Re: [PATCH] printk: CPU backtrace not printing on panic

[John Ogness]

Hi Ryo,

On 2024-07-29, takakura@xxxxxxxxxxxxx wrote:
> Or can we use cpu_sync for checking wheather non-panicked CPUs can
> write to ringbuffer and let nbcon_atomic_flush_pending() do the
> printing

Yes, I like this. Let the non-panic CPUs write their messages, but keep
the actual printing to the panic CPU.

Right after the call to panic_other_cpus_shutdown() we plan [0] on
calling printk_legacy_allow_panic_sync(), which will handle flushing the
legacy consoles.

> something like:
>
> ----- BEGIN -----
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 7e207092576b..eed76e3e061b 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -252,8 +252,10 @@ 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) {
>  		trigger_all_cpu_backtrace();
> +		nbcon_atomic_flush_pending();
> +	}
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index d0bff0b0abfd..b8132801ea07 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2354,7 +2354,7 @@ 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() && !__printk_cpu_sync_owner())
>  		return 0;
>  
>  	if (level == LOGLEVEL_SCHED) {
> @@ -4511,6 +4511,11 @@ void __printk_cpu_sync_wait(void)
>  }
>  EXPORT_SYMBOL(__printk_cpu_sync_wait);
>  
> +bool __printk_cpu_sync_owner(void)
> +{
> +	return (atomic_read(&printk_cpu_sync_owner) == raw_smp_processor_id());
> +}
> +
> ----- END -----

Note that printing on the non-panic CPUs is guaranteed for nbcon
consoles because of the other_cpu_in_panic() in
nbcon_context_try_acquire_direct() and for the legacy consoles because
the cpu_sync holder must defer printing [1].

I am curious what Petr has to say.

John

[0] https://lore.kernel.org/lkml/20240527063749.391035-26-john.ogness@xxxxxxxxxxxxx

[1] https://lore.kernel.org/lkml/87plrcqyii.fsf@xxxxxxxxxxxxxxxxxxxxx