Re: Perf: BUG: kernel NULL pointer dereference, address: 0000000000000198

[Ravi Bangoria]

On 03-Nov-22 6:03 PM, Peter Zijlstra wrote:
> On Thu, Nov 03, 2022 at 05:15:30PM +0530, Ravi Bangoria wrote:
> 
>> Sorry was distracted a bit. So, this seems to be happening because of
>> race between amd_pmu_enable_all() and perf event NMI. Something like:
>>
>>   amd_pmu_enable_all()
>>   {
>>       if (!test_bit(idx, cpuc->active_mask))
>>
>>   --->/* perf NMI entry */
>>       ...
>>       x86_pmu_stop() {
>>           __clear_bit(hwc->idx, cpuc->active_mask);
>>           cpuc->events[hwc->idx] = NULL;
>>       }
>>       ...
>>   <---/* perf NMI exit */
>>
>>       amd_pmu_enable_event(cpuc->events[idx]);
>>   }
> 
> Hmm, do you have more information?

I've extracted function graph logs from crash dump and uploaded it here:
https://github.com/BangoriaRavi/function_graph/blob/main/trace.function_graph.dat

crash was on CPU1.

> Something like that would require
> calling amd_pmu_enable_all() while it is already active -- and that
> seems suspect at first glance.
> 
> That is, you shouldn't be getting an NMI for @idx before
> amd_pmu_enable_event().

I too was wondering about this. Will try to get some more data tomorrow.

Thanks,
Ravi