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

[Valentin Schneider]

On 25/02/21 09:45, Peter Zijlstra wrote:
> On Wed, Feb 24, 2021 at 03:34:36PM +0000, Valentin Schneider wrote:
>> 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.
>
> Right, so I didn't really think too hard about the intermediate states,
> given it's all pretty buggered until at least 5. But yeah, double
> complete is harmless.
>
> Specifically, the refcount the stopper has should avoid the stack from
> getting released.

Aye that should be fine, it really was just the double complete which I
was unsure about.