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

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Message-ID: <20241108190835.GA11231@redhat.com>
Date: Fri, 8 Nov 2024 20:08:36 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"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.

Sorry, currently I don't have time to even read the emails from lkml.

Plus I wasn't cc'ed so I don't understand the intent at all, but ...

On 11/08, Frederic Weisbecker wrote:
>
> > @@ -232,13 +222,24 @@ void task_work_run(void)
> >  		 * But it can remove another entry from the ->next list.
> >  		 */
> >  		raw_spin_lock_irq(&task->pi_lock);
> > +		do {
> > +			head = NULL;
> > +			if (work) {
> > +				head = READ_ONCE(work->next);
> > +			} else {
> > +				if (task->flags & PF_EXITING)
> > +					head = &work_exited;
> > +				else
> > +					break;
> > +			}
> > +		} while (!try_cmpxchg(&task->task_works, &work, head));
> >  		raw_spin_unlock_irq(&task->pi_lock);
>
> And having more than one task work should be sufficiently rare
> that we don't care about doing the locking + cmpxchg() for each
> of them pending.

Please see
https://lore.kernel.org/all/1440816150.8932.123.camel@edumazet-glaptop2.roam.corp.google.com/
and the whole thread.

I don't think raw_spin_lock_irq + cmpxchg for each work is a good
idea, but quite possibly I misunderstood this change.

Oleg.