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

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Date:	Thu, 02 May 2013 15:02:26 +0300
From:	Imre Deak <imre.deak@...el.com>
To:	David Howells <dhowells@...hat.com>
Cc:	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()

On Thu, 2013-05-02 at 11:29 +0100, David Howells wrote:
> 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 trying to understand what sequence do you mean. I can think of the
following - as an example - in case of starting a transaction that will
set a completion flag:

waiter                                               completion handler
start transaction
                                                     set completion_flag
ret = wait_event_timeout(timeout, completion_flag)

In this case ret will be timeout which is the original behavior, so
should be ok. One exception is if timeout=0 to begin with, since then -
after this change - ret will be 1. But I can't see how that use case is
useful. I guess I'm missing something, could you elaborate?

--Imre

> 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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