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

[Jinchao Wang <wangjinchao600@gmail.com>]

On Tue, Sep 16, 2025 at 11:57:11AM +0200, Petr Mladek wrote:
> 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.
Thanks for feedback, I will do it later.

BTW, this series was merged to -mm branch already.

> 
> 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

-- 
Jinchao