Date:	Thu, 02 May 2013 15:34:56 +0300
From:	Imre Deak <imre.deak@...el.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Daniel Vetter <daniel.vetter@...ll.ch>,
	David Howells <dhowells@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dave Jones <davej@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] wait: fix false timeouts when using wait_event_timeout()

On Thu, 2013-05-02 at 14:23 +0200, Jens Axboe wrote:
> On Thu, May 02 2013, Daniel Vetter wrote:
> > On Thu, May 2, 2013 at 12:29 PM, David Howells <dhowells@...hat.com> wrote:
> > >> Fix this by returning at least 1 if the condition becomes true. This
> > >> semantic is in line with what wait_for_condition_timeout() does; see
> > >> commit bb10ed09 - "sched: fix wait_for_completion_timeout() spurious
> > >> failure under heavy load".
> > >
> > > But now you can't distinguish the timer expiring first, if the thread doing
> > > the waiting gets delayed sufficiently long for the event to happen.
> > 
> > That can already happen, e.g.
> > 
> > 1. wakeup happens and condition is true.
> > 2. we compute remaining jiffies > 0
> > -> preempt
> > 3. now wait_for_event_timeout returns.
> > 
> > Only difference is that the delay/preempt happens in between 1. and
> > 2., and then suddenly the wake up didn't happen in time (with the
> > current return code semantics).
> > 
> > So imo the current behaviour is simply a bug and will miss timely
> > wakeups in some cases.
> > 
> > The other way round, namely wait_for_event_timeout taking longer than
> > the timeout is expected (and part of the interface for every timeout
> > function). So all current callers already need to be able to cope with
> > random preemption/delays pushing the total time before the call to
> > wait_for_event and checking the return value over the timeout, even
> > when condition was signalled in time.
> > 
> > If there's any case which relies on accurate timeout detection that
> > simply won't work with wait_for_event (they need an nmi or a hw
> > timestamp counter or something similar).
> 
> I seriously doubt that anyone is depending on any sort of accuracy on
> the return. 1 jiffy is not going to make or break anything - in fact,
> jiffies could be incremented nsecs after the initial call. So a
> granularity of at least 1 is going to be expected in any case.
> 
> The important bit here is that the API should behave as expected. And
> the most logical way to code that is to check the return value. I can
> easily see people forgetting to re-check the condition, hence you get a
> bug. The fact that you and the original reporter already had accidents
> with this is a clear sign that the logical way to use the API is not the
> correct one.
> 
> IMHO, the change definitely makes sense.

Ok, so taking courage of this answer ;P How about also the following?

diff --git a/kernel/timer.c b/kernel/timer.c
index dbf7a78..5a62456 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1515,7 +1515,11 @@ signed long __sched schedule_timeout(signed long
timeout)
 		}
 	}
 
-	expire = timeout + jiffies;
+	/*
+	 * We can't be sure how close we are to the next tick, so +1 to
+	 * guarantee that we wait at least timeout amount.
+	 */
+	expire = timeout + jiffies + 1;
 
 	setup_timer_on_stack(&timer, process_timeout, (unsigned long)current);
 	__mod_timer(&timer, expire, false, TIMER_NOT_PINNED);


It'd plug a similar hole for wait_event_timeout() and similar users, who
don't compensate for the above..

--Imre


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