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

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Date:   Wed, 10 Mar 2021 09:57:30 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 Tue, Mar 09 2021 at 13:01, Thomas Gleixner wrote:
> On Fri, Mar 05 2021 at 11:57, Oleg Nesterov wrote:
>> On 03/04, Thomas Gleixner wrote:
>>> On Wed, Mar 03 2021 at 16:37, Oleg Nesterov wrote:
>>> >> +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()
>>
>> In the last case "fromslab" is true, sigqueue_from_cache() won't be called.
>>
>>> and on the free side we have a bunch of callers which do not hold
>>> sighand::siglock either.
>>
>> Where?

Bah. I confused myself. Let me start over with that.

Thanks,

        tglx