Re: [PATCH v2 24/24] perf: Make perf_pmu_unregister() useable

[Ravi Bangoria]

On 17-Feb-25 1:54 PM, Ravi Bangoria wrote:
> Hi Peter,
> 
>>> Apparently not, it ends up with:
>>>
>>>   ------------[ cut here ]------------
>>>   WARNING: CPU: 145 PID: 5459 at kernel/events/core.c:281 event_function+0xd2/0xf0
>>>   WARNING: CPU: 145 PID: 5459 at kernel/events/core.c:286 event_function+0xd6/0xf0
>>
>>>    remote_function+0x4f/0x70
>>>    generic_exec_single+0x7f/0x160
>>>    smp_call_function_single+0x110/0x160
>>>    event_function_call+0x98/0x1d0
>>>    _perf_event_disable+0x41/0x70
>>>    perf_event_for_each_child+0x40/0x90
>>>    _perf_ioctl+0xac/0xb00
>>>    perf_ioctl+0x45/0x80
>>
>> Took me a long while trying to blame this on the 'event->parent =
>> NULL;', but AFAICT this is a new, unrelated issue.
>>
>> What I think happens is this perf_ioctl(DISABLE) vs pmu_detach_events()
>> race, where the crux is that perf_ioctl() path does not take
>> event2->mutex which allows the following interleave:
> 
> This one was only with perf_fuzzer, so pmu_detach_events() code path was
> not invoked.

I think the issue is, unaccount_event() gets called for the child event
after the child is detached. Since event->parent is NULL, unaccount_event()
abruptly corrupts the perf_sched_work.

I haven't verified it. Will do it tomorrow.

Thanks,
Ravi