linux-kernel - Re: [PATCH v3 3/4] perf: Remove perf_swevent_get_recursion_context() from perf_pending

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Date: Tue, 9 Apr 2024 12:35:46 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Jiri Olsa <jolsa@...nel.org>, Marco Elver <elver@...gle.com>,
	Mark Rutland <mark.rutland@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH v3 3/4] perf: Remove perf_swevent_get_recursion_context()
 from perf_pending_task().

Le Tue, Apr 09, 2024 at 08:25:01AM +0200, Sebastian Andrzej Siewior a écrit :
> On 2024-04-09 00:06:00 [+0200], Frederic Weisbecker wrote:
> > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > index e0b2da8de485f..5400f7ed2f98b 100644
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -6785,14 +6785,6 @@ static void perf_pending_irq(struct irq_work *entry)
> > >  static void perf_pending_task(struct callback_head *head)
> > >  {
> > >  	struct perf_event *event = container_of(head, struct perf_event, pending_task);
> > > -	int rctx;
> > > -
> > > -	/*
> > > -	 * If we 'fail' here, that's OK, it means recursion is already disabled
> > > -	 * and we won't recurse 'further'.
> > > -	 */
> > > -	preempt_disable_notrace();
> > > -	rctx = perf_swevent_get_recursion_context();
> > >  
> > >  	if (event->pending_work) {
> > >  		event->pending_work = 0;
> > > @@ -6800,10 +6792,6 @@ static void perf_pending_task(struct callback_head *head)
> > >  		local_dec(&event->ctx->nr_pending);
> > >  	}
> > >  
> > > -	if (rctx >= 0)
> > > -		perf_swevent_put_recursion_context(rctx);
> > > -	preempt_enable_notrace();
> > 
> > Well, if a software event happens during perf_sigtrap(), the task work
> > may be requeued endlessly and the task may get stuck in task_work_run()...
> 
> The last time I checked it had no users in the task context. How would
> that happen?

I guess many tracepoint events would do the trick. Such as trace_lock_acquire()
for example.

Thanks.