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

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Date:   Fri, 12 Mar 2021 20:25:16 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Oleg Nesterov <oleg@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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 Fri, Mar 12 2021 at 17:18, Oleg Nesterov wrote:
> 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().

Yes, it was me being lazy. Lemme open code it as it's actually resulting
in a locked instruction.

Thanks,

        tglx