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

John Ogness
2024-05-02T21:38:53Z

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