Re: wait_for_completion_timeout() spurious failure under heavy load?

[Oleg Nesterov <oleg@tv-sign.ru>]

On 06/20, Oleg Nesterov wrote:
>
> On 06/20, Ingo Molnar wrote:
> >
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -4405,6 +4405,16 @@ do_wait_for_common(struct completion *x, long timeout, int state)
> >  			spin_unlock_irq(&x->wait.lock);
> >  			timeout = schedule_timeout(timeout);
> >  			spin_lock_irq(&x->wait.lock);
> > +
> > +			/*
> > +			 * If the completion has arrived meanwhile
> > +			 * then return 1 jiffy time left:
> > +			 */
> > +			if (x->done && !timeout) {
> > +				timeout = 1;
> > +				break;
> > +			}
> > +
> >  			if (!timeout) {
> >  				__remove_wait_queue(&x->wait, &wait);
> >  				return timeout;
> 
> This is the real nitpick, but I wonder what is the right behaviour
> of wait_for_completion_timeout(x, 0) when x->done != 0. Perhaps we
> can return 1 in that case too, just for the consistency?
> 
> IOW, how about the patch below? this also makes the code a bit
> simpler because we factor out __remove_wait_queue().

Even better, we can kill the first __remove_wait_queue() as well.

Oleg.

--- kernel/sched.c
+++ kernel/sched.c
@@ -4739,22 +4739,20 @@ do_wait_for_common(struct completion *x,
 			     signal_pending(current)) ||
 			    (state == TASK_KILLABLE &&
 			     fatal_signal_pending(current))) {
-				__remove_wait_queue(&x->wait, &wait);
-				return -ERESTARTSYS;
+				timeout = -ERESTARTSYS;
+				break;
 			}
 			__set_current_state(state);
 			spin_unlock_irq(&x->wait.lock);
 			timeout = schedule_timeout(timeout);
 			spin_lock_irq(&x->wait.lock);
-			if (!timeout) {
-				__remove_wait_queue(&x->wait, &wait);
-				return timeout;
-			}
-		} while (!x->done);
+		} while (!x->done && timeout);
 		__remove_wait_queue(&x->wait, &wait);
+		if (!x->done)
+			return timeout;
 	}
 	x->done--;
-	return timeout;
+	return timeout ?: 1;
 }
 
 static long __sched