Re: [PATCH 2/6] sched: Simplify migration_cpu_stop()

[Valentin Schneider <valentin.schneider@arm.com>]

On 24/02/21 13:24, Peter Zijlstra wrote:
> @@ -1950,31 +1931,20 @@ static int migration_cpu_stop(void *data
>                       goto out;
>
>               if (pending) {
> -			p->migration_pending = NULL;
> +			if (p->migration_pending == pending)
> +				p->migration_pending = NULL;
>                       complete = true;
>               }
>
> -		/* migrate_enable() --  we must not race against SCA */
> -		if (dest_cpu < 0) {
> -			/*
> -			 * When this was migrate_enable() but we no longer
> -			 * have a @pending, a concurrent SCA 'fixed' things
> -			 * and we should be valid again. Nothing to do.
> -			 */
> -			if (!pending) {
> -				WARN_ON_ONCE(!cpumask_test_cpu(task_cpu(p), &p->cpus_mask));
> -				goto out;
> -			}
> -

This is fixed by 5+6, but at this patch I think you can have double
completions - I thought this was an issue, but briefly looking at
completion stuff it might not. In any case, consider:

  task_cpu(p) == Y

  SCA(p, X);
                 SCA(p, Y);


SCA(p, Y) will uninstall SCA(p, X)'s pending and complete.

migration/Y kicked by SCA(p, X) will grab arg->pending, which is still
SCA(p, X)'s pending and also complete.

> +		if (dest_cpu < 0)
>                       dest_cpu = cpumask_any_distribute(&p->cpus_mask);
> -		}
>
>               if (task_on_rq_queued(p))
>                       rq = __migrate_task(rq, &rf, p, dest_cpu);
>               else
>                       p->wake_cpu = dest_cpu;
>
> -	} else if (dest_cpu < 0 || pending) {
> +	} else if (pending) {
>               /*
>                * This happens when we get migrated between migrate_enable()'s
>                * preempt_enable() and scheduling the stopper task. At that