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

[Ravi Bangoria <ravi.bangoria@amd.com>]

On 05-Feb-25 3:51 PM, 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.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Another race between perf_event_init_task() failure path and
perf_pmu_unregister():

        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()

Thanks,
Ravi