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

[George Kennedy <george.kennedy@oracle.com>]

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@mail.gmail.com/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