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

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Message-ID: <CAKMK7uEL0kbS1j=gEu_GzUevseeKQ6JuZWPptr96JumCYurfnQ@mail.gmail.com>
Date:	Thu, 2 May 2013 11:36:56 +0200
From:	Daniel Vetter <daniel.vetter@...ll.ch>
To:	Imre Deak <imre.deak@...el.com>
Cc:	"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 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.

Acked-by:  Daniel Vetter <daniel.vetter@...ll.ch>
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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