[PATCH printk v4 0/6] printk: remove safe buffers

[PATCH printk v4 0/6] printk: remove safe buffers

[John Ogness]

Hi,

Here is v4 of a series to remove the safe buffers. v3 can be
found here [0]. The safe buffers are no longer needed because
messages can be stored directly into the log buffer from any
context.

However, the safe buffers also provided a form of recursion
protection. For that reason, explicit recursion protection is
implemented for this series.

The safe buffers also implicitly provided serialization
between multiple CPUs executing in NMI context. This was
particularly necessary for the nmi_backtrace() output. This
serializiation is now preserved by using the printk cpulock.

With the removal of the safe buffers, there is no need for
extra NMI enter/exit tracking. So this is also removed
(which includes removing the config option CONFIG_PRINTK_NMI).

And finally, there are a few places in the kernel that need to
specify code blocks where all printk calls are to be deferred
printing. Previously the NMI tracking API was being (mis)used
for this purpose. This series introduces an official and
explicit interface for such cases. (Note that all deferred
printing will be removed anyway, once printing kthreads are
introduced.)

Changes since v3:

- Remove safe context tracking in vprintk().

- Add safe context tracking for @console_owner usage since that
  is also a component of the printing code.

- Refactor syslog_print() so that it is easier to understand
  and follow the locking logic.

- Introduce printk_deferred_enter/exit functions to be used by
  code that needs to specify code block where all printk calls
  are to be deferred printing.

John Ogness

[0] https://lore.kernel.org/lkml/20210624111148.5190-1-john.ogness@linutronix.de

John Ogness (6):
  lib/nmi_backtrace: explicitly serialize banner and regs
  printk: track/limit recursion
  printk: remove safe buffers
  printk: remove NMI tracking
  printk: convert @syslog_lock to mutex
  printk: syslog: close window between wait and read

 arch/arm/kernel/smp.c          |   4 +-
 arch/powerpc/kernel/traps.c    |   1 -
 arch/powerpc/kernel/watchdog.c |   5 -
 arch/powerpc/kexec/crash.c     |   2 +-
 include/linux/hardirq.h        |   2 -
 include/linux/printk.h         |  41 ++--
 init/Kconfig                   |   5 -
 kernel/kexec_core.c            |   1 -
 kernel/panic.c                 |   3 -
 kernel/printk/internal.h       |  25 ---
 kernel/printk/printk.c         | 268 ++++++++++++++----------
 kernel/printk/printk_safe.c    | 364 +--------------------------------
 kernel/trace/trace.c           |   4 +-
 lib/nmi_backtrace.c            |  13 +-
 14 files changed, 194 insertions(+), 544 deletions(-)


base-commit: 70333dec446292cd896cd051d2ebd6808b328949
-- 
2.20.1


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[PATCH printk v4 4/6] printk: remove NMI tracking

[John Ogness]

All NMI contexts are handled the same as the safe context: store the
message and defer printing. There is no need to have special NMI
context tracking for this. Using in_nmi() is enough.

There are several parts of the kernel that are manually calling into
the printk NMI context tracking in order to cause general printk
deferred printing:

    arch/arm/kernel/smp.c
    arch/powerpc/kexec/crash.c
    kernel/trace/trace.c

For these users, provide a new function pair
printk_deferred_enter/exit that explicitly achieves the same
objective.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 arch/arm/kernel/smp.c       |  4 ++--
 arch/powerpc/kexec/crash.c  |  2 +-
 include/linux/hardirq.h     |  2 --
 include/linux/printk.h      | 31 +++++++++++++++++++------------
 init/Kconfig                |  5 -----
 kernel/printk/internal.h    |  8 --------
 kernel/printk/printk_safe.c | 37 +------------------------------------
 kernel/trace/trace.c        |  4 ++--
 8 files changed, 25 insertions(+), 68 deletions(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index c7bb168b0d97..842427ff2b3c 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -667,9 +667,9 @@ static void do_handle_IPI(int ipinr)
 		break;
 
 	case IPI_CPU_BACKTRACE:
-		printk_nmi_enter();
+		printk_deferred_enter();
 		nmi_cpu_backtrace(get_irq_regs());
-		printk_nmi_exit();
+		printk_deferred_exit();
 		break;
 
 	default:
diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
index 0196d0c211ac..1070378c8e35 100644
--- a/arch/powerpc/kexec/crash.c
+++ b/arch/powerpc/kexec/crash.c
@@ -313,7 +313,7 @@ void default_machine_crash_shutdown(struct pt_regs *regs)
 	int (*old_handler)(struct pt_regs *regs);
 
 	/* Avoid hardlocking with irresponsive CPU holding logbuf_lock */
-	printk_nmi_enter();
+	printk_deferred_enter();
 
 	/*
 	 * This function is only called after the system
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 69bc86ea382c..76878b357ffa 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -116,7 +116,6 @@ extern void rcu_nmi_exit(void);
 	do {							\
 		lockdep_off();					\
 		arch_nmi_enter();				\
-		printk_nmi_enter();				\
 		BUG_ON(in_nmi() == NMI_MASK);			\
 		__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
 	} while (0)
@@ -135,7 +134,6 @@ extern void rcu_nmi_exit(void);
 	do {							\
 		BUG_ON(!in_nmi());				\
 		__preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET);	\
-		printk_nmi_exit();				\
 		arch_nmi_exit();				\
 		lockdep_on();					\
 	} while (0)
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 664612f75dac..03f7ccedaf18 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -149,18 +149,6 @@ static inline __printf(1, 2) __cold
 void early_printk(const char *s, ...) { }
 #endif
 
-#ifdef CONFIG_PRINTK_NMI
-extern void printk_nmi_enter(void);
-extern void printk_nmi_exit(void);
-extern void printk_nmi_direct_enter(void);
-extern void printk_nmi_direct_exit(void);
-#else
-static inline void printk_nmi_enter(void) { }
-static inline void printk_nmi_exit(void) { }
-static inline void printk_nmi_direct_enter(void) { }
-static inline void printk_nmi_direct_exit(void) { }
-#endif /* PRINTK_NMI */
-
 struct dev_printk_info;
 
 #ifdef CONFIG_PRINTK
@@ -180,6 +168,16 @@ 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);
+/*
+ * 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
+
 /*
  * Please don't use printk_ratelimit(), because it shares ratelimiting state
  * with all other unrelated printk_ratelimit() callsites.  Instead use
@@ -223,6 +221,15 @@ int printk_deferred(const char *s, ...)
 {
 	return 0;
 }
+
+static inline void printk_deferred_enter(void)
+{
+}
+
+static inline void printk_deferred_exit(void)
+{
+}
+
 static inline int printk_ratelimit(void)
 {
 	return 0;
diff --git a/init/Kconfig b/init/Kconfig
index a61c92066c2e..9c0510693543 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1506,11 +1506,6 @@ config PRINTK
 	  very difficult to diagnose system problems, saying N here is
 	  strongly discouraged.
 
-config PRINTK_NMI
-	def_bool y
-	depends on PRINTK
-	depends on HAVE_NMI
-
 config BUG
 	bool "BUG() support" if EXPERT
 	default y
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 6cc35c5de890..b6d310c72fc9 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -6,12 +6,6 @@
 
 #ifdef CONFIG_PRINTK
 
-#define PRINTK_SAFE_CONTEXT_MASK	0x007ffffff
-#define PRINTK_NMI_DIRECT_CONTEXT_MASK	0x008000000
-#define PRINTK_NMI_CONTEXT_MASK		0xff0000000
-
-#define PRINTK_NMI_CONTEXT_OFFSET	0x010000000
-
 __printf(4, 0)
 int vprintk_store(int facility, int level,
 		  const struct dev_printk_info *dev_info,
@@ -19,8 +13,6 @@ 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);
 
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 29c580dac93d..ef0f9a2044da 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -4,12 +4,9 @@
  */
 
 #include <linux/preempt.h>
-#include <linux/spinlock.h>
-#include <linux/debug_locks.h>
 #include <linux/kdb.h>
 #include <linux/smp.h>
 #include <linux/cpumask.h>
-#include <linux/irq_work.h>
 #include <linux/printk.h>
 #include <linux/kprobes.h>
 
@@ -17,35 +14,6 @@
 
 static DEFINE_PER_CPU(int, printk_context);
 
-#ifdef CONFIG_PRINTK_NMI
-void noinstr printk_nmi_enter(void)
-{
-	this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
-}
-
-void noinstr printk_nmi_exit(void)
-{
-	this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
-}
-
-/*
- * Marks a code that might produce many messages in NMI context
- * and the risk of losing them is more critical than eventual
- * reordering.
- */
-void printk_nmi_direct_enter(void)
-{
-	if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
-		this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
-}
-
-void printk_nmi_direct_exit(void)
-{
-	this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
-}
-
-#endif /* CONFIG_PRINTK_NMI */
-
 /* Can be preempted by NMI. */
 void __printk_safe_enter(void)
 {
@@ -70,10 +38,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	 * Use the main logbuf even in NMI. But avoid calling console
 	 * drivers that might have their own locks.
 	 */
-	if (this_cpu_read(printk_context) &
-	    (PRINTK_NMI_DIRECT_CONTEXT_MASK |
-	     PRINTK_NMI_CONTEXT_MASK |
-	     PRINTK_SAFE_CONTEXT_MASK)) {
+	if (this_cpu_read(printk_context) || in_nmi()) {
 		int len;
 
 		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d23a09d3eb37..b30ad20d251f 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9647,7 +9647,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
 	tracing_off();
 
 	local_irq_save(flags);
-	printk_nmi_direct_enter();
+	printk_deferred_enter();
 
 	/* Simulate the iterator */
 	trace_init_global_iter(&iter);
@@ -9729,7 +9729,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
 		atomic_dec(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
 	}
 	atomic_dec(&dump_running);
-	printk_nmi_direct_exit();
+	printk_deferred_exit();
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(ftrace_dump);
-- 
2.20.1


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Re: [PATCH printk v4 4/6] printk: remove NMI tracking

[Petr Mladek]

On Thu 2021-07-15 21:39:57, John Ogness wrote:
> All NMI contexts are handled the same as the safe context: store the
> message and defer printing. There is no need to have special NMI
> context tracking for this. Using in_nmi() is enough.
> 
> There are several parts of the kernel that are manually calling into
> the printk NMI context tracking in order to cause general printk
> deferred printing:
> 
>     arch/arm/kernel/smp.c
>     arch/powerpc/kexec/crash.c
>     kernel/trace/trace.c
> 
> For these users, provide a new function pair
> printk_deferred_enter/exit that explicitly achieves the same
> objective.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> ---
>  arch/arm/kernel/smp.c       |  4 ++--
>  arch/powerpc/kexec/crash.c  |  2 +-
>  include/linux/hardirq.h     |  2 --
>  include/linux/printk.h      | 31 +++++++++++++++++++------------
>  init/Kconfig                |  5 -----
>  kernel/printk/internal.h    |  8 --------
>  kernel/printk/printk_safe.c | 37 +------------------------------------
>  kernel/trace/trace.c        |  4 ++--
>  8 files changed, 25 insertions(+), 68 deletions(-)
> 
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index c7bb168b0d97..842427ff2b3c 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -667,9 +667,9 @@ static void do_handle_IPI(int ipinr)
>  		break;
>  
>  	case IPI_CPU_BACKTRACE:
> -		printk_nmi_enter();
> +		printk_deferred_enter();
>  		nmi_cpu_backtrace(get_irq_regs());
> -		printk_nmi_exit();
> +		printk_deferred_exit();
>  		break;
>  
>  	default:
> diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
> index 0196d0c211ac..1070378c8e35 100644
> --- a/arch/powerpc/kexec/crash.c
> +++ b/arch/powerpc/kexec/crash.c
> @@ -313,7 +313,7 @@ void default_machine_crash_shutdown(struct pt_regs *regs)
>  	int (*old_handler)(struct pt_regs *regs);
>  
>  	/* Avoid hardlocking with irresponsive CPU holding logbuf_lock */
> -	printk_nmi_enter();
> +	printk_deferred_enter();
>  
>  	/*
>  	 * This function is only called after the system
> diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> index 69bc86ea382c..76878b357ffa 100644
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -116,7 +116,6 @@ extern void rcu_nmi_exit(void);
>  	do {							\
>  		lockdep_off();					\
>  		arch_nmi_enter();				\
> -		printk_nmi_enter();				\
>  		BUG_ON(in_nmi() == NMI_MASK);			\
>  		__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
>  	} while (0)
> @@ -135,7 +134,6 @@ extern void rcu_nmi_exit(void);
>  	do {							\
>  		BUG_ON(!in_nmi());				\
>  		__preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET);	\
> -		printk_nmi_exit();				\
>  		arch_nmi_exit();				\
>  		lockdep_on();					\
>  	} while (0)
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 664612f75dac..03f7ccedaf18 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -149,18 +149,6 @@ static inline __printf(1, 2) __cold
>  void early_printk(const char *s, ...) { }
>  #endif
>  
> -#ifdef CONFIG_PRINTK_NMI
> -extern void printk_nmi_enter(void);
> -extern void printk_nmi_exit(void);
> -extern void printk_nmi_direct_enter(void);
> -extern void printk_nmi_direct_exit(void);
> -#else
> -static inline void printk_nmi_enter(void) { }
> -static inline void printk_nmi_exit(void) { }
> -static inline void printk_nmi_direct_enter(void) { }
> -static inline void printk_nmi_direct_exit(void) { }
> -#endif /* PRINTK_NMI */
> -
>  struct dev_printk_info;
>  
>  #ifdef CONFIG_PRINTK
> @@ -180,6 +168,16 @@ 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);
> +/*
> + * 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
> +
>  /*
>   * Please don't use printk_ratelimit(), because it shares ratelimiting state
>   * with all other unrelated printk_ratelimit() callsites.  Instead use
> @@ -223,6 +221,15 @@ int printk_deferred(const char *s, ...)
>  {
>  	return 0;
>  }
> +
> +static inline void printk_deferred_enter(void)
> +{
> +}
> +
> +static inline void printk_deferred_exit(void)
> +{
> +}
> +
>  static inline int printk_ratelimit(void)
>  {
>  	return 0;
> diff --git a/init/Kconfig b/init/Kconfig
> index a61c92066c2e..9c0510693543 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1506,11 +1506,6 @@ config PRINTK
>  	  very difficult to diagnose system problems, saying N here is
>  	  strongly discouraged.
>  
> -config PRINTK_NMI
> -	def_bool y
> -	depends on PRINTK
> -	depends on HAVE_NMI
> -
>  config BUG
>  	bool "BUG() support" if EXPERT
>  	default y
> diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
> index 6cc35c5de890..b6d310c72fc9 100644
> --- a/kernel/printk/internal.h
> +++ b/kernel/printk/internal.h
> @@ -6,12 +6,6 @@
>  
>  #ifdef CONFIG_PRINTK
>  
> -#define PRINTK_SAFE_CONTEXT_MASK	0x007ffffff
> -#define PRINTK_NMI_DIRECT_CONTEXT_MASK	0x008000000
> -#define PRINTK_NMI_CONTEXT_MASK		0xff0000000
> -
> -#define PRINTK_NMI_CONTEXT_OFFSET	0x010000000
> -
>  __printf(4, 0)
>  int vprintk_store(int facility, int level,
>  		  const struct dev_printk_info *dev_info,
> @@ -19,8 +13,6 @@ 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);
>  
> diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
> index 29c580dac93d..ef0f9a2044da 100644
> --- a/kernel/printk/printk_safe.c
> +++ b/kernel/printk/printk_safe.c
> @@ -4,12 +4,9 @@
>   */
>  
>  #include <linux/preempt.h>
> -#include <linux/spinlock.h>
> -#include <linux/debug_locks.h>
>  #include <linux/kdb.h>
>  #include <linux/smp.h>
>  #include <linux/cpumask.h>
> -#include <linux/irq_work.h>
>  #include <linux/printk.h>
>  #include <linux/kprobes.h>
>  
> @@ -17,35 +14,6 @@
>  
>  static DEFINE_PER_CPU(int, printk_context);
>  
> -#ifdef CONFIG_PRINTK_NMI
> -void noinstr printk_nmi_enter(void)
> -{
> -	this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
> -}
> -
> -void noinstr printk_nmi_exit(void)
> -{
> -	this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
> -}
> -
> -/*
> - * Marks a code that might produce many messages in NMI context
> - * and the risk of losing them is more critical than eventual
> - * reordering.
> - */
> -void printk_nmi_direct_enter(void)
> -{
> -	if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
> -		this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
> -}
> -
> -void printk_nmi_direct_exit(void)
> -{
> -	this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
> -}
> -
> -#endif /* CONFIG_PRINTK_NMI */
> -
>  /* Can be preempted by NMI. */
>  void __printk_safe_enter(void)
>  {
> @@ -70,10 +38,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
>  	 * Use the main logbuf even in NMI. But avoid calling console
>  	 * drivers that might have their own locks.
>  	 */
> -	if (this_cpu_read(printk_context) &
> -	    (PRINTK_NMI_DIRECT_CONTEXT_MASK |
> -	     PRINTK_NMI_CONTEXT_MASK |
> -	     PRINTK_SAFE_CONTEXT_MASK)) {
> +	if (this_cpu_read(printk_context) || in_nmi()) {
>  		int len;
>  
>  		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index d23a09d3eb37..b30ad20d251f 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -9647,7 +9647,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
>  	tracing_off();
>  
>  	local_irq_save(flags);
> -	printk_nmi_direct_enter();
> +	printk_deferred_enter();

I would prefer to do not manipulate the printk context here anymore,
as it was done in v3.

printk_nmi_direct_enter() was added here by the commit the commit
03fc7f9c99c1e7ae2925d4 ("printk/nmi: Prevent deadlock when
accessing the main log buffer in NMI"). It was _not_ about console
handling. The reason was to modify the default behavior under NMI
and store the messages directly into the main log buffer.

When I think about it. The original fix was not correct. We should have
modified the context only when ftrace_dump() was really called under NMI:

	if (in_nmi())
		printk_nmi_direct_enter();

By other words. We should try to show the messages on the console
when ftrace_dump()/panic() is not called from NMI. It will help
to see all messages even when the ftrace buffers are bigger
than printk() ones.

And we do not need any special handling here for NMI. vprintk()
in printk/printk_safe.c will do the right thing for us.

Best Regards,
Petr

PS: There is no need to re-send the entire patchset if this was
    the only problem. Feel free to send v4.1 version of this
    patch only.

   That said, I still have to look at 5th and 6th patch.

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Re: [PATCH printk v4 4/6] printk: remove NMI tracking

[John Ogness]

On 2021-07-21, Petr Mladek <pmladek@suse.com> wrote:
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -9647,7 +9647,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
>>  	tracing_off();
>>  
>>  	local_irq_save(flags);
>> -	printk_nmi_direct_enter();
>> +	printk_deferred_enter();
>
> I would prefer to do not manipulate the printk context here anymore,
> as it was done in v3.
>
> printk_nmi_direct_enter() was added here by the commit the commit
> 03fc7f9c99c1e7ae2925d4 ("printk/nmi: Prevent deadlock when
> accessing the main log buffer in NMI"). It was _not_ about console
> handling. The reason was to modify the default behavior under NMI
> and store the messages directly into the main log buffer.
>
> When I think about it. The original fix was not correct. We should have
> modified the context only when ftrace_dump() was really called under NMI:
>
> 	if (in_nmi())
> 		printk_nmi_direct_enter();
>
> By other words. We should try to show the messages on the console
> when ftrace_dump()/panic() is not called from NMI. It will help
> to see all messages even when the ftrace buffers are bigger
> than printk() ones.
>
> And we do not need any special handling here for NMI. vprintk()
> in printk/printk_safe.c will do the right thing for us.

Agreed. We need to mention this behavior change in the commit
message. Perhaps this as the commit message:

All NMI contexts are handled the same as the safe context: store the
message and defer printing. There is no need to have special NMI
context tracking for this. Using in_nmi() is enough.

There are several parts of the kernel that are manually calling into
the printk NMI context tracking in order to cause general printk
deferred printing:

    arch/arm/kernel/smp.c
    arch/powerpc/kexec/crash.c
    kernel/trace/trace.c

For arm/kernel/smp.c and powerpc/kexec/crash.c, provide a new
function pair printk_deferred_enter/exit that explicitly achieves the
same objective.

For ftrace, remove general printk deferring. This general deferrment
was added in commit 03fc7f9c99c1 ("printk/nmi: Prevent deadlock when
accessing the main log buffer in NMI"), but really should have only
been deferred when in NMI context. Since vprintk() now checks for
NMI context when deciding to defer, ftrace does not need any special
handling.

Signed-off-by: John Ogness <john.ogness@linutronix.de>

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Re: [PATCH printk v4 4/6] printk: remove NMI tracking

[Petr Mladek]

On Wed 2021-07-21 14:52:15, John Ogness wrote:
> On 2021-07-21, Petr Mladek <pmladek@suse.com> wrote:
> >> --- a/kernel/trace/trace.c
> >> +++ b/kernel/trace/trace.c
> >> @@ -9647,7 +9647,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
> >>  	tracing_off();
> >>  
> >>  	local_irq_save(flags);
> >> -	printk_nmi_direct_enter();
> >> +	printk_deferred_enter();
> >
> > I would prefer to do not manipulate the printk context here anymore,
> > as it was done in v3.
> >
> > printk_nmi_direct_enter() was added here by the commit the commit
> > 03fc7f9c99c1e7ae2925d4 ("printk/nmi: Prevent deadlock when
> > accessing the main log buffer in NMI"). It was _not_ about console
> > handling. The reason was to modify the default behavior under NMI
> > and store the messages directly into the main log buffer.
> >
> > When I think about it. The original fix was not correct. We should have
> > modified the context only when ftrace_dump() was really called under NMI:
> >
> > 	if (in_nmi())
> > 		printk_nmi_direct_enter();
> >
> > By other words. We should try to show the messages on the console
> > when ftrace_dump()/panic() is not called from NMI. It will help
> > to see all messages even when the ftrace buffers are bigger
> > than printk() ones.
> >
> > And we do not need any special handling here for NMI. vprintk()
> > in printk/printk_safe.c will do the right thing for us.
> 
> Agreed. We need to mention this behavior change in the commit
> message. Perhaps this as the commit message:
>
> All NMI contexts are handled the same as the safe context: store the
> message and defer printing. There is no need to have special NMI
> context tracking for this. Using in_nmi() is enough.
> 
> There are several parts of the kernel that are manually calling into
> the printk NMI context tracking in order to cause general printk
> deferred printing:
> 
>     arch/arm/kernel/smp.c
>     arch/powerpc/kexec/crash.c
>     kernel/trace/trace.c
> 
> For arm/kernel/smp.c and powerpc/kexec/crash.c, provide a new
> function pair printk_deferred_enter/exit that explicitly achieves the
> same objective.
> 
> For ftrace, remove general printk deferring. This general deferrment
> was added in commit 03fc7f9c99c1 ("printk/nmi: Prevent deadlock when
> accessing the main log buffer in NMI"), but really should have only
> been deferred when in NMI context. Since vprintk() now checks for
> NMI context when deciding to defer, ftrace does not need any special
> handling.

I would make it less focused on the deferring part and try to explain
the original purpose here, something like:

"For ftrace, remove the printk context manipulation completely. It was
added in commit 03fc7f9c99c1 ("printk/nmi: Prevent deadlock when
accessing the main log buffer in NMI"). The purpose was to enforce
storing messages directly into the ring buffer even in NMI context.
It really should have only modified the behavior in NMI context.
There is no need for a special behavior any longer. All messages are
always stored directly now. The console deferring is handled
transparently in vprintk()."

But I do not resist on it. And feel free to make it shorter.

Best Regards,
Petr

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Re: [PATCH printk v4 4/6] printk: remove NMI tracking

[John Ogness]

On 2021-07-21, Petr Mladek <pmladek@suse.com> wrote:
> On Wed 2021-07-21 14:52:15, John Ogness wrote:
>> On 2021-07-21, Petr Mladek <pmladek@suse.com> wrote:
>> >> --- a/kernel/trace/trace.c
>> >> +++ b/kernel/trace/trace.c
>> >> @@ -9647,7 +9647,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
>> >>  	tracing_off();
>> >>  
>> >>  	local_irq_save(flags);
>> >> -	printk_nmi_direct_enter();
>> >> +	printk_deferred_enter();
>> >
>> > I would prefer to do not manipulate the printk context here anymore,
>> > as it was done in v3.
>> >
>> > printk_nmi_direct_enter() was added here by the commit the commit
>> > 03fc7f9c99c1e7ae2925d4 ("printk/nmi: Prevent deadlock when
>> > accessing the main log buffer in NMI"). It was _not_ about console
>> > handling. The reason was to modify the default behavior under NMI
>> > and store the messages directly into the main log buffer.
>> >
>> > When I think about it. The original fix was not correct. We should have
>> > modified the context only when ftrace_dump() was really called under NMI:
>> >
>> > 	if (in_nmi())
>> > 		printk_nmi_direct_enter();
>> >
>> > By other words. We should try to show the messages on the console
>> > when ftrace_dump()/panic() is not called from NMI. It will help
>> > to see all messages even when the ftrace buffers are bigger
>> > than printk() ones.
>> >
>> > And we do not need any special handling here for NMI. vprintk()
>> > in printk/printk_safe.c will do the right thing for us.
>> 
>> Agreed. We need to mention this behavior change in the commit
>> message. Perhaps this as the commit message:
>>
>> All NMI contexts are handled the same as the safe context: store the
>> message and defer printing. There is no need to have special NMI
>> context tracking for this. Using in_nmi() is enough.
>> 
>> There are several parts of the kernel that are manually calling into
>> the printk NMI context tracking in order to cause general printk
>> deferred printing:
>> 
>>     arch/arm/kernel/smp.c
>>     arch/powerpc/kexec/crash.c
>>     kernel/trace/trace.c
>> 
>> For arm/kernel/smp.c and powerpc/kexec/crash.c, provide a new
>> function pair printk_deferred_enter/exit that explicitly achieves the
>> same objective.
>> 
>> For ftrace, remove general printk deferring. This general deferrment
>> was added in commit 03fc7f9c99c1 ("printk/nmi: Prevent deadlock when
>> accessing the main log buffer in NMI"), but really should have only
>> been deferred when in NMI context. Since vprintk() now checks for
>> NMI context when deciding to defer, ftrace does not need any special
>> handling.
>
> I would make it less focused on the deferring part and try to explain
> the original purpose here, something like:
>
> "For ftrace, remove the printk context manipulation completely. It was
> added in commit 03fc7f9c99c1 ("printk/nmi: Prevent deadlock when
> accessing the main log buffer in NMI"). The purpose was to enforce
> storing messages directly into the ring buffer even in NMI context.
> It really should have only modified the behavior in NMI context.
> There is no need for a special behavior any longer. All messages are
> always stored directly now. The console deferring is handled
> transparently in vprintk()."

Your wording is OK for me.

John Ogness

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Re: [PATCH printk v4 0/6] printk: remove safe buffers

[Petr Mladek]

On Thu 2021-07-15 21:39:53, John Ogness wrote:
> Hi,
> 
> Here is v4 of a series to remove the safe buffers. v3 can be
> found here [0]. The safe buffers are no longer needed because
> messages can be stored directly into the log buffer from any
> context.
> 
> However, the safe buffers also provided a form of recursion
> protection. For that reason, explicit recursion protection is
> implemented for this series.
> 
> The safe buffers also implicitly provided serialization
> between multiple CPUs executing in NMI context. This was
> particularly necessary for the nmi_backtrace() output. This
> serializiation is now preserved by using the printk cpulock.
> 
> With the removal of the safe buffers, there is no need for
> extra NMI enter/exit tracking. So this is also removed
> (which includes removing the config option CONFIG_PRINTK_NMI).
> 
> And finally, there are a few places in the kernel that need to
> specify code blocks where all printk calls are to be deferred
> printing. Previously the NMI tracking API was being (mis)used
> for this purpose. This series introduces an official and
> explicit interface for such cases. (Note that all deferred
> printing will be removed anyway, once printing kthreads are
> introduced.)
> 
> John Ogness (6):
>   lib/nmi_backtrace: explicitly serialize banner and regs
>   printk: track/limit recursion
>   printk: remove safe buffers
>   printk: remove NMI tracking
>   printk: convert @syslog_lock to mutex
>   printk: syslog: close window between wait and read

The entire patchset has been committed into printk/linux.git,
branch rework/printk_safe-removal.

Note that I have updated the 4th patch as discussed, see
https://lore.kernel.org/r/20210721120026.y3dqno24ahw4sazy@pathway.suse.cz
https://lore.kernel.org/r/20210721130852.zrjnti6b3fwjgdzj@pathway.suse.cz

Best Regards,
Petr

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