Re: [PATCH v3 7/7] perf: Make perf_pmu_unregister() useable

[Ravi Bangoria]

Peter,

On 12-Mar-25 6:27 PM, Peter Zijlstra wrote:
> On Mon, Mar 10, 2025 at 10:16:09PM +0530, Ravi Bangoria wrote:
>> On 08-Mar-25 1:03 AM, Peter Zijlstra wrote:
>>> Previously it was only safe to call perf_pmu_unregister() if there
>>> were no active events of that pmu around -- which was impossible to
>>> guarantee since it races all sorts against perf_init_event().
>>>
>>> Rework the whole thing by:
>>>
>>>  - keeping track of all events for a given pmu
>>>
>>>  - 'hiding' the pmu from perf_init_event()
>>>
>>>  - waiting for the appropriate (s)rcu grace periods such that all
>>>    prior references to the PMU will be completed
>>>
>>>  - detaching all still existing events of that pmu (see first point)
>>>    and moving them to a new REVOKED state.
>>>
>>>  - actually freeing the pmu data.
>>>
>>> Where notably the new REVOKED state must inhibit all event actions
>>> from reaching code that wants to use event->pmu.
>>
>> I think perf_event_init_task() failure path can still race with
>> perf_pmu_unregister() and trigger a WARN():
>>
>>         CPU 1                                          CPU 2
>>
>>   perf_event_init_task()
>>     perf_event_free_task()
>>       perf_free_event(event1)
>>       /* event1->refcount is 1 */
>>                                                   perf_pmu_unregister()
>>                                                     pmu_detach_events()
>>                                                       pmu_get_event(pmu) /* Picks event1 */
>>                                                         atomic_long_inc_not_zero(&event1->refcount) /* event1 */
>>       /* event1->refcount became 2 (by CPU 2) */
>>         free_event(event1)
>>           WARN()
>>

Please ignore this.

I missed to realize that you have already replaced perf_free_event() with
perf_event_exit_event() and also replaced free_event() with put_event()
in perf_event_exit_event().

Thanks,
Ravi