Re: [PATCH] panic: call hardlockup_detector_perf_stop in panic

[Petr Mladek <pmladek@suse.com>]

Adding Peter Zijlstra into Cc.

The nested panic() should return. But panic() was never supposed to
return. It seems that it is not marked as noreturn but I am not sure
whether some tricks are not hidden somewhere, in objtool, or...

On Wed 2025-08-20 14:22:52, Jinchao Wang wrote:
> On 8/19/25 23:01, Petr Mladek wrote:
> > On Wed 2025-07-30 11:06:33, Wang Jinchao wrote:
> > > When a panic happens, it blocks the cpu, which may
> > > trigger the hardlockup detector if some dump is slow.
> > > So call hardlockup_detector_perf_stop() to disable
> > > hardlockup dector.
> > 
> > Could you please provide more details, especially the log showing
> > the problem?
> 
> Here's what happened: I configured the kernel to use efi-pstore for kdump
> logging while enabling the perf hard lockup detector (NMI). Perhaps the
> efi-pstore was slow and there were too many logs. When the first panic was
> triggered, the pstore dump callback in kmsg_dump()->dumper->dump() took a
> long time, which triggered the NMI watchdog. Then emergency_restart()
> triggered the machine restart before the efi-pstore operation finished.
> The function call flow looked like this:
> 
> ```c
> real panic() {
> 	kmsg_dump() {
> 		...
> 		pstore_dump() {
> 			start_dump();
> 			... // long time operation triggers NMI watchdog
> 			nmi panic() {
> 				...
> 				emergency_restart(); //pstore unfinished
> 			}
> 			...
> 			finish_dump(); // never reached
> 		}
> 	}
> }
> ```
> 
> This created a nested panic situation where the second panic interrupted
> the crash dump process, causing the loss of the original panic information.

I believe that we should prevent the nested panic() in the first
place. There already is the following code:

void vpanic(const char *fmt, va_list args)
{
[...]
	 * Only one CPU is allowed to execute the panic code from here. For
	 * multiple parallel invocations of panic, all other CPUs either
	 * stop themself or will wait until they are stopped by the 1st CPU
	 * with smp_send_stop().
	 *
	 * cmpxchg success means this is the 1st CPU which comes here,
	 * so go ahead.
	 * `old_cpu == this_cpu' means we came from nmi_panic() which sets
	 * panic_cpu to this CPU.  In this case, this is also the 1st CPU.
	 */
	old_cpu = PANIC_CPU_INVALID;
	this_cpu = raw_smp_processor_id();

	/* atomic_try_cmpxchg updates old_cpu on failure */
	if (atomic_try_cmpxchg(&panic_cpu, &old_cpu, this_cpu)) {
		/* go ahead */
	} else if (old_cpu != this_cpu)
		panic_smp_self_stop();


We should improve it to detect nested panic() call as well,
something like:

	this_cpu = raw_smp_processor_id();
	/* Bail out in a nested panic(). Let the outer one finish the job. */
	if (this_cpu == atomic_read(&panic_cpu))
		return;

	/* atomic_try_cmpxchg updates old_cpu on failure */
	old_cpu = PANIC_CPU_INVALID;
	if (atomic_try_cmpxchg(&panic_cpu, &old_cpu, this_cpu)) {
		/* go ahead */
	} else if (old_cpu != this_cpu)
		panic_smp_self_stop();


> > That said, it might make sense to disable the hardlockup
> > detector during panic. But I do not like the proposed way,
> > see below.
> > 
> > > --- a/kernel/panic.c
> > > +++ b/kernel/panic.c
> > > @@ -339,6 +339,7 @@ void panic(const char *fmt, ...)
> > >   	 */
> > >   	local_irq_disable();
> > >   	preempt_disable_notrace();
> > > +	hardlockup_detector_perf_stop();
> > 
> > Anyway, it does not look safe. panic() might be called in any context,
> > including NMI, and I see:
> > 
> >   + hardlockup_detector_perf_stop()
> >     + perf_event_disable()
> >       + perf_event_ctx_lock()
> >         + mutex_lock_nested()
> > 
> > This might cause deadlock when called in NMI, definitely.
> > 
> > Alternative:
> > 
> > A conservative approach would be to update watchdog_hardlockup_check()
> > so that it does nothing when panic_in_progress() returns true. It
> > would even work for both hardlockup detectors implementation.
> Yes, I think it is a better solution.
> I didn't find panic_in_progress() but found hardlockup_detector_perf_stop()
> available instead :)
> I will send another patch.

OK.

Best Regards,
Petr