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Message-ID: <ZhUt8XMndGSwNuwx@localhost.localdomain>
Date: Tue, 9 Apr 2024 14:00:49 +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 12:54:05PM +0200, Sebastian Andrzej Siewior a écrit :
> On 2024-04-09 12:35:46 [+0200], Frederic Weisbecker wrote:
> > > > > @@ -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.
>
> So the perf_trace_buf_alloc() is invoked from that trace point and
> avoids the recursion. And any trace event from within perf_sigtrap()
> would trigger the endless loop?
No sure I'm following:
1) event->perf_event_overflow() -> task_work_add()
//return to userspace
2) task_work_run() -> perf_pending_task() -> perf_sigtrap() -> tracepoint event
-> perf_event_overflow() -> task_work_add()
3) task_work_run() -> perf_pending_task() -> etc...
What am I missing?
Thanks.
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