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References: <20200921163557.234036895@infradead.org> <20200921163845.830487105@infradead.org> <jhj3637lzdm.mognet@arm.com> <20200925084339.GU2628@hirez.programming.kicks-ass.net>
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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, mingo@kernel.org, linux-kernel@vger.kernel.org,
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        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vincent.donnefort@arm.com
Subject: Re: [PATCH 8/9] sched: Fix migrate_disable() vs set_cpus_allowed_ptr()
In-reply-to: <20200925084339.GU2628@hirez.programming.kicks-ass.net>
Date:   Fri, 25 Sep 2020 11:07:47 +0100
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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.