linux-kernel - Re: [PATCH] signal: Allow RT tasks to cache one sigqueue struct

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Date:   Thu, 04 Mar 2021 22:10:03 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Oleg Nesterov <oleg@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        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>
Subject: Re: [PATCH] signal: Allow RT tasks to cache one sigqueue struct

On Wed, Mar 03 2021 at 16:37, Oleg Nesterov wrote:
> On 03/03, Sebastian Andrzej Siewior wrote:
>>
>> +static struct sigqueue *sigqueue_from_cache(struct task_struct *t)
>> +{
>> +	struct sigqueue *q = t->sigqueue_cache;
>> +
>> +	if (q && cmpxchg(&t->sigqueue_cache, q, NULL) == q)
>> +		return q;
>> +	return NULL;
>> +}
>> +
>> +static bool sigqueue_add_cache(struct task_struct *t, struct sigqueue *q)
>> +{
>> +	if (!t->sigqueue_cache && cmpxchg(&t->sigqueue_cache, NULL, q) == NULL)
>> +		return true;
>> +	return false;
>> +}
>
> Do we really need cmpxchg? It seems they are always called with
> spinlock held.

With which spinlock held?

__send_signal()         <- sighand::siglock held
  __sigqueue_alloc()

alloc_posix_timer()
  sigqueue_alloc()      <- No lock held
    __sigqueue_alloc()

and on the free side we have a bunch of callers which do not hold
sighand::siglock either. So the cmpxchg() is required.

Thanks,

        tglx