Re: [PATCH v2 1/9] panic: Introduce helper functions for panic state

[Petr Mladek <pmladek@suse.com>]

On Mon 2025-08-25 10:29:29, Jinchao Wang wrote:
> This patch introduces four new helper functions to abstract the
> management of the panic_cpu variable. These functions will be used in
> subsequent patches to refactor existing code.
> 
> The direct use of panic_cpu can be error-prone and ambiguous, as it
> requires manual checks to determine which CPU is handling the panic.
> The new helpers clarify intent:
> 
> panic_try_start():
> Atomically sets the current CPU as the panicking CPU.
> 
> panic_reset():
> Reset panic_cpu to PANIC_CPU_INVALID.
> 
> panic_in_progress():
> Checks if a panic has been triggered.
> 
> panic_on_this_cpu():
> Returns true if the current CPU is the panic originator.
> 
> panic_on_other_cpu():
> Returns true if a panic is on another CPU.
> 
> This change lays the groundwork for improved code readability
> and robustness in the panic handling subsystem.
> 
> Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
> ---
>  include/linux/panic.h  |  6 +++++
>  kernel/panic.c         | 53 ++++++++++++++++++++++++++++++++++++++++++
>  kernel/printk/printk.c |  5 ----
>  3 files changed, 59 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/panic.h b/include/linux/panic.h
> index 7be742628c25..6f972a66c13e 100644
> --- a/include/linux/panic.h
> +++ b/include/linux/panic.h
> @@ -43,6 +43,12 @@ void abort(void);
>  extern atomic_t panic_cpu;
>  #define PANIC_CPU_INVALID	-1
>  
> +bool panic_try_start(void);
> +void panic_reset(void);
> +bool panic_in_progress(void);
> +bool panic_on_this_cpu(void);
> +bool panic_on_other_cpu(void);
> +
>  /*
>   * Only to be used by arch init code. If the user over-wrote the default
>   * CONFIG_PANIC_TIMEOUT, honor it.
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 72fcbb5a071b..eacb0c972110 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -294,6 +294,59 @@ void __weak crash_smp_send_stop(void)
>  
>  atomic_t panic_cpu = ATOMIC_INIT(PANIC_CPU_INVALID);
>  
> +bool panic_try_start(void)
> +{
> +	int old_cpu, this_cpu;
> +
> +	/*
> +	 * Only one CPU is allowed to execute the crash_kexec() code as with
> +	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
> +	 * may stop each other.  To exclude them, we use panic_cpu here too.
> +	 */
> +	old_cpu = PANIC_CPU_INVALID;
> +	this_cpu = raw_smp_processor_id();
> +
> +	return atomic_try_cmpxchg(&panic_cpu, &old_cpu, this_cpu);
> +}
> +EXPORT_SYMBOL(panic_try_start);
> +
> +void panic_reset(void)
> +{
> +	atomic_set(&panic_cpu, PANIC_CPU_INVALID);
> +}
> +EXPORT_SYMBOL(panic_reset);
> +
> +bool panic_in_progress(void)
> +{
> +	return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
> +}
> +EXPORT_SYMBOL(panic_in_progress);
> +
> +/* Return true if a panic is in progress on the current CPU. */
> +bool panic_on_this_cpu(void)
> +{
> +	/*
> +	 * We can use raw_smp_processor_id() here because it is impossible for
> +	 * the task to be migrated to the panic_cpu, or away from it. If
> +	 * panic_cpu has already been set, and we're not currently executing on
> +	 * that CPU, then we never will be.
> +	 */
> +	return unlikely(atomic_read(&panic_cpu) == raw_smp_processor_id());
> +}
> +EXPORT_SYMBOL(panic_on_this_cpu);
> +
> +/*
> + * Return true if a panic is in progress on a remote CPU.
> + *
> + * On true, the local CPU should immediately release any printing resources
> + * that may be needed by the panic CPU.
> + */
> +bool panic_on_other_cpu(void)
> +{
> +	return (panic_in_progress() && !this_cpu_in_panic());
> +}
> +EXPORT_SYMBOL(panic_on_other_cpu);
> +
>  /*
>   * A variant of panic() called from NMI context. We return if we've already
>   * panicked on this CPU. If another CPU already panicked, loop in
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 0efbcdda9aab..5fe35f377b79 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -345,11 +345,6 @@ static void __up_console_sem(unsigned long ip)
>  }
>  #define up_console_sem() __up_console_sem(_RET_IP_)
>  
> -static bool panic_in_progress(void)
> -{
> -	return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
> -}
> -
>  /* Return true if a panic is in progress on the current CPU. */
>  bool this_cpu_in_panic(void)
>  {

All the functions are trivial. It would make sense to define
them in linux/panic.h. Then the callers would benefit
from the (unlikely) prediction macro...

It can be done in a followup path.

Otherwise, the patch looks good. I think that it is too late
but feel free to use:

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

Best Regards,
Petr