linux-kernel - Re: [PATCH v4 2/6] perf: Enqueue SIGTRAP always via task

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Message-ID: <20241029172126.5XY8vLBH@linutronix.de>
Date: Tue, 29 Oct 2024 18:21:26 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: "Lai, Yi" <yi1.lai@...ux.intel.com>, 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>,
	Daniel Bristot de Oliveira <bristot@...nel.org>,
	Ian Rogers <irogers@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Jiri Olsa <jolsa@...nel.org>, Kan Liang <kan.liang@...ux.intel.com>,
	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>, yi1.lai@...el.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH v4 2/6] perf: Enqueue SIGTRAP always via task_work.

On 2024-10-28 13:21:39 [+0100], Frederic Weisbecker wrote:
> Ah the perf_pending_task work is pending but perf_pending_task_sync()
> fails to cancel there:
> 
> 	/*
> 	 * If the task is queued to the current task's queue, we
> 	 * obviously can't wait for it to complete. Simply cancel it.
> 	 */
> 	if (task_work_cancel(current, head)) {
> 		event->pending_work = 0;
> 		local_dec(&event->ctx->nr_no_switch_fast);
> 		return;
> 	}
> 
> And that's because the work is not anymore on the task work
> list in task->task_works. Instead it's in the executing list
> in task_work_run(). It's a blind spot for task_work_cancel()
> if the current task is already running the task works. And it
> does since it's running the fput delayed work.
> 
> Something like this untested?

Tested. Not sure if this is a good idea.
Couldn't we take the ->next pointer and add it to current::task_works
instead?
That patch in ZtYyXG4fYbUdoBpk@...ilion.home gets rid of that
rcuwait_wait_event().

Sebastian