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Date:   Sat, 8 Oct 2022 16:08:18 +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 Sat, Oct 08, 2022 at 03:51:24PM +0200, Peter Zijlstra wrote:
> 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.

I've settled for:

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