linux-kernel - Re: [PATCH] wait: fix false timeouts when using wait_event

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Date:	Wed, 08 May 2013 12:49:48 +0300
From:	Imre Deak <imre.deak@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Daniel Vetter <daniel.vetter@...ll.ch>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Dave Jones <davej@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	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 Tue, 2013-05-07 at 16:12 -0700, Andrew Morton wrote:
> On Thu, 2 May 2013 11:36:56 +0200 Daniel Vetter <daniel.vetter@...ll.ch> wrote:
> 
> > On Thu, May 2, 2013 at 10:58 AM, Imre Deak <imre.deak@...el.com> wrote:
> > > Many callers of the wait_event_timeout() and
> > > wait_event_interruptible_timeout() expect that the return value will be
> > > positive if the specified condition becomes true before the timeout
> > > elapses. However, at the moment this isn't guaranteed. If the wake-up
> > > handler is delayed enough, the time remaining until timeout will be
> > > calculated as 0 - and passed back as a return value - even if the
> > > condition became true before the timeout has passed.
> > >
> > > 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".
> > >
> > > Signed-off-by: Imre Deak <imre.deak@...el.com>
> > 
> > We have 3 instances of this bug in drm/i915. One case even where we
> > switch between the interruptible and not interruptible
> > wait_event_timeout variants, foolishly presuming they have the same
> > semantics. I very much like this.
> 
> Let's think about scheduling this fix.
> 
> Are any of the bugs which we expect this patch fixes serious enough to
> warrant merging it into 3.10?  And -stable?

There is at least [1], but I'm sure there is more similar reports about
i915. I'd vote for -stable at least.

--Imre

[1] https://bugs.freedesktop.org/show_bug.cgi?id=64133


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