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

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Message-ID: <20210312161148.GA25946@redhat.com>
Date:   Fri, 12 Mar 2021 17:11:51 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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().

Oleg.