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

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Message-ID: <20241216191937.GC374@redhat.com>
Date: Mon, 16 Dec 2024 20:19:37 +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.

On 12/13, Frederic Weisbecker wrote:
>
> So which way do you prefer do solve the initial problem?

Ah. please do what you think/feel is right.

I can't suggest a better fix anyway.

> > > > 			do {
> > > > 	-			next = work->next;
> > > > 	+			next = READ_ONCE(work->next);
> > > > 				work->func(work);
> > > > 				work = next;
> > > > 				cond_resched();
> > > >
> > > > Perhaps it makes sense before the patch from Sebastian even if that patch
> > > > removes this do/while loop ?
> > >
> > > Hmm, can work->next be modified concurrently here?
> >
> > work->func(work) can, say, do kfree(work) or do another task_work_add(X,
> > work).
>
> Right but then isn't it serialized program order, from the compiler point of view?

Hmm, indeed, you are right. In this case the compiler can't assume it can
"defer" work->next. Thanks for correcting me!

Oleg.