Re: [PATCH 1/6] sched: Unbreak wakeups

[Davidlohr Bueso]

On Wed, 02 Jun 2021, Peter Zijlstra wrote:

> Remove broken task->state references and let wake_up_process() DTRT.
>
> The anti-pattern in these patches breaks the ordering of ->state vs
> COND as described in the comment near set_current_state() and can lead
> to missed wakeups:
>
> 	(OoO load, observes RUNNING)<-.
> 	for (;;) {                    |
> 	  t->state = UNINTERRUPTIBLE; |
> 	  smp_mb();          ,-----> ,' (OoO load, observed !COND)
>                             |       |
> 	                     |       |	COND = 1;
> 			     |	     `- if (t->state != RUNNING)
>                             |		  wake_up_process(t); // not done
> 	  if (COND) ---------'
> 	    break;
> 	  schedule(); // forever waiting
> 	}
> 	t->state = TASK_RUNNING;
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Reviewed-by: Davidlohr Bueso <dbueso@xxxxxxx>