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

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Message-ID: <15077.1367490569@warthog.procyon.org.uk>
Date:	Thu, 02 May 2013 11:29:29 +0100
From:	David Howells <dhowells@...hat.com>
To:	Imre Deak <imre.deak@...el.com>
Cc:	dhowells@...hat.com, Daniel Vetter <daniel.vetter@...ll.ch>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dave Jones <davej@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Lukas Czerner <lczerner@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] wait: fix false timeouts when using wait_event_timeout()

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.

Fun.

> 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.

I'm not sure there's a good answer - except maybe making the timer expiry
handler check the condition (which would likely get really yucky really
quickly).

David
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