Re: [PATCH 8/9] sched: Fix migrate_disable() vs set_cpus_allowed_ptr()

[Valentin Schneider]

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.