linux-kernel - Re: [PATCH v3 2/4] perf: Enqueue SIGTRAP always via task

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Date: Wed, 10 Apr 2024 16:06:33 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
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 2/4] perf: Enqueue SIGTRAP always via task_work.

On 2024-04-10 16:00:17 [+0200], Frederic Weisbecker wrote:
> Le Wed, Apr 10, 2024 at 03:47:02PM +0200, Sebastian Andrzej Siewior a écrit :
> > On 2024-04-10 13:37:05 [+0200], Frederic Weisbecker wrote:
> > > > Couldn't we either flush _or_ remove the task_work in perf_release()?
> > > 
> > > Right so the problem in perf_release() is that we may be dealing with task works
> > > of other tasks than current. In that case, task_work_cancel() is fine if it
> > > successes. But if it fails, you don't have the guarantee that the task work
> > > isn't concurrently running or about to run. And you have no way to know about
> > > that. So then you need some sort of flushing indeed.
> > 
> > Since perf_release() preemptible, a wait/sleep for completion would be
> > best (instead of flushing).
> 
> Like this then?
> 
> https://lore.kernel.org/all/202403310406.TPrIela8-lkp@intel.com/T/#m63c28147d8ac06b21c64d7784d49f892e06c0e50

Kind of, yes. Do we have more than one waiter? If not, maybe that
rcuwait would work then.
Otherwise (>1 waiter) we did establish that we may need a per-task
counter for recursion handling so preempt-disable shouldn't be a problem
then. The pending_work_wq must not be used outside of task context (means
no hardirq or something like that).

Sebastian