linux-kernel - Re: [PATCH v3 7/7] perf: Make perf_pmu

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Message-ID: <0f36ca48-09ab-4fca-8a8d-7ed3b2abc056@amd.com>
Date: Mon, 10 Mar 2025 21:05:45 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, lucas.demarchi@...el.com, linux-kernel@...r.kernel.org,
 acme@...nel.org, namhyung@...nel.org, mark.rutland@....com,
 alexander.shishkin@...ux.intel.com, jolsa@...nel.org, irogers@...gle.com,
 adrian.hunter@...el.com, kan.liang@...ux.intel.com,
 Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH v3 7/7] perf: Make perf_pmu_unregister() useable

> @@ -207,6 +207,7 @@ static void perf_ctx_unlock(struct perf_
>  }
>  
>  #define TASK_TOMBSTONE ((void *)-1L)
> +#define EVENT_TOMBSTONE ((void *)-1L)
>  
>  static bool is_kernel_event(struct perf_event *event)
>  {
> @@ -2348,6 +2349,11 @@ static void perf_child_detach(struct per
>  
>  	sync_child_event(event);
>  	list_del_init(&event->child_list);
> +	/*
> +	 * Cannot set to NULL, as that would confuse the situation vs
> +	 * not being a child event. See for example unaccount_event().
> +	 */
> +	event->parent = EVENT_TOMBSTONE;

This will cause issues where we do `event = event->parent`. No? For ex:

  perf_pmu_unregister()
    ...
      perf_event_exit_event()
        perf_event_wakeup()
          ring_buffer_wakeup()
            if (event->parent)
              event = event->parent;

Thanks,
Ravi