Re: [PATCH] wait: fix false timeouts when using wait_event_timeout()

[Imre Deak]

On Thu, 2013-05-02 at 14:54 +0200, Jens Axboe wrote:
> On Thu, May 02 2013, Imre Deak wrote:
> > 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@xxxxxxxxxx> 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..
> 
> Any jiffy based API is going to have this issue. I think it's different
> from the original patch, which just makes the API potentially return
> something that is confusing.

Yea, at least those that take a relative time. Usually the timeout is
given with a big overhead, so there it won't be a problem. But I also
found users like drivers/acpi/ec.c, that will pass a timeout of 1
jiffies, which may then result in premature timeouts.

> So not sure on the above, sorry.

Ok. A WARN to wait_event_timeout for the above case could still be a
useful guard..

--Imre

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