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

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Date:   Tue, 16 Mar 2021 13:36:47 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 Sat, Mar 13 2021 at 17:49, Oleg Nesterov wrote:
> On 03/12, Thomas Gleixner wrote:
>>
>> On Fri, Mar 12 2021 at 20:26, Thomas Gleixner wrote:
>> > On Fri, Mar 12 2021 at 17:11, Oleg Nesterov wrote:
>> >> On 03/11, Thomas Gleixner wrote:
>> >>>
>> >>> @@ -456,7 +460,12 @@ static void __sigqueue_free(struct sigqu
>> >>>  		return;
>> >>>  	if (atomic_dec_and_test(&q->user->sigpending))
>> >>>  		free_uid(q->user);
>> >>> -	kmem_cache_free(sigqueue_cachep, q);
>> >>> +
>> >>> +	/* Cache one sigqueue per task */
>> >>> +	if (!current->sigqueue_cache)
>> >>> +		current->sigqueue_cache = q;
>> >>> +	else
>> >>> +		kmem_cache_free(sigqueue_cachep, q);
>> >>>  }
>> >>
>> >> This doesn't look right, note that __exit_signal() does
>> >> flush_sigqueue(&sig->shared_pending) at the end, after exit_task_sighand()
>> >> was already called.
>> >>
>> >> I'd suggest to not add the new exit_task_sighand() helper and simply free
>> >> current->sigqueue_cache at the end of __exit_signal().
>> >
>> > Ooops. Thanks for spotting this!
>>
>> Hrm.
>>
>> The task which is released is obviously not current, so even if there
>> are still sigqueues in shared_pending then they wont end up in the
>> released tasks sigqueue_cache. They can only ever end up in
>> current->sigqueue_cache.
>
> The task which is released can be "current" if this task autoreaps.
> For example, if its parent ignores SIGCHLD. In this case exit_notify()
> does release_task(current).

Bah. Let me stare at it moar.