[dm-devel] [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 at infradead.org>

Reviewed-by: Davidlohr Bueso <dbueso at suse.de>