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Date:   Sat, 8 Oct 2022 15:51:24 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Marco Elver <elver@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        kasan-dev@googlegroups.com, Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH] perf: Fix missing SIGTRAPs
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On Fri, Oct 07, 2022 at 03:58:03PM +0200, Marco Elver wrote:
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 9319af6013f1..7de83c42d312 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2285,9 +2285,10 @@ event_sched_out(struct perf_event *event,
>  			 */
>  			local_dec(&event->ctx->nr_pending);
>  		} else {
> -			WARN_ON_ONCE(event->pending_work);
> -			event->pending_work = 1;
> -			task_work_add(current, &event->pending_task, TWA_RESUME);
> +			if (!event->pending_work) {
> +				event->pending_work = 1;
> +				task_work_add(current, &event->pending_task, TWA_RESUME);
> +			}
			  else {
				local_dec(&event->ctx->nr_pending);
			}
>  		}
>  	}

That whole thing can be written much saner like:

	if (event->pending_sigtrap) {
		event->pending_sigtrap = 0;
		if (state != PERF_EVENT_STATE_OFF &&
		    !event->pending_work) {
			event->pending_work = 1;
			local_inc(&event->ctx->nr_pending);
			task_work_add(current, &event->pending_task, TWA_RESUME);
		}
		local_dec(&event->ctx->nr_pending);
	}

Except now we have two nr_pending ops -- I'm torn.