Re: [PATCH] perf/x86/amd: check event before enable to avoid GPF

[George Kennedy]

Hi Ravi,

On 6/4/2024 9:40 AM, Ravi Bangoria wrote:
> > On 6/4/2024 9:16 AM, Ravi Bangoria wrote:
> > > > > > > Events can be deleted and the entry can be NULL.
> > > > > > Can you please also explain "how".
> > > > > It looks like x86_pmu_stop() is clearing the bit in active_mask and setting the events entry to NULL (and doing it in the correct order) for the same events index that amd_pmu_enable_all() is trying to enable.
> > > > > > > Check event for NULL in amd_pmu_enable_all() before enable to avoid a GPF.
> > > > > > > This appears to be an AMD only issue.
> > > > > > >
> > > > > > > Syzkaller reported a GPF in amd_pmu_enable_all.
> > > > > > Can you please provide a bug report link? Also, any reproducer?
> > > > > The Syzkaller reproducer can be found in this link:
> > > > > https://lore.kernel.org/netdev/CAMt6jhyec7-TSFpr3F+_ikjpu39WV3jnCBBGwpzpBrPx55w20g@xxxxxxxxxxxxxx/T/#u
> > > > > > > @@ -760,7 +760,8 @@ static void amd_pmu_enable_all(int added)
> > > > > > >             if (!test_bit(idx, cpuc->active_mask))
> > > > > > >                 continue;
> > > > > > >     -        amd_pmu_enable_event(cpuc->events[idx]);
> > > > > > > +        if (cpuc->events[idx])
> > > > > > > +            amd_pmu_enable_event(cpuc->events[idx]);
> > > > > > What if cpuc->events[idx] becomes NULL after if (cpuc->events[idx]) but
> > > > > > before amd_pmu_enable_event(cpuc->events[idx])?
> > > > > Good question, but the crash has not reproduced with the proposed fix in hours of testing. It usually reproduces within minutes without the fix.
> > > > Also, a similar fix is done in __intel_pmu_enable_all() in arch/x86/events/intel/core.c except that a WARN_ON_ONCE is done as well.
> > > > See: https://elixir.bootlin.com/linux/v6.10-rc1/source/arch/x86/events/intel/core.c#L2256
> > > There are subtle differences between Intel and AMD pmu implementation.
> > > __intel_pmu_enable_all() enables all event with single WRMSR whereas
> > > amd_pmu_enable_all() loops over each PMC and enables it individually.
> > >
> > > The WARN_ON_ONCE() is important because it will warn about potential
> > > sw bug somewhere else.
> > We could add a similar WARN_ON_ONCE() to the proposed patch.
> Sure, that would help in future. But for current splat, can you please
> try to rootcause the underlying race condition?

Were you able to reproduce the crash on the AMD machine?

Thanks,
George
> Thanks,
> Ravi