Re: wait_for_completion_timeout() spurious failure under heavy load?

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

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().

Oleg.

--- kernel/sched.c
+++ kernel/sched.c
@@ -4746,15 +4746,13 @@ do_wait_for_common(struct completion *x,
 			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 0;
 	}
 	x->done--;
-	return timeout;
+	return timeout ?: 1;
 }
 
 static long __sched