 105 insertions(+), 5 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 5a93825871de..91bc0ee43f8d 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -78,6 +78,8 @@ void defer_console_output(void);
 
 u16 printk_parse_prefix(const char *text, int *level,
 			enum printk_info_flags *flags);
+void console_lock_spinning_enable(void);
+int console_lock_spinning_disable_and_check(int cookie);
 
 u64 nbcon_seq_read(struct console *con);
 void nbcon_seq_force(struct console *con, u64 seq);
@@ -86,6 +88,8 @@ void nbcon_init(struct console *con, u64 init_seq);
 void nbcon_free(struct console *con);
 enum nbcon_prio nbcon_get_default_prio(void);
 void nbcon_atomic_flush_pending(void);
+bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
+				   int cookie);
 
 /*
  * Check if the given console is currently capable and allowed to print
@@ -142,6 +146,8 @@ static inline void nbcon_init(struct console *con, u64 init_seq) { }
 static inline void nbcon_free(struct console *con) { }
 static inline enum nbcon_prio nbcon_get_default_prio(void) { return NBCON_PRIO_NONE; }
 static inline void nbcon_atomic_flush_pending(void) { }
+static inline bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
+						 int cookie) { return false; }
 
 static inline bool console_is_usable(struct console *con, short flags) { return false; }
 
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 7532eba55d15..abd3835a3466 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -953,6 +953,93 @@ enum nbcon_prio nbcon_get_default_prio(void)
 }
 
 /*
+ * nbcon_atomic_emit_one - Print one record for an nbcon console using the
+ *				write_atomic() callback
+ * @wctxt:	An initialized write context struct to use for this context
+ *
+ * Return:	True, when a record has been printed and there are still
+ *		pending records. The caller might want to continue flushing.
+ *
+ *		False, when there is no pending record, or when the console
+ *		context cannot be acquired, or the ownership has been lost.
+ *		The caller should give up. Either the job is done, cannot be
+ *		done, or will be handled by the owning context.
+ *
+ * This is an internal helper to handle the locking of the console before
+ * calling nbcon_emit_next_record().
+ */
+static bool nbcon_atomic_emit_one(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	if (!nbcon_context_try_acquire(ctxt))
+		return false;
+
+	/*
+	 * nbcon_emit_next_record() returns false when the console was
+	 * handed over or taken over. In both cases the context is no
+	 * longer valid.
+	 *
+	 * The higher priority printing context takes over responsibility
+	 * to print the pending records.
+	 */
+	if (!nbcon_emit_next_record(wctxt))
+		return false;
+
+	nbcon_context_release(ctxt);
+
+	return ctxt->backlog;
+}
+
+/**
+ * nbcon_legacy_emit_next_record - Print one record for an nbcon console
+ *					in legacy contexts
+ * @con:	The console to print on
+ * @handover:	Will be set to true if a printk waiter has taken over the
+ *		console_lock, in which case the caller is no longer holding
+ *		both the console_lock and the SRCU read lock. Otherwise it
+ *		is set to false.
+ * @cookie:	The cookie from the SRCU read lock.
+ *
+ * Context:	Any context except NMI.
+ * Return:	True, when a record has been printed and there are still
+ *		pending records. The caller might want to continue flushing.
+ *
+ *		False, when there is no pending record, or when the console
+ *		context cannot be acquired, or the ownership has been lost.
+ *		The caller should give up. Either the job is done, cannot be
+ *		done, or will be handled by the owning context.
+ *
+ * This function is meant to be called by console_flush_all() to print records
+ * on nbcon consoles from legacy context (printing via console unlocking).
+ * Essentially it is the nbcon version of console_emit_next_record().
+ */
+bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
+				   int cookie)
+{
+	struct nbcon_write_context wctxt = { };
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
+	unsigned long flags;
+	bool progress;
+
+	/* Use the same procedure as console_emit_next_record(). */
+	printk_safe_enter_irqsave(flags);
+	console_lock_spinning_enable();
+	stop_critical_timings();
+
+	ctxt->console	= con;
+	ctxt->prio	= nbcon_get_default_prio();
+
+	progress = nbcon_atomic_emit_one(&wctxt);
+
+	start_critical_timings();
+	*handover = console_lock_spinning_disable_and_check(cookie);
+	printk_safe_exit_irqrestore(flags);
+
+	return progress;
+}
+
+/**
  * __nbcon_atomic_flush_pending_con - Flush specified nbcon console using its
  *					write_atomic() callback
  * @con:			The nbcon console to flush
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e1cf0f7d6ff2..d17b0e057e22 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1860,7 +1860,7 @@ static bool console_waiter;
  * there may be a waiter spinning (like a spinlock). Also it must be
  * ready to hand over the lock at the end of the section.
  */
-static void console_lock_spinning_enable(void)
+void console_lock_spinning_enable(void)
 {
 	/*
 	 * Do not use spinning in panic(). The panic CPU wants to keep the lock.
@@ -1899,7 +1899,7 @@ static void console_lock_spinning_enable(void)
  *
  * Return: 1 if the lock rights were passed, 0 otherwise.
  */
-static int console_lock_spinning_disable_and_check(int cookie)
+int console_lock_spinning_disable_and_check(int cookie)
 {
 	int waiter;
 
@@ -2951,13 +2951,20 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 		cookie = console_srcu_read_lock();
 		for_each_console_srcu(con) {
 			short flags = console_srcu_read_flags(con);
+			u64 printk_seq;
 			bool progress;
 
 			if (!console_is_usable(con, flags))
 				continue;
 			any_usable = true;
 
-			progress = console_emit_next_record(con, handover, cookie);
+			if (flags & CON_NBCON) {
+				progress = nbcon_legacy_emit_next_record(con, handover, cookie);
+				printk_seq = nbcon_seq_read(con);
+			} else {
+				progress = console_emit_next_record(con, handover, cookie);
+				printk_seq = con->seq;
+			}
 
 			/*
 			 * If a handover has occurred, the SRCU read lock
@@ -2967,8 +2974,8 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 				return false;
 
 			/* Track the next of the highest seq flushed. */
-			if (con->seq > *next_seq)
-				*next_seq = con->seq;
+			if (printk_seq > *next_seq)
+				*next_seq = printk_seq;
 
 			if (!progress)
 				continue;
-- 
2.39.2

Re: [PATCH printk v5 21/30] printk: nbcon: Use nbcon consoles in console_flush_all()

[Petr Mladek]

On Thu 2024-05-02 23:44:30, John Ogness wrote:
> Allow nbcon consoles to print messages in the legacy printk()
> caller context (printing via unlock) by integrating them into
> console_flush_all(). The write_atomic() callback is used for
> printing.
> 
> Provide nbcon_legacy_emit_next_record(), which acts as the
> nbcon variant of console_emit_next_record(). Call this variant
> within console_flush_all() for nbcon consoles. Since nbcon
> consoles use their own @nbcon_seq variable to track the next
> record to print, this also must be appropriately handled in
> console_flush_all().
> 
> Note that the legacy printing logic uses @handover to detect
> handovers for printing all consoles. For nbcon consoles,
> handovers/takeovers occur on a per-console basis and thus do
> not cause the console_flush_all() loop to abort.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

[PATCH printk v5 22/30] printk: nbcon: Add unsafe flushing on panic

[John Ogness]

Add nbcon_atomic_flush_unsafe() to flush all nbcon consoles
using the write_atomic() callback and allowing unsafe hostile
takeovers. Call this at the end of panic() as a final attempt
to flush any pending messages.

Note that legacy consoles use unsafe methods for flushing
from the beginning of panic (see bust_spinlocks()). Therefore,
systems using both legacy and nbcon consoles may still fail to
see panic messages due to unsafe legacy console usage.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 include/linux/printk.h |  5 +++++
 kernel/panic.c         |  1 +
 kernel/printk/nbcon.c  | 32 +++++++++++++++++++++++++-------
 3 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index d0a1106388d1..439decc792b6 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -197,6 +197,7 @@ extern asmlinkage void dump_stack(void) __cold;
 void printk_trigger_flush(void);
 extern bool nbcon_driver_try_acquire(struct console *con);
 extern void nbcon_driver_release(struct console *con);
+void nbcon_atomic_flush_unsafe(void);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)
@@ -286,6 +287,10 @@ static inline void nbcon_driver_release(struct console *con)
 {
 }
 
+static inline void nbcon_atomic_flush_unsafe(void)
+{
+}
+
 #endif
 
 bool this_cpu_in_panic(void);
diff --git a/kernel/panic.c b/kernel/panic.c
index f22d8f33ea14..c039f8e1ddae 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -453,6 +453,7 @@ void panic(const char *fmt, ...)
 	 * Explicitly flush the kernel log buffer one last time.
 	 */
 	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
+	nbcon_atomic_flush_unsafe();
 
 	local_irq_enable();
 	for (i = 0; ; i += PANIC_TIMER_STEP) {
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index abd3835a3466..8f6b3858ab27 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1044,6 +1044,7 @@ bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
  *					write_atomic() callback
  * @con:			The nbcon console to flush
  * @stop_seq:			Flush up until this record
+ * @allow_unsafe_takeover:	True, to allow unsafe hostile takeovers
  *
  * Return:	0 if @con was flushed up to @stop_seq Otherwise, error code on
  *		failure.
@@ -1062,7 +1063,8 @@ bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
  * returned, it cannot be expected that the unfinalized record will become
  * available.
  */
-static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
+static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq,
+					    bool allow_unsafe_takeover)
 {
 	struct nbcon_write_context wctxt = { };
 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
@@ -1071,6 +1073,7 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
 	ctxt->console			= con;
 	ctxt->spinwait_max_us		= 2000;
 	ctxt->prio			= nbcon_get_default_prio();
+	ctxt->allow_unsafe_takeover	= allow_unsafe_takeover;
 
 	if (!nbcon_context_try_acquire(ctxt))
 		return -EPERM;
@@ -1100,13 +1103,15 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
  *					write_atomic() callback
  * @con:			The nbcon console to flush
  * @stop_seq:			Flush up until this record
+ * @allow_unsafe_takeover:	True, to allow unsafe hostile takeovers
  *
  * This will stop flushing before @stop_seq if another context has ownership.
  * That context is then responsible for the flushing. Likewise, if new records
  * are added while this context was flushing and there is no other context
  * to handle the printing, this context must also flush those records.
  */
-static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
+static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq,
+					   bool allow_unsafe_takeover)
 {
 	unsigned long flags;
 	int err;
@@ -1120,7 +1125,7 @@ static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
 	 */
 	local_irq_save(flags);
 
-	err = __nbcon_atomic_flush_pending_con(con, stop_seq);
+	err = __nbcon_atomic_flush_pending_con(con, stop_seq, allow_unsafe_takeover);
 
 	local_irq_restore(flags);
 
@@ -1144,8 +1149,9 @@ static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
  * __nbcon_atomic_flush_pending - Flush all nbcon consoles using their
  *					write_atomic() callback
  * @stop_seq:			Flush up until this record
+ * @allow_unsafe_takeover:	True, to allow unsafe hostile takeovers
  */
-static void __nbcon_atomic_flush_pending(u64 stop_seq)
+static void __nbcon_atomic_flush_pending(u64 stop_seq, bool allow_unsafe_takeover)
 {
 	struct console *con;
 	int cookie;
@@ -1163,7 +1169,7 @@ static void __nbcon_atomic_flush_pending(u64 stop_seq)
 		if (nbcon_seq_read(con) >= stop_seq)
 			continue;
 
-		nbcon_atomic_flush_pending_con(con, stop_seq);
+		nbcon_atomic_flush_pending_con(con, stop_seq, allow_unsafe_takeover);
 	}
 	console_srcu_read_unlock(cookie);
 }
@@ -1178,7 +1184,19 @@ static void __nbcon_atomic_flush_pending(u64 stop_seq)
  */
 void nbcon_atomic_flush_pending(void)
 {
-	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb));
+	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), false);
+}
+
+/**
+ * nbcon_atomic_flush_unsafe - Flush all nbcon consoles using their
+ *	write_atomic() callback and allowing unsafe hostile takeovers
+ *
+ * Flush the backlog up through the currently newest record. Unsafe hostile
+ * takeovers will be performed, if necessary.
+ */
+void nbcon_atomic_flush_unsafe(void)
+{
+	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), true);
 }
 
 /**
@@ -1307,7 +1325,7 @@ void nbcon_driver_release(struct console *con)
 	cookie = console_srcu_read_lock();
 	if (console_is_usable(con, console_srcu_read_flags(con)) &&
 	    prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
-		__nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb));
+		__nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb), false);
 	}
 	console_srcu_read_unlock(cookie);
 }
-- 
2.39.2

[PATCH printk v5 23/30] printk: Avoid console_lock dance if no legacy or boot consoles

[John Ogness]

Currently the console lock is used to attempt legacy-type
printing even if there are no legacy or boot consoles registered.
If no such consoles are registered, the console lock does not
need to be taken.

Add tracking of legacy console registration and use it with
boot console tracking to avoid unnecessary code paths, i.e.
do not use the console lock if there are no boot consoles
and no legacy consoles.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 78 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 63 insertions(+), 15 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index d17b0e057e22..0cb6ae5fd7e3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -463,6 +463,13 @@ static int console_msg_format = MSG_FORMAT_DEFAULT;
 /* syslog_lock protects syslog_* variables and write access to clear_seq. */
 static DEFINE_MUTEX(syslog_lock);
 
+/*
+ * Specifies if a legacy console is registered. If legacy consoles are
+ * present, it is necessary to perform the console lock/unlock dance
+ * whenever console flushing should occur.
+ */
+static bool have_legacy_console;
+
 /*
  * Specifies if a boot console is registered. If boot consoles are present,
  * nbcon consoles cannot print simultaneously and must be synchronized by
@@ -471,6 +478,14 @@ static DEFINE_MUTEX(syslog_lock);
  */
 static bool have_boot_console;
 
+/*
+ * Specifies if the console lock/unlock dance is needed for console
+ * printing. If @have_boot_console is true, the nbcon consoles will
+ * be printed serially along with the legacy consoles because nbcon
+ * consoles cannot print simultaneously with boot consoles.
+ */
+#define printing_via_unlock (have_legacy_console || have_boot_console)
+
 #ifdef CONFIG_PRINTK
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
 /* All 3 protected by @syslog_lock. */
@@ -2339,7 +2354,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
 
 	/* If called from the scheduler, we can not call up(). */
-	if (!in_sched) {
+	if (!in_sched && printing_via_unlock) {
 		/*
 		 * The caller may be holding system-critical or
 		 * timing-sensitive locks. Disable preemption during
@@ -2648,7 +2663,7 @@ void resume_console(void)
  */
 static int console_cpu_notify(unsigned int cpu)
 {
-	if (!cpuhp_tasks_frozen) {
+	if (!cpuhp_tasks_frozen && printing_via_unlock) {
 		/* If trylock fails, someone else is doing the printing */
 		if (console_trylock())
 			console_unlock();
@@ -3189,7 +3204,8 @@ void console_flush_on_panic(enum con_flush_mode mode)
 
 	nbcon_atomic_flush_pending();
 
-	console_flush_all(false, &next_seq, &handover);
+	if (printing_via_unlock)
+		console_flush_all(false, &next_seq, &handover);
 }
 
 /*
@@ -3523,6 +3539,7 @@ void register_console(struct console *newcon)
 	if (newcon->flags & CON_NBCON) {
 		nbcon_init(newcon, init_seq);
 	} else {
+		have_legacy_console = true;
 		newcon->seq = init_seq;
 	}
 
@@ -3598,6 +3615,7 @@ EXPORT_SYMBOL(register_console);
 /* Must be called under console_list_lock(). */
 static int unregister_console_locked(struct console *console)
 {
+	bool found_legacy_con = false;
 	bool found_boot_con = false;
 	unsigned long flags;
 	struct console *c;
@@ -3665,9 +3683,13 @@ static int unregister_console_locked(struct console *console)
 	for_each_console(c) {
 		if (c->flags & CON_BOOT)
 			found_boot_con = true;
+		if (!(c->flags & CON_NBCON))
+			found_legacy_con = true;
 	}
 	if (!found_boot_con)
 		have_boot_console = found_boot_con;
+	if (!found_legacy_con)
+		have_legacy_console = found_legacy_con;
 
 	return res;
 }
@@ -3828,22 +3850,34 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 	seq = prb_next_reserve_seq(prb);
 
 	/* Flush the consoles so that records up to @seq are printed. */
-	console_lock();
-	console_unlock();
+	if (printing_via_unlock) {
+		console_lock();
+		console_unlock();
+	}
 
 	for (;;) {
 		unsigned long begin_jiffies;
 		unsigned long slept_jiffies;
-
-		diff = 0;
+		bool use_console_lock = printing_via_unlock;
 
 		/*
-		 * Hold the console_lock to guarantee safe access to
-		 * console->seq. Releasing console_lock flushes more
-		 * records in case @seq is still not printed on all
-		 * usable consoles.
+		 * Ensure the compiler does not optimize @use_console_lock to
+		 * be @printing_via_unlock since the latter can change at any
+		 * time.
 		 */
-		console_lock();
+		barrier();
+
+		diff = 0;
+
+		if (use_console_lock) {
+			/*
+			 * Hold the console_lock to guarantee safe access to
+			 * console->seq. Releasing console_lock flushes more
+			 * records in case @seq is still not printed on all
+			 * usable consoles.
+			 */
+			console_lock();
+		}
 
 		cookie = console_srcu_read_lock();
 		for_each_console_srcu(c) {
@@ -3863,6 +3897,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 			if (flags & CON_NBCON) {
 				printk_seq = nbcon_seq_read(c);
 			} else {
+				WARN_ON_ONCE(!use_console_lock);
 				printk_seq = c->seq;
 			}
 
@@ -3874,7 +3909,8 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 		if (diff != last_diff && reset_on_progress)
 			remaining_jiffies = timeout_jiffies;
 
-		console_unlock();
+		if (use_console_lock)
+			console_unlock();
 
 		/* Note: @diff is 0 if there are no usable consoles. */
 		if (diff == 0 || remaining_jiffies == 0)
@@ -3944,6 +3980,7 @@ static void __wake_up_klogd(int val)
 		return;
 
 	preempt_disable();
+
 	/*
 	 * Guarantee any new records can be seen by tasks preparing to wait
 	 * before this context checks if the wait queue is empty.
@@ -3955,11 +3992,22 @@ static void __wake_up_klogd(int val)
 	 *
 	 * This pairs with devkmsg_read:A and syslog_print:A.
 	 */
-	if (wq_has_sleeper(&log_wait) || /* LMM(__wake_up_klogd:A) */
-	    (val & PRINTK_PENDING_OUTPUT)) {
+	if (!wq_has_sleeper(&log_wait)) /* LMM(__wake_up_klogd:A) */
+		val &= ~PRINTK_PENDING_WAKEUP;
+
+	/*
+	 * Simple read is safe. register_console() would flush a newly
+	 * registered legacy console when writing the message about it
+	 * being enabled.
+	 */
+	if (!printing_via_unlock)
+		val &= ~PRINTK_PENDING_OUTPUT;
+
+	if (val) {
 		this_cpu_or(printk_pending, val);
 		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
 	}
+
 	preempt_enable();
 }
 
-- 
2.39.2

[PATCH printk v5 24/30] printk: Track nbcon consoles

[John Ogness]

Add a global flag @have_nbcon_console to identify if any nbcon
consoles are registered. This will be used in follow-up commits
to preserve legacy behavior when no nbcon consoles are registered.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0cb6ae5fd7e3..31ed4450d9a6 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -470,6 +470,11 @@ static DEFINE_MUTEX(syslog_lock);
  */
 static bool have_legacy_console;
 
+/*
+ * Specifies if an nbcon console is registered.
+ */
+static bool have_nbcon_console;
+
 /*
  * Specifies if a boot console is registered. If boot consoles are present,
  * nbcon consoles cannot print simultaneously and must be synchronized by
@@ -3537,6 +3542,7 @@ void register_console(struct console *newcon)
 	init_seq = get_init_console_seq(newcon, bootcon_registered);
 
 	if (newcon->flags & CON_NBCON) {
+		have_nbcon_console = true;
 		nbcon_init(newcon, init_seq);
 	} else {
 		have_legacy_console = true;
@@ -3616,6 +3622,7 @@ EXPORT_SYMBOL(register_console);
 static int unregister_console_locked(struct console *console)
 {
 	bool found_legacy_con = false;
+	bool found_nbcon_con = false;
 	bool found_boot_con = false;
 	unsigned long flags;
 	struct console *c;
@@ -3683,13 +3690,18 @@ static int unregister_console_locked(struct console *console)
 	for_each_console(c) {
 		if (c->flags & CON_BOOT)
 			found_boot_con = true;
-		if (!(c->flags & CON_NBCON))
+
+		if (c->flags & CON_NBCON)
+			found_nbcon_con = true;
+		else
 			found_legacy_con = true;
 	}
 	if (!found_boot_con)
 		have_boot_console = found_boot_con;
 	if (!found_legacy_con)
 		have_legacy_console = found_legacy_con;
+	if (!found_nbcon_con)
+		have_nbcon_console = found_nbcon_con;
 
 	return res;
 }
-- 
2.39.2

[PATCH printk v5 25/30] printk: Coordinate direct printing in panic

[John Ogness]

Perform printing by nbcon consoles on the panic CPU from the
printk() caller context in order to get panic messages printed
as soon as possible.

If legacy and nbcon consoles are registered, the legacy consoles
will no longer perform direct printing on the panic CPU until
after the backtrace has been stored. This will give the safe
nbcon consoles a chance to print the panic messages before
allowing the unsafe legacy consoles to print.

If no nbcon consoles are registered, there is no change in
behavior (i.e. legacy consoles will always attempt to print
from the printk() caller context).

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 include/linux/printk.h |  5 ++++
 kernel/panic.c         |  2 ++
 kernel/printk/printk.c | 62 ++++++++++++++++++++++++++++++++++++------
 3 files changed, 61 insertions(+), 8 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 439decc792b6..bbd00a310f6b 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -195,6 +195,7 @@ void show_regs_print_info(const char *log_lvl);
 extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold;
 extern asmlinkage void dump_stack(void) __cold;
 void printk_trigger_flush(void);
+void printk_legacy_allow_panic_sync(void);
 extern bool nbcon_driver_try_acquire(struct console *con);
 extern void nbcon_driver_release(struct console *con);
 void nbcon_atomic_flush_unsafe(void);
@@ -278,6 +279,10 @@ static inline void printk_trigger_flush(void)
 {
 }
 
+static inline void printk_legacy_allow_panic_sync(void)
+{
+}
+
 static inline bool nbcon_driver_try_acquire(struct console *con)
 {
 	return false;
diff --git a/kernel/panic.c b/kernel/panic.c
index c039f8e1ddae..de8115c829cf 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -364,6 +364,8 @@ void panic(const char *fmt, ...)
 
 	panic_other_cpus_shutdown(_crash_kexec_post_notifiers);
 
+	printk_legacy_allow_panic_sync();
+
 	/*
 	 * Run any panic handlers, including those that might need to
 	 * add information to the kmsg dump output.
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 31ed4450d9a6..eb1d7f0b10bf 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -471,7 +471,9 @@ static DEFINE_MUTEX(syslog_lock);
 static bool have_legacy_console;
 
 /*
- * Specifies if an nbcon console is registered.
+ * Specifies if an nbcon console is registered. If nbcon consoles are present,
+ * synchronous printing of legacy consoles will not occur during panic until
+ * the backtrace has been stored to the ringbuffer.
  */
 static bool have_nbcon_console;
 
@@ -2330,12 +2332,29 @@ int vprintk_store(int facility, int level,
 	return ret;
 }
 
+static bool legacy_allow_panic_sync;
+
+/*
+ * This acts as a one-way switch to allow legacy consoles to print from
+ * the printk() caller context on a panic CPU. It also attempts to flush
+ * the legacy consoles in this context.
+ */
+void printk_legacy_allow_panic_sync(void)
+{
+	legacy_allow_panic_sync = true;
+
+	if (printing_via_unlock && !in_nmi()) {
+		if (console_trylock())
+			console_unlock();
+	}
+}
+
 asmlinkage int vprintk_emit(int facility, int level,
 			    const struct dev_printk_info *dev_info,
 			    const char *fmt, va_list args)
 {
+	bool do_trylock_unlock = printing_via_unlock;
 	int printed_len;
-	bool in_sched = false;
 
 	/* Suppress unimportant messages after panic happens */
 	if (unlikely(suppress_printk))
@@ -2351,15 +2370,42 @@ asmlinkage int vprintk_emit(int facility, int level,
 
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
-		in_sched = true;
+		/* If called from the scheduler, we can not call up(). */
+		do_trylock_unlock = false;
 	}
 
 	printk_delay(level);
 
 	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
 
-	/* If called from the scheduler, we can not call up(). */
-	if (!in_sched && printing_via_unlock) {
+	if (have_nbcon_console && !have_boot_console) {
+		bool is_panic_context = this_cpu_in_panic();
+
+		/*
+		 * In panic, the legacy consoles are not allowed to print from
+		 * the printk calling context unless explicitly allowed. This
+		 * gives the safe nbcon consoles a chance to print out all the
+		 * panic messages first. This restriction only applies if
+		 * there are nbcon consoles registered.
+		 */
+		if (is_panic_context)
+			do_trylock_unlock &= legacy_allow_panic_sync;
+
+		/*
+		 * There are situations where nbcon atomic printing should
+		 * happen in the printk() caller context:
+		 *
+		 * - When this CPU is in panic.
+		 *
+		 * Note that if boot consoles are registered, the console
+		 * lock/unlock dance must be relied upon instead because nbcon
+		 * consoles cannot print simultaneously with boot consoles.
+		 */
+		if (is_panic_context)
+			nbcon_atomic_flush_pending();
+	}
+
+	if (do_trylock_unlock) {
 		/*
 		 * The caller may be holding system-critical or
 		 * timing-sensitive locks. Disable preemption during
@@ -2379,10 +2425,10 @@ asmlinkage int vprintk_emit(int facility, int level,
 		preempt_enable();
 	}
 
-	if (in_sched)
-		defer_console_output();
-	else
+	if (do_trylock_unlock)
 		wake_up_klogd();
+	else
+		defer_console_output();
 
 	return printed_len;
 }
-- 
2.39.2

[PATCH printk v5 26/30] printk: nbcon: Implement emergency sections

[John Ogness]

From: Thomas Gleixner <tglx@linutronix.de>

In emergency situations (something has gone wrong but the
system continues to operate), usually important information
(such as a backtrace) is generated via printk(). Each
individual printk record has little meaning. It is the
collection of printk messages that is most often needed by
developers and users.

In order to help ensure that the collection of printk messages
in an emergency situation are all stored to the ringbuffer as
quickly as possible, disable console output for that CPU while
it is in the emergency situation. The consoles need to be
flushed when exiting the emergency situation.

Add per-CPU emergency nesting tracking because an emergency
can arise while in an emergency situation.

Add functions to mark the beginning and end of emergency
sections where the urgent messages are generated.

Do not print if the current CPU is in an emergency state.

When exiting all emergency nesting, flush nbcon consoles
directly using their atomic callback. Legacy consoles are
triggered for flushing via irq_work because it is not known
if the context was safe for a trylock on the console lock.

Note that the emergency state is not system-wide. While one CPU
is in an emergency state, another CPU may continue to print
console messages.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
---
 include/linux/console.h  |   6 ++
 kernel/printk/internal.h |  11 ++++
 kernel/printk/nbcon.c    | 116 +++++++++++++++++++++++++++++++++++++++
 kernel/printk/printk.c   |  25 +++++----
 4 files changed, 146 insertions(+), 12 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 71d0638aa818..1b25eda4e7c2 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -535,10 +535,16 @@ static inline bool console_is_registered(const struct console *con)
 	hlist_for_each_entry(con, &console_list, node)
 
 #ifdef CONFIG_PRINTK
+extern void nbcon_cpu_emergency_enter(void);
+extern void nbcon_cpu_emergency_exit(void);
+extern void nbcon_cpu_emergency_flush(void);
 extern bool nbcon_can_proceed(struct nbcon_write_context *wctxt);
 extern bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt);
 extern bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt);
 #else
+static inline void nbcon_cpu_emergency_enter(void) { }
+static inline void nbcon_cpu_emergency_exit(void) { }
+static inline void nbcon_cpu_emergency_flush(void) { }
 static inline bool nbcon_can_proceed(struct nbcon_write_context *wctxt) { return false; }
 static inline bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt) { return false; }
 static inline bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt) { return false; }
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 91bc0ee43f8d..5519239fa5d2 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -153,6 +153,17 @@ static inline bool console_is_usable(struct console *con, short flags) { return
 
 #endif /* CONFIG_PRINTK */
 
+extern bool have_boot_console;
+extern bool have_legacy_console;
+
+/*
+ * Specifies if the console lock/unlock dance is needed for console
+ * printing. If @have_boot_console is true, the nbcon consoles will
+ * be printed serially along with the legacy consoles because nbcon
+ * consoles cannot print simultaneously with boot consoles.
+ */
+#define printing_via_unlock (have_legacy_console || have_boot_console)
+
 extern struct printk_buffers printk_shared_pbufs;
 
 /**
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 8f6b3858ab27..991e2702915c 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -936,6 +936,29 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 	return nbcon_context_exit_unsafe(ctxt);
 }
 
+/* Track the nbcon emergency nesting per CPU. */
+static DEFINE_PER_CPU(unsigned int, nbcon_pcpu_emergency_nesting);
+static unsigned int early_nbcon_pcpu_emergency_nesting __initdata;
+
+/**
+ * nbcon_get_cpu_emergency_nesting - Get the per CPU emergency nesting pointer
+ *
+ * Return:	Either a pointer to the per CPU emergency nesting counter of
+ *		the current CPU or to the init data during early boot.
+ */
+static __ref unsigned int *nbcon_get_cpu_emergency_nesting(void)
+{
+	/*
+	 * The value of __printk_percpu_data_ready gets set in normal
+	 * context and before SMP initialization. As a result it could
+	 * never change while inside an nbcon emergency section.
+	 */
+	if (!printk_percpu_data_ready())
+		return &early_nbcon_pcpu_emergency_nesting;
+
+	return this_cpu_ptr(&nbcon_pcpu_emergency_nesting);
+}
+
 /**
  * nbcon_get_default_prio - The appropriate nbcon priority to use for nbcon
  *				printing on the current CPU
@@ -946,9 +969,15 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
  */
 enum nbcon_prio nbcon_get_default_prio(void)
 {
+	unsigned int *cpu_emergency_nesting;
+
 	if (this_cpu_in_panic())
 		return NBCON_PRIO_PANIC;
 
+	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
+	if (*cpu_emergency_nesting)
+		return NBCON_PRIO_EMERGENCY;
+
 	return NBCON_PRIO_NORMAL;
 }
 
@@ -1199,6 +1228,93 @@ void nbcon_atomic_flush_unsafe(void)
 	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), true);
 }
 
+/**
+ * nbcon_cpu_emergency_enter - Enter an emergency section where printk()
+ *				messages for that CPU are only stored
+ *
+ * Upon exiting the emergency section, all stored messages are flushed.
+ *
+ * Context:	Any context. Disables preemption.
+ *
+ * When within an emergency section, no printing occurs on that CPU. This
+ * is to allow all emergency messages to be dumped into the ringbuffer before
+ * flushing the ringbuffer. The actual printing occurs when exiting the
+ * outermost emergency section.
+ */
+void nbcon_cpu_emergency_enter(void)
+{
+	unsigned int *cpu_emergency_nesting;
+
+	preempt_disable();
+
+	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
+	(*cpu_emergency_nesting)++;
+}
+
+/**
+ * nbcon_cpu_emergency_exit - Exit an emergency section and flush the
+ *				stored messages
+ *
+ * Flushing only occurs when exiting all nesting for the CPU.
+ *
+ * Context:	Any context. Enables preemption.
+ */
+void nbcon_cpu_emergency_exit(void)
+{
+	unsigned int *cpu_emergency_nesting;
+	bool do_trigger_flush = false;
+
+	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
+
+	/*
+	 * Flush the messages before enabling preemtion to see them ASAP.
+	 *
+	 * Reduce the risk of potential softlockup by using the
+	 * flush_pending() variant which ignores messages added later. It is
+	 * called before decrementing the counter so that the printing context
+	 * for the emergency messages is NBCON_PRIO_EMERGENCY.
+	 */
+	if (*cpu_emergency_nesting == 1) {
+		nbcon_atomic_flush_pending();
+		do_trigger_flush = true;
+	}
+
+	(*cpu_emergency_nesting)--;
+
+	if (WARN_ON_ONCE(*cpu_emergency_nesting < 0))
+		*cpu_emergency_nesting = 0;
+
+	preempt_enable();
+
+	if (do_trigger_flush)
+		printk_trigger_flush();
+}
+
+/**
+ * nbcon_cpu_emergency_flush - Explicitly flush consoles while
+ *				within emergency context
+ *
+ * Both nbcon and legacy consoles are flushed.
+ *
+ * It should be used only when there are too many messages printed
+ * in emergency context, for example, printing backtraces of all
+ * CPUs or processes. It is typically needed when the watchdogs
+ * need to be touched as well.
+ */
+void nbcon_cpu_emergency_flush(void)
+{
+	/* The explicit flush is needed only in the emergency context. */
+	if (*(nbcon_get_cpu_emergency_nesting()) == 0)
+		return;
+
+	nbcon_atomic_flush_pending();
+
+	if (printing_via_unlock && !in_nmi()) {
+		if (console_trylock())
+			console_unlock();
+	}
+}
+
 /**
  * nbcon_alloc - Allocate buffers needed by the nbcon console
  * @con:	Console to allocate buffers for
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index eb1d7f0b10bf..487742eeacf3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -468,7 +468,7 @@ static DEFINE_MUTEX(syslog_lock);
  * present, it is necessary to perform the console lock/unlock dance
  * whenever console flushing should occur.
  */
-static bool have_legacy_console;
+bool have_legacy_console;
 
 /*
  * Specifies if an nbcon console is registered. If nbcon consoles are present,
@@ -483,15 +483,7 @@ static bool have_nbcon_console;
  * the console lock. This is because boot consoles and nbcon consoles may
  * have mapped the same hardware.
  */
-static bool have_boot_console;
-
-/*
- * Specifies if the console lock/unlock dance is needed for console
- * printing. If @have_boot_console is true, the nbcon consoles will
- * be printed serially along with the legacy consoles because nbcon
- * consoles cannot print simultaneously with boot consoles.
- */
-#define printing_via_unlock (have_legacy_console || have_boot_console)
+bool have_boot_console;
 
 #ifdef CONFIG_PRINTK
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
@@ -2412,16 +2404,25 @@ asmlinkage int vprintk_emit(int facility, int level,
 		 * printing of all remaining records to all consoles so that
 		 * this context can return as soon as possible. Hopefully
 		 * another printk() caller will take over the printing.
+		 *
+		 * Also, nbcon_get_default_prio() requires migration disabled.
 		 */
 		preempt_disable();
+
 		/*
 		 * Try to acquire and then immediately release the console
 		 * semaphore. The release will print out buffers. With the
 		 * spinning variant, this context tries to take over the
 		 * printing from another printing context.
+		 *
+		 * Skip it in EMERGENCY priority. The console will be
+		 * explicitly flushed when exiting the emergency section.
 		 */
-		if (console_trylock_spinning())
-			console_unlock();
+		if (nbcon_get_default_prio() != NBCON_PRIO_EMERGENCY) {
+			if (console_trylock_spinning())
+				console_unlock();
+		}
+
 		preempt_enable();
 	}
 
-- 
2.39.2

Re: [PATCH printk v5 26/30] printk: nbcon: Implement emergency sections

[John Ogness]

On 2024-05-02, John Ogness <john.ogness@linutronix.de> wrote:
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index 8f6b3858ab27..991e2702915c 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> +void nbcon_cpu_emergency_exit(void)
> +{
> +	unsigned int *cpu_emergency_nesting;
> +	bool do_trigger_flush = false;
> +
> +	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
> +
> +	/*
> +	 * Flush the messages before enabling preemtion to see them ASAP.
> +	 *
> +	 * Reduce the risk of potential softlockup by using the
> +	 * flush_pending() variant which ignores messages added later. It is
> +	 * called before decrementing the counter so that the printing context
> +	 * for the emergency messages is NBCON_PRIO_EMERGENCY.
> +	 */
> +	if (*cpu_emergency_nesting == 1) {
> +		nbcon_atomic_flush_pending();
> +		do_trigger_flush = true;
> +	}
> +
> +	(*cpu_emergency_nesting)--;
> +
> +	if (WARN_ON_ONCE(*cpu_emergency_nesting < 0))
> +		*cpu_emergency_nesting = 0;

I see two issues here. First, this is unsigned. kernel test robot
reported:

> kernel/printk/nbcon.c:1540 nbcon_cpu_emergency_exit() warn: unsigned
> '*cpu_emergency_nesting' is never less than zero.

Also, in this situation, we are allowing a brief window of activated
emergency mode by allowing it to become !0 before correcting
it. Instead, we should avoid illegally decrementing. I suggest:

void nbcon_cpu_emergency_exit(void)
{
	unsigned int *cpu_emergency_nesting;
	bool do_trigger_flush = false;

	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();

	/*
	 * Flush the messages before enabling preemtion to see them ASAP.
	 *
	 * Reduce the risk of potential softlockup by using the
	 * flush_pending() variant which ignores messages added later. It is
	 * called before decrementing the counter so that the printing context
	 * for the emergency messages is NBCON_PRIO_EMERGENCY.
	 */
	if (*cpu_emergency_nesting == 1) {
		nbcon_atomic_flush_pending();
		do_trigger_flush = true;
	}

	if (!WARN_ON_ONCE(*cpu_emergency_nesting == 0))
		(*cpu_emergency_nesting)--;

	preempt_enable();

	if (do_trigger_flush)
		printk_trigger_flush();
}

John

Re: [PATCH printk v5 26/30] printk: nbcon: Implement emergency sections

[Petr Mladek]

On Mon 2024-05-06 12:45:59, John Ogness wrote:
> On 2024-05-02, John Ogness <john.ogness@linutronix.de> wrote:
> > diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> > index 8f6b3858ab27..991e2702915c 100644
> > --- a/kernel/printk/nbcon.c
> > +++ b/kernel/printk/nbcon.c
> > +void nbcon_cpu_emergency_exit(void)
> > +{
> > +	unsigned int *cpu_emergency_nesting;
> > +	bool do_trigger_flush = false;
> > +
> > +	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
> > +
> > +	/*
> > +	 * Flush the messages before enabling preemtion to see them ASAP.
> > +	 *
> > +	 * Reduce the risk of potential softlockup by using the
> > +	 * flush_pending() variant which ignores messages added later. It is
> > +	 * called before decrementing the counter so that the printing context
> > +	 * for the emergency messages is NBCON_PRIO_EMERGENCY.
> > +	 */
> > +	if (*cpu_emergency_nesting == 1) {
> > +		nbcon_atomic_flush_pending();
> > +		do_trigger_flush = true;
> > +	}
> > +
> > +	(*cpu_emergency_nesting)--;
> > +
> > +	if (WARN_ON_ONCE(*cpu_emergency_nesting < 0))
> > +		*cpu_emergency_nesting = 0;
> 
> I see two issues here. First, this is unsigned. kernel test robot
> reported:
> 
> > kernel/printk/nbcon.c:1540 nbcon_cpu_emergency_exit() warn: unsigned
> > '*cpu_emergency_nesting' is never less than zero.
> 
> Also, in this situation, we are allowing a brief window of activated
> emergency mode by allowing it to become !0 before correcting
> it. Instead, we should avoid illegally decrementing. I suggest:

Great catch!

> void nbcon_cpu_emergency_exit(void)
> {
> 	unsigned int *cpu_emergency_nesting;
> 	bool do_trigger_flush = false;
> 
> 	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
> 
> 	/*
> 	 * Flush the messages before enabling preemtion to see them ASAP.
> 	 *
> 	 * Reduce the risk of potential softlockup by using the
> 	 * flush_pending() variant which ignores messages added later. It is
> 	 * called before decrementing the counter so that the printing context
> 	 * for the emergency messages is NBCON_PRIO_EMERGENCY.
> 	 */
> 	if (*cpu_emergency_nesting == 1) {
> 		nbcon_atomic_flush_pending();
> 		do_trigger_flush = true;
> 	}
> 
> 	if (!WARN_ON_ONCE(*cpu_emergency_nesting == 0))
> 		(*cpu_emergency_nesting)--;

Looks good.

> 	preempt_enable();
> 
> 	if (do_trigger_flush)
> 		printk_trigger_flush();
> }

Best Regards,
Petr

Re: [PATCH printk v5 26/30] printk: nbcon: Implement emergency sections

[Petr Mladek]

On Thu 2024-05-02 23:44:35, John Ogness wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> In emergency situations (something has gone wrong but the
> system continues to operate), usually important information
> (such as a backtrace) is generated via printk(). Each
> individual printk record has little meaning. It is the
> collection of printk messages that is most often needed by
> developers and users.
> 
> In order to help ensure that the collection of printk messages
> in an emergency situation are all stored to the ringbuffer as
> quickly as possible, disable console output for that CPU while
> it is in the emergency situation. The consoles need to be
> flushed when exiting the emergency situation.
> 
> Add per-CPU emergency nesting tracking because an emergency
> can arise while in an emergency situation.
> 
> Add functions to mark the beginning and end of emergency
> sections where the urgent messages are generated.
> 
> Do not print if the current CPU is in an emergency state.
> 
> When exiting all emergency nesting, flush nbcon consoles
> directly using their atomic callback. Legacy consoles are
> triggered for flushing via irq_work because it is not known
> if the context was safe for a trylock on the console lock.
> 
> Note that the emergency state is not system-wide. While one CPU
> is in an emergency state, another CPU may continue to print
> console messages.
> 
> Co-developed-by: John Ogness <john.ogness@linutronix.de>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>

> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1199,6 +1228,93 @@ void nbcon_atomic_flush_unsafe(void)
>  	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), true);
>  }
>  
> +/**
> + * nbcon_cpu_emergency_enter - Enter an emergency section where printk()
> + *				messages for that CPU are only stored
> + *
> + * Upon exiting the emergency section, all stored messages are flushed.
> + *
> + * Context:	Any context. Disables preemption.
> + *
> + * When within an emergency section, no printing occurs on that CPU. This
> + * is to allow all emergency messages to be dumped into the ringbuffer before
> + * flushing the ringbuffer. The actual printing occurs when exiting the
> + * outermost emergency section.
> + */
> +void nbcon_cpu_emergency_enter(void)
> +{
> +	unsigned int *cpu_emergency_nesting;
> +
> +	preempt_disable();
> +
> +	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
> +	(*cpu_emergency_nesting)++;
> +}
> +
> +/**
> + * nbcon_cpu_emergency_exit - Exit an emergency section and flush the
> + *				stored messages
> + *
> + * Flushing only occurs when exiting all nesting for the CPU.
> + *
> + * Context:	Any context. Enables preemption.
> + */
> +void nbcon_cpu_emergency_exit(void)
> +{
> +	unsigned int *cpu_emergency_nesting;
> +	bool do_trigger_flush = false;
> +
> +	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
> +
> +	/*
> +	 * Flush the messages before enabling preemtion to see them ASAP.
> +	 *
> +	 * Reduce the risk of potential softlockup by using the
> +	 * flush_pending() variant which ignores messages added later. It is
> +	 * called before decrementing the counter so that the printing context
> +	 * for the emergency messages is NBCON_PRIO_EMERGENCY.
> +	 */
> +	if (*cpu_emergency_nesting == 1) {
> +		nbcon_atomic_flush_pending();
> +		do_trigger_flush = true;
> +	}

The commit message says:

  "Legacy consoles are triggered for flushing via irq_work because
   it is not known if the context was safe for a trylock on the
   console lock."

I do not feel completely comfortable with this. If printk() knows
when it is safe to call console_trylock() then we should know as
well.

IMHO, we could just call the below implemented nbcon_cpu_emergency_flush() here.

> +
> +	(*cpu_emergency_nesting)--;
> +
> +	if (WARN_ON_ONCE(*cpu_emergency_nesting < 0))
> +		*cpu_emergency_nesting = 0;
> +
> +	preempt_enable();
> +
> +	if (do_trigger_flush)
> +		printk_trigger_flush();
> +}
> +
> +/**
> + * nbcon_cpu_emergency_flush - Explicitly flush consoles while
> + *				within emergency context
> + *
> + * Both nbcon and legacy consoles are flushed.
> + *
> + * It should be used only when there are too many messages printed
> + * in emergency context, for example, printing backtraces of all
> + * CPUs or processes. It is typically needed when the watchdogs
> + * need to be touched as well.
> + */
> +void nbcon_cpu_emergency_flush(void)
> +{
> +	/* The explicit flush is needed only in the emergency context. */
> +	if (*(nbcon_get_cpu_emergency_nesting()) == 0)
> +		return;
> +
> +	nbcon_atomic_flush_pending();
> +
> +	if (printing_via_unlock && !in_nmi()) {
> +		if (console_trylock())
> +			console_unlock();
> +	}

Hmm, we should also check whether we are in printk_safe context.
We could implement:

bool is_printk_deferred(void)
{
	/*
	 * The per-CPU variable @printk_context can be read safely in any
	 * context. The CPU migration always disabled when set.
	 */
	return this_cpu_read(printk_context) || in_nmi();
}

How does that sound?

Best Regards,
Petr

[PATCH printk v5 27/30] panic: Mark emergency section in warn

[John Ogness]

From: Thomas Gleixner <tglx@linutronix.de>

Mark the full contents of __warn() as an emergency section. In
this section, the CPU will not perform console output for the
printk() calls. Instead, a flushing of the console output is
triggered when exiting the emergency section.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/panic.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/panic.c b/kernel/panic.c
index de8115c829cf..ee03193f9495 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -667,6 +667,8 @@ struct warn_args {
 void __warn(const char *file, int line, void *caller, unsigned taint,
 	    struct pt_regs *regs, struct warn_args *args)
 {
+	nbcon_cpu_emergency_enter();
+
 	disable_trace_on_warning();
 
 	if (file)
@@ -697,6 +699,8 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
 
 	/* Just a warning, don't kill lockdep. */
 	add_taint(taint, LOCKDEP_STILL_OK);
+
+	nbcon_cpu_emergency_exit();
 }
 
 #ifdef CONFIG_BUG
-- 
2.39.2

[PATCH printk v5 28/30] panic: Mark emergency section in oops

[John Ogness]

Mark an emergency section beginning with oops_enter() until the
end of oops_exit(). In this section, the CPU will not perform
console output for the printk() calls. Instead, a flushing of the
console output is triggered when exiting the emergency section.

The very end of oops_exit() performs a kmsg_dump(). This is not
included in the emergency section because it is another
flushing mechanism that should occur after the consoles have
been triggered to flush.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/panic.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/panic.c b/kernel/panic.c
index ee03193f9495..3754a2471b4f 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -634,6 +634,7 @@ bool oops_may_print(void)
  */
 void oops_enter(void)
 {
+	nbcon_cpu_emergency_enter();
 	tracing_off();
 	/* can't trust the integrity of the kernel anymore: */
 	debug_locks_off();
@@ -656,6 +657,7 @@ void oops_exit(void)
 {
 	do_oops_enter_exit();
 	print_oops_end_marker();
+	nbcon_cpu_emergency_exit();
 	kmsg_dump(KMSG_DUMP_OOPS);
 }
 
-- 
2.39.2

[PATCH printk v5 29/30] rcu: Mark emergency sections in rcu stalls

[John Ogness]

Mark emergency sections wherever multiple lines of
rcu stall information are generated. In an emergency
section the CPU will not perform console output for the
printk() calls. Instead, a flushing of the console
output is triggered when exiting the emergency section.
This allows the full message block to be stored as
quickly as possible in the ringbuffer.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/rcu/tree_exp.h   |  9 +++++++++
 kernel/rcu/tree_stall.h | 11 +++++++++++
 2 files changed, 20 insertions(+)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 6b83537480b1..bc1d8733c08f 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -7,6 +7,7 @@
  * Authors: Paul E. McKenney <paulmck@linux.ibm.com>
  */
 
+#include <linux/console.h>
 #include <linux/lockdep.h>
 
 static void rcu_exp_handler(void *unused);
@@ -571,6 +572,9 @@ static void synchronize_rcu_expedited_wait(void)
 			return;
 		if (rcu_stall_is_suppressed())
 			continue;
+
+		nbcon_cpu_emergency_enter();
+
 		j = jiffies;
 		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_EXP, (void *)(j - jiffies_start));
 		trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
@@ -612,6 +616,7 @@ static void synchronize_rcu_expedited_wait(void)
 			}
 			pr_cont("\n");
 		}
+		nbcon_cpu_emergency_flush();
 		rcu_for_each_leaf_node(rnp) {
 			for_each_leaf_node_possible_cpu(rnp, cpu) {
 				mask = leaf_node_cpu_bit(rnp, cpu);
@@ -624,6 +629,9 @@ static void synchronize_rcu_expedited_wait(void)
 			rcu_exp_print_detail_task_stall_rnp(rnp);
 		}
 		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
+
+		nbcon_cpu_emergency_exit();
+
 		panic_on_rcu_stall();
 	}
 }
@@ -792,6 +800,7 @@ static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
 		 */
 		touch_nmi_watchdog();
 		sched_show_task(t);
+		nbcon_cpu_emergency_flush();
 	}
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 5d666428546b..1ca0826545c1 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -7,6 +7,7 @@
  * Author: Paul E. McKenney <paulmck@linux.ibm.com>
  */
 
+#include <linux/console.h>
 #include <linux/kvm_para.h>
 #include <linux/rcu_notifier.h>
 
@@ -260,6 +261,7 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
 		 */
 		touch_nmi_watchdog();
 		sched_show_task(t);
+		nbcon_cpu_emergency_flush();
 	}
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
@@ -522,6 +524,7 @@ static void print_cpu_stall_info(int cpu)
 	       falsepositive ? " (false positive?)" : "");
 
 	print_cpu_stat_info(cpu);
+	nbcon_cpu_emergency_flush();
 }
 
 /* Complain about starvation of grace-period kthread.  */
@@ -604,6 +607,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 	if (rcu_stall_is_suppressed())
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	/*
 	 * OK, time to rat on our buddy...
 	 * See Documentation/RCU/stallwarn.rst for info on how to debug
@@ -655,6 +660,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 	rcu_check_gp_kthread_expired_fqs_timer();
 	rcu_check_gp_kthread_starvation();
 
+	nbcon_cpu_emergency_exit();
+
 	panic_on_rcu_stall();
 
 	rcu_force_quiescent_state();  /* Kick them all. */
@@ -675,6 +682,8 @@ static void print_cpu_stall(unsigned long gps)
 	if (rcu_stall_is_suppressed())
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	/*
 	 * OK, time to rat on ourselves...
 	 * See Documentation/RCU/stallwarn.rst for info on how to debug
@@ -703,6 +712,8 @@ static void print_cpu_stall(unsigned long gps)
 			   jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
+	nbcon_cpu_emergency_exit();
+
 	panic_on_rcu_stall();
 
 	/*
-- 
2.39.2

Re: [PATCH printk v5 29/30] rcu: Mark emergency sections in rcu stalls

[Petr Mladek]

On Thu 2024-05-02 23:44:38, John Ogness wrote:
> Mark emergency sections wherever multiple lines of
> rcu stall information are generated. In an emergency
> section the CPU will not perform console output for the
> printk() calls. Instead, a flushing of the console
> output is triggered when exiting the emergency section.
> This allows the full message block to be stored as
> quickly as possible in the ringbuffer.
>
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -612,6 +616,7 @@ static void synchronize_rcu_expedited_wait(void)
>  			}
>  			pr_cont("\n");
>  		}
> +		nbcon_cpu_emergency_flush();

It would make more sense to do the flush inside the cycle after each
dump_cpu_task(). Something like:

		rcu_for_each_leaf_node(rnp) {
			for_each_leaf_node_possible_cpu(rnp, cpu) {
				mask = leaf_node_cpu_bit(rnp, cpu);
				if (!(READ_ONCE(rnp->expmask) & mask))
					continue;
				preempt_disable(); // For smp_processor_id() in dump_cpu_task().
				dump_cpu_task(cpu);
				preempt_enable();
+				nbcon_cpu_emergency_flush();
			}
			rcu_exp_print_detail_task_stall_rnp(rnp);
		}


Or maybe, it is limited onto to few CPUs by rcu_for_each_leaf_node(rnp)?


>  		rcu_for_each_leaf_node(rnp) {
>  			for_each_leaf_node_possible_cpu(rnp, cpu) {
>  				mask = leaf_node_cpu_bit(rnp, cpu);

Otherwise, it looks good to me.

Best Regards,
Petr

[PATCH printk v5 30/30] lockdep: Mark emergency sections in lockdep splats

[John Ogness]

Mark emergency sections wherever multiple lines of
lock debugging output are generated. In an emergency
section the CPU will not perform console output for the
printk() calls. Instead, a flushing of the console
output is triggered when exiting the emergency section.
This allows the full message block to be stored as
quickly as possible in the ringbuffer.

Note that debug_show_all_locks() and
lockdep_print_held_locks() rely on their callers to
enter the emergency section. This is because these
functions can also be called in non-emergency
situations (such as sysrq).

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/locking/lockdep.c | 84 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 82 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 151bd3de5936..c06842e037d8 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -56,6 +56,7 @@
 #include <linux/kprobes.h>
 #include <linux/lockdep.h>
 #include <linux/context_tracking.h>
+#include <linux/console.h>
 
 #include <asm/sections.h>
 
@@ -574,8 +575,10 @@ static struct lock_trace *save_trace(void)
 		if (!debug_locks_off_graph_unlock())
 			return NULL;
 
+		nbcon_cpu_emergency_enter();
 		print_lockdep_off("BUG: MAX_STACK_TRACE_ENTRIES too low!");
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 
 		return NULL;
 	}
@@ -888,11 +891,13 @@ look_up_lock_class(const struct lockdep_map *lock, unsigned int subclass)
 	if (unlikely(subclass >= MAX_LOCKDEP_SUBCLASSES)) {
 		instrumentation_begin();
 		debug_locks_off();
+		nbcon_cpu_emergency_enter();
 		printk(KERN_ERR
 			"BUG: looking up invalid subclass: %u\n", subclass);
 		printk(KERN_ERR
 			"turning off the locking correctness validator.\n");
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 		instrumentation_end();
 		return NULL;
 	}
@@ -969,11 +974,13 @@ static bool assign_lock_key(struct lockdep_map *lock)
 	else {
 		/* Debug-check: all keys must be persistent! */
 		debug_locks_off();
+		nbcon_cpu_emergency_enter();
 		pr_err("INFO: trying to register non-static key.\n");
 		pr_err("The code is fine but needs lockdep annotation, or maybe\n");
 		pr_err("you didn't initialize this object before use?\n");
 		pr_err("turning off the locking correctness validator.\n");
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 		return false;
 	}
 
@@ -1317,8 +1324,10 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
 			return NULL;
 		}
 
+		nbcon_cpu_emergency_enter();
 		print_lockdep_off("BUG: MAX_LOCKDEP_KEYS too low!");
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 		return NULL;
 	}
 	nr_lock_classes++;
@@ -1350,11 +1359,13 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
 	if (verbose(class)) {
 		graph_unlock();
 
+		nbcon_cpu_emergency_enter();
 		printk("\nnew class %px: %s", class->key, class->name);
 		if (class->name_version > 1)
 			printk(KERN_CONT "#%d", class->name_version);
 		printk(KERN_CONT "\n");
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 
 		if (!graph_lock()) {
 			return NULL;
@@ -1393,8 +1404,10 @@ static struct lock_list *alloc_list_entry(void)
 		if (!debug_locks_off_graph_unlock())
 			return NULL;
 
+		nbcon_cpu_emergency_enter();
 		print_lockdep_off("BUG: MAX_LOCKDEP_ENTRIES too low!");
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 		return NULL;
 	}
 	nr_list_entries++;
@@ -2040,6 +2053,8 @@ static noinline void print_circular_bug(struct lock_list *this,
 
 	depth = get_lock_depth(target);
 
+	nbcon_cpu_emergency_enter();
+
 	print_circular_bug_header(target, depth, check_src, check_tgt);
 
 	parent = get_lock_parent(target);
@@ -2058,6 +2073,8 @@ static noinline void print_circular_bug(struct lock_list *this,
 
 	printk("\nstack backtrace:\n");
 	dump_stack();
+
+	nbcon_cpu_emergency_exit();
 }
 
 static noinline void print_bfs_bug(int ret)
@@ -2570,6 +2587,8 @@ print_bad_irq_dependency(struct task_struct *curr,
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("=====================================================\n");
 	pr_warn("WARNING: %s-safe -> %s-unsafe lock order detected\n",
@@ -2619,11 +2638,13 @@ print_bad_irq_dependency(struct task_struct *curr,
 	pr_warn(" and %s-irq-unsafe lock:\n", irqclass);
 	next_root->trace = save_trace();
 	if (!next_root->trace)
-		return;
+		goto out;
 	print_shortest_lock_dependencies(forwards_entry, next_root);
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+out:
+	nbcon_cpu_emergency_exit();
 }
 
 static const char *state_names[] = {
@@ -2988,6 +3009,8 @@ print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("============================================\n");
 	pr_warn("WARNING: possible recursive locking detected\n");
@@ -3010,6 +3033,8 @@ print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+
+	nbcon_cpu_emergency_exit();
 }
 
 /*
@@ -3607,6 +3632,8 @@ static void print_collision(struct task_struct *curr,
 			struct held_lock *hlock_next,
 			struct lock_chain *chain)
 {
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("============================\n");
 	pr_warn("WARNING: chain_key collision\n");
@@ -3623,6 +3650,8 @@ static void print_collision(struct task_struct *curr,
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+
+	nbcon_cpu_emergency_exit();
 }
 #endif
 
@@ -3713,8 +3742,10 @@ static inline int add_chain_cache(struct task_struct *curr,
 		if (!debug_locks_off_graph_unlock())
 			return 0;
 
+		nbcon_cpu_emergency_enter();
 		print_lockdep_off("BUG: MAX_LOCKDEP_CHAINS too low!");
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 		return 0;
 	}
 	chain->chain_key = chain_key;
@@ -3731,8 +3762,10 @@ static inline int add_chain_cache(struct task_struct *curr,
 		if (!debug_locks_off_graph_unlock())
 			return 0;
 
+		nbcon_cpu_emergency_enter();
 		print_lockdep_off("BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!");
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 		return 0;
 	}
 
@@ -3971,6 +4004,8 @@ print_usage_bug(struct task_struct *curr, struct held_lock *this,
 	if (!debug_locks_off() || debug_locks_silent)
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("================================\n");
 	pr_warn("WARNING: inconsistent lock state\n");
@@ -3999,6 +4034,8 @@ print_usage_bug(struct task_struct *curr, struct held_lock *this,
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+
+	nbcon_cpu_emergency_exit();
 }
 
 /*
@@ -4033,6 +4070,8 @@ print_irq_inversion_bug(struct task_struct *curr,
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("========================================================\n");
 	pr_warn("WARNING: possible irq lock inversion dependency detected\n");
@@ -4073,11 +4112,13 @@ print_irq_inversion_bug(struct task_struct *curr,
 	pr_warn("\nthe shortest dependencies between 2nd lock and 1st lock:\n");
 	root->trace = save_trace();
 	if (!root->trace)
-		return;
+		goto out;
 	print_shortest_lock_dependencies(other, root);
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+out:
+	nbcon_cpu_emergency_exit();
 }
 
 /*
@@ -4154,6 +4195,8 @@ void print_irqtrace_events(struct task_struct *curr)
 {
 	const struct irqtrace_events *trace = &curr->irqtrace;
 
+	nbcon_cpu_emergency_enter();
+
 	printk("irq event stamp: %u\n", trace->irq_events);
 	printk("hardirqs last  enabled at (%u): [<%px>] %pS\n",
 		trace->hardirq_enable_event, (void *)trace->hardirq_enable_ip,
@@ -4167,6 +4210,8 @@ void print_irqtrace_events(struct task_struct *curr)
 	printk("softirqs last disabled at (%u): [<%px>] %pS\n",
 		trace->softirq_disable_event, (void *)trace->softirq_disable_ip,
 		(void *)trace->softirq_disable_ip);
+
+	nbcon_cpu_emergency_exit();
 }
 
 static int HARDIRQ_verbose(struct lock_class *class)
@@ -4687,10 +4732,12 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 	 * We must printk outside of the graph_lock:
 	 */
 	if (ret == 2) {
+		nbcon_cpu_emergency_enter();
 		printk("\nmarked lock as {%s}:\n", usage_str[new_bit]);
 		print_lock(this);
 		print_irqtrace_events(curr);
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 	}
 
 	return ret;
@@ -4731,6 +4778,8 @@ print_lock_invalid_wait_context(struct task_struct *curr,
 	if (debug_locks_silent)
 		return 0;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("=============================\n");
 	pr_warn("[ BUG: Invalid wait context ]\n");
@@ -4750,6 +4799,8 @@ print_lock_invalid_wait_context(struct task_struct *curr,
 	pr_warn("stack backtrace:\n");
 	dump_stack();
 
+	nbcon_cpu_emergency_exit();
+
 	return 0;
 }
 
@@ -4954,6 +5005,8 @@ print_lock_nested_lock_not_held(struct task_struct *curr,
 	if (debug_locks_silent)
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("==================================\n");
 	pr_warn("WARNING: Nested lock was not taken\n");
@@ -4974,6 +5027,8 @@ print_lock_nested_lock_not_held(struct task_struct *curr,
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+
+	nbcon_cpu_emergency_exit();
 }
 
 static int __lock_is_held(const struct lockdep_map *lock, int read);
@@ -5019,11 +5074,13 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	debug_class_ops_inc(class);
 
 	if (very_verbose(class)) {
+		nbcon_cpu_emergency_enter();
 		printk("\nacquire class [%px] %s", class->key, class->name);
 		if (class->name_version > 1)
 			printk(KERN_CONT "#%d", class->name_version);
 		printk(KERN_CONT "\n");
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 	}
 
 	/*
@@ -5150,6 +5207,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 #endif
 	if (unlikely(curr->lockdep_depth >= MAX_LOCK_DEPTH)) {
 		debug_locks_off();
+		nbcon_cpu_emergency_enter();
 		print_lockdep_off("BUG: MAX_LOCK_DEPTH too low!");
 		printk(KERN_DEBUG "depth: %i  max: %lu!\n",
 		       curr->lockdep_depth, MAX_LOCK_DEPTH);
@@ -5157,6 +5215,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 		lockdep_print_held_locks(current);
 		debug_show_all_locks();
 		dump_stack();
+		nbcon_cpu_emergency_exit();
 
 		return 0;
 	}
@@ -5176,6 +5235,8 @@ static void print_unlock_imbalance_bug(struct task_struct *curr,
 	if (debug_locks_silent)
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("=====================================\n");
 	pr_warn("WARNING: bad unlock balance detected!\n");
@@ -5192,6 +5253,8 @@ static void print_unlock_imbalance_bug(struct task_struct *curr,
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+
+	nbcon_cpu_emergency_exit();
 }
 
 static noinstr int match_held_lock(const struct held_lock *hlock,
@@ -5895,6 +5958,8 @@ static void print_lock_contention_bug(struct task_struct *curr,
 	if (debug_locks_silent)
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("=================================\n");
 	pr_warn("WARNING: bad contention detected!\n");
@@ -5911,6 +5976,8 @@ static void print_lock_contention_bug(struct task_struct *curr,
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+
+	nbcon_cpu_emergency_exit();
 }
 
 static void
@@ -6524,6 +6591,8 @@ print_freed_lock_bug(struct task_struct *curr, const void *mem_from,
 	if (debug_locks_silent)
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("=========================\n");
 	pr_warn("WARNING: held lock freed!\n");
@@ -6536,6 +6605,8 @@ print_freed_lock_bug(struct task_struct *curr, const void *mem_from,
 
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+
+	nbcon_cpu_emergency_exit();
 }
 
 static inline int not_in_range(const void* mem_from, unsigned long mem_len,
@@ -6582,6 +6653,8 @@ static void print_held_locks_bug(void)
 	if (debug_locks_silent)
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	pr_warn("\n");
 	pr_warn("====================================\n");
 	pr_warn("WARNING: %s/%d still has locks held!\n",
@@ -6591,6 +6664,8 @@ static void print_held_locks_bug(void)
 	lockdep_print_held_locks(current);
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+
+	nbcon_cpu_emergency_exit();
 }
 
 void debug_check_no_locks_held(void)
@@ -6616,6 +6691,7 @@ void debug_show_all_locks(void)
 		if (!p->lockdep_depth)
 			continue;
 		lockdep_print_held_locks(p);
+		nbcon_cpu_emergency_flush();
 		touch_nmi_watchdog();
 		touch_all_softlockup_watchdogs();
 	}
@@ -6648,6 +6724,7 @@ asmlinkage __visible void lockdep_sys_exit(void)
 	if (unlikely(curr->lockdep_depth)) {
 		if (!debug_locks_off())
 			return;
+		nbcon_cpu_emergency_enter();
 		pr_warn("\n");
 		pr_warn("================================================\n");
 		pr_warn("WARNING: lock held when returning to user space!\n");
@@ -6656,6 +6733,7 @@ asmlinkage __visible void lockdep_sys_exit(void)
 		pr_warn("%s/%d is leaving the kernel with locks still held!\n",
 				curr->comm, curr->pid);
 		lockdep_print_held_locks(curr);
+		nbcon_cpu_emergency_exit();
 	}
 
 	/*
@@ -6672,6 +6750,7 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
 	bool rcu = warn_rcu_enter();
 
 	/* Note: the following can be executed concurrently, so be careful. */
+	nbcon_cpu_emergency_enter();
 	pr_warn("\n");
 	pr_warn("=============================\n");
 	pr_warn("WARNING: suspicious RCU usage\n");
@@ -6710,6 +6789,7 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
 	lockdep_print_held_locks(curr);
 	pr_warn("\nstack backtrace:\n");
 	dump_stack();
+	nbcon_cpu_emergency_exit();
 	warn_rcu_exit(rcu);
 }
 EXPORT_SYMBOL_GPL(lockdep_rcu_suspicious);
-- 
2.39.2

Re: [PATCH printk v5 30/30] lockdep: Mark emergency sections in lockdep splats

[Petr Mladek]

On Thu 2024-05-02 23:44:39, John Ogness wrote:
> Mark emergency sections wherever multiple lines of
> lock debugging output are generated. In an emergency
> section the CPU will not perform console output for the
> printk() calls. Instead, a flushing of the console
> output is triggered when exiting the emergency section.
> This allows the full message block to be stored as
> quickly as possible in the ringbuffer.
> 
> Note that debug_show_all_locks() and
> lockdep_print_held_locks() rely on their callers to
> enter the emergency section. This is because these
> functions can also be called in non-emergency
> situations (such as sysrq).
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr