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

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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>



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