linux-kernel - Re: [patch V2 3/3] signal: Allow tasks to cache one sigqueue struct

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Message-ID: <20210312161830.GA27820@redhat.com>
Date:   Fri, 12 Mar 2021 17:18:31 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [patch V2 3/3] signal: Allow tasks to cache one sigqueue struct

On 03/12, Sebastian Andrzej Siewior wrote:
>
> On 2021-03-11 14:20:39 [+0100], Thomas Gleixner wrote:
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -433,7 +433,11 @@ static struct sigqueue *
> >  	rcu_read_unlock();
> >
> >  	if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) {
> > -		q = kmem_cache_alloc(sigqueue_cachep, gfp_flags);
> > +		/* Preallocation does not hold sighand::siglock */
> > +		if (sigqueue_flags || !t->sigqueue_cache)
> > +			q = kmem_cache_alloc(sigqueue_cachep, gfp_flags);
> > +		else
> > +			q = xchg(&t->sigqueue_cache, NULL);
>
> Could it happen that two tasks saw t->sigqueue_cache != NULL, the first
> one got the pointer via xchg() and the second got NULL via xchg()?

It is called with sighand::siglock held, we don't even need xchg().

Oleg.