linux-kernel - Re: [patch V4 2/2] signal: Allow tasks to cache one sigqueue struct

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Date:   Tue, 23 Mar 2021 19:04:42 +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 V4 2/2] signal: Allow tasks to cache one sigqueue struct

On 03/22, Thomas Gleixner wrote:
>
> +static void sigqueue_cache_or_free(struct sigqueue *q, bool cache)
> +{
> +	/*
> +	 * Cache one sigqueue per task. This pairs with the consumer side
> +	 * in __sigqueue_alloc() and needs READ/WRITE_ONCE() to prevent the
> +	 * compiler from store tearing and to tell KCSAN that the data race
> +	 * is intentional when run without holding current->sighand->siglock,
> +	 * which is fine as current obviously cannot run __sigqueue_free()
> +	 * concurrently.
> +	 */
> +	if (cache && !READ_ONCE(current->sigqueue_cache))
> +		WRITE_ONCE(current->sigqueue_cache, q);
> +	else
> +		kmem_cache_free(sigqueue_cachep, q);
> +}
> +
> +void exit_task_sigqueue_cache(struct task_struct *tsk)
> +{
> +	/* Race free because @tsk is mopped up */
> +	struct sigqueue *q = tsk->sigqueue_cache;
> +
> +	if (q) {
> +		tsk->sigqueue_cache = NULL;
> +		/* If task is self reaping, don't cache it back */
> +		sigqueue_cache_or_free(q, tsk != current);
                                          ^^^^^^^^^^^^^^
Still not right or I am totally confused.

tsk != current can be true if an exiting (and autoreaping) sub-thread
releases its group leader.

IOW. Suppose a process has 2 threads, its parent ignores SIGCHLD.

The group leader L exits. Then its sub-thread T exits too and calls
release_task(T). In this case the tsk != current is false.

But after that T calls release_task(L) and L != T is true.

I'd suggest to free tsk->sigqueue_cache in __exit_signal() unconditionally and
remove the "bool cache" argument from sigqueue_cache_or_free().

Oleg.