linux-kernel - Re: wait_for_completion_timeout() spurious failure under heavy load?

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Message-ID: <20080620152124.GC17373@elte.hu>
Date:	Fri, 20 Jun 2008 17:21:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Jiri Slaby <jirislaby@...il.com>,
	Roland Dreier <rdreier@...co.com>,
	linux-kernel@...r.kernel.org, Eli Cohen <eli@....mellanox.co.il>,
	general@...ts.openfabrics.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: wait_for_completion_timeout() spurious failure under heavy
	load?


* Oleg Nesterov <oleg@...sign.ru> wrote:

> > 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.

nice, thanks - applied it in the form below to tip/sched/urgent.

	Ingo

------------------>
commit 6b8464474776dccf619283ee5510b0b795382dfb
Author: Oleg Nesterov <oleg@...sign.ru>
Date:   Fri Jun 20 18:32:20 2008 +0400

    sched: refactor wait_for_completion_timeout()
    
    Simplify the code and fix the boundary condition of
    wait_for_completion_timeout(,0).
    
    We can kill the first __remove_wait_queue() as well.
    
    Signed-off-by: Ingo Molnar <mingo@...e.hu>

diff --git a/kernel/sched.c b/kernel/sched.c
index 577f160..bebf978 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4398,32 +4398,20 @@ do_wait_for_common(struct completion *x, long timeout, int state)
 			     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 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;
-			}
-		} 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
--
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