From: Ryo Takakura <takakura@valinux.co.jp>

commit 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing
to ringbuffer") disabled non-panic CPUs to further write messages to
ringbuffer after panicked.

Since the commit, non-panicked CPU's are not allowed to write to
ring buffer after panicked and CPU backtrace which is triggered
after panicked to sample non-panicked CPUs' backtrace no longer
serves its function as it has nothing to print.

Fix the issue by allowing non-panicked CPUs to write into ringbuffer
while CPU backtrace is in flight.

Fixes: 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing to ringbuffer")
Signed-off-by: Ryo Takakura <takakura@valinux.co.jp>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 include/linux/panic.h  | 1 +
 kernel/panic.c         | 8 +++++++-
 kernel/printk/printk.c | 2 +-
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/panic.h b/include/linux/panic.h
index 3130e0b51..54d90b6c5 100644
--- a/include/linux/panic.h
+++ b/include/linux/panic.h
@@ -16,6 +16,7 @@ extern void oops_enter(void);
 extern void oops_exit(void);
 extern bool oops_may_print(void);
 
+extern bool panic_triggering_all_cpu_backtrace;
 extern int panic_timeout;
 extern unsigned long panic_print;
 extern int panic_on_oops;
diff --git a/kernel/panic.c b/kernel/panic.c
index f861bedc1..2a0449144 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -64,6 +64,8 @@ unsigned long panic_on_taint;
 bool panic_on_taint_nousertaint = false;
 static unsigned int warn_limit __read_mostly;
 
+bool panic_triggering_all_cpu_backtrace;
+
 int panic_timeout = CONFIG_PANIC_TIMEOUT;
 EXPORT_SYMBOL_GPL(panic_timeout);
 
@@ -253,8 +255,12 @@ void check_panic_on_warn(const char *origin)
  */
 static void panic_other_cpus_shutdown(bool crash_kexec)
 {
-	if (panic_print & PANIC_PRINT_ALL_CPU_BT)
+	if (panic_print & PANIC_PRINT_ALL_CPU_BT) {
+		/* Temporary allow non-panic CPUs to write their backtraces. */
+		panic_triggering_all_cpu_backtrace = true;
 		trigger_all_cpu_backtrace();
+		panic_triggering_all_cpu_backtrace = false;
+	}
 
 	/*
 	 * Note that smp_send_stop() is the usual SMP shutdown function,
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 054c0e778..c22b07049 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2316,7 +2316,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 	 * non-panic CPUs are generating any messages, they will be
 	 * silently dropped.
 	 */
-	if (other_cpu_in_panic())
+	if (other_cpu_in_panic() && !panic_triggering_all_cpu_backtrace)
 		return 0;
 
 	if (level == LOGLEVEL_SCHED) {
-- 
2.34.1

On Mon 2024-08-12 16:27:03, takakura@valinux.co.jp wrote:
> From: Ryo Takakura <takakura@valinux.co.jp>
> 
> commit 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing
> to ringbuffer") disabled non-panic CPUs to further write messages to
> ringbuffer after panicked.
> 
> Since the commit, non-panicked CPU's are not allowed to write to
> ring buffer after panicked and CPU backtrace which is triggered
> after panicked to sample non-panicked CPUs' backtrace no longer
> serves its function as it has nothing to print.
> 
> Fix the issue by allowing non-panicked CPUs to write into ringbuffer
> while CPU backtrace is in flight.
> 
> Fixes: 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing to ringbuffer")
> Signed-off-by: Ryo Takakura <takakura@valinux.co.jp>
> Reviewed-by: Petr Mladek <pmladek@suse.com>

JFYI, I have pushed this patch into printk/linux.git, branch for-6.11-fixup.
I am going to create pull request after it spends at least one or two
days in linux-next.

The 2nd patch is more complicated. It depends on another patchset
integrating con->write_atomic() callback. 

Best Regards,
Petr

Hi Petr and John,

On Tue 2024-08-13 13:48, Petr Mladek wrote:
>On Mon 2024-08-12 16:27:03, takakura@valinux.co.jp wrote:
>> From: Ryo Takakura <takakura@valinux.co.jp>
>> 
>> commit 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing
>> to ringbuffer") disabled non-panic CPUs to further write messages to
>> ringbuffer after panicked.
>> 
>> Since the commit, non-panicked CPU's are not allowed to write to
>> ring buffer after panicked and CPU backtrace which is triggered
>> after panicked to sample non-panicked CPUs' backtrace no longer
>> serves its function as it has nothing to print.
>> 
>> Fix the issue by allowing non-panicked CPUs to write into ringbuffer
>> while CPU backtrace is in flight.
>> 
>> Fixes: 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing to ringbuffer")
>> Signed-off-by: Ryo Takakura <takakura@valinux.co.jp>
>> Reviewed-by: Petr Mladek <pmladek@suse.com>
>
>JFYI, I have pushed this patch into printk/linux.git, branch for-6.11-fixup.
>I am going to create pull request after it spends at least one or two
>days in linux-next.

Thanks!!

>The 2nd patch is more complicated. It depends on another patchset
>integrating con->write_atomic() callback. 
>
>Best Regards,
>Petr

Sincerely,
Ryo Takakura