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

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Date:   Fri, 12 Mar 2021 12:35:40 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        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 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()?

>  	} else {
>  		print_dropped_signal(sig);
>  	}
> @@ -472,12 +481,19 @@ void flush_sigqueue(struct sigpending *q
>  }
>  
>  /*
> - * Called from __exit_signal. Flush tsk->pending and clear tsk->sighand.
> + * Called from __exit_signal. Flush tsk->pending, clear tsk->sighand and
> + * free tsk->sigqueue_cache.
>   */
>  void exit_task_sighand(struct task_struct *tsk)
>  {
> +	struct sigqueue *q;
> +
>  	flush_sigqueue(&tsk->pending);
>  	tsk->sighand = NULL;
> +
> +	q = xchg(&tsk->sigqueue_cache, NULL);
> +	if (q)
> +		kmem_cache_free(sigqueue_cachep, q);

Do we need this xchg() here? Only the task itself adds something here
and the task is on its way out so it should not add an entry to the
cache.

>  }
>  
>  /*

Sebastian