linux-kernel - Re: [PATCH 8/9] sched: Fix migrate_disable() vs set_cpus_allowed

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Date:   Fri, 25 Sep 2020 11:07:47 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...nel.org, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, qais.yousef@....com, swood@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vincent.donnefort@....com
Subject: Re: [PATCH 8/9] sched: Fix migrate_disable() vs set_cpus_allowed_ptr()


On 25/09/20 09:43, Peter Zijlstra wrote:
> On Thu, Sep 24, 2020 at 08:59:33PM +0100, Valentin Schneider wrote:
>
>> > +	if (task_running(rq, p) || p->state == TASK_WAKING) {
>> > +
>> > +		task_rq_unlock(rq, p, rf);
>> > +		stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
>> > +
>>
>> Shouldn't we check for is_migrate_disabled(p) before doing any of that?
>> migration_cpu_stop() does check for it, is there something that prevents us
>> from acting on it earlier than that?
>
> Since migrate_disable() / ->migration_disabled is only touched from the
> current task, you can only reliably read it from the same CPU.
>
> Hence I only look at it when the stop task has pinned the task, because
> at that point I know it's stable.
>
> Doing it earlier gives races, races give me head-aches. This is a slow
> path, I don't care about performance.

Makes sense, thanks.