Re: [PATCH] Race condition in del_timer_sync (2.5)

[Tejun Huh]

On Tue, Sep 02, 2003 at 08:33:50AM +0200, Ingo Molnar wrote:
> 
> On Tue, 2 Sep 2003, Tejun Huh wrote:
> 
> >  This patch fixes a race between del_timer_sync and recursive timers.
> > Current implementation allows the value of timer->base that is used for
> > timer_pending test to be fetched before finishing running_timer test, so
> > it's possible for a recursive time to be pending after del_timer_sync.  
> > Adding smp_rmb before timer_pending removes the race.
> 
> good catch. Have you ever trigger this bug, or did you find it via code
> review?

 I found it via code review.  Actually, I haven't succeeded to boot
even once with 2.5 on my test box.  It prints a lot of messages about
scheduling while atomic and eventually panics.  I'm trying to identify
what's causing the problem.  Wish me luck. :-)

> just to explore the scope of this problem a bit more: at first glance all
> other timer_pending() uses seem to be safe. del_timer_sync()'s
> timer_pending() use is special, because it's next to the ->running_timer
> check without any barriers inbetween - so we could indeed in theory end up
> with having the two reads reordered and a freshly added timer (on another
> CPU) not being recognized properly. Also, this is the only timer API call
> that guarantees the complete stopping of a timer.
> 
> 	Ingo

 Yeap, in other cases, when timer_pending() fails to see timer->base,
it can be thought as the test took place before the timer is added,
and leaving pending timer behind is semantically ok.  But because a
recusive timer without some other entity trying to add it should
actually be removed by timer_del_sync(), timer_pending() test should
be barriered from running_timer test.

 I'll submit the patch to Linus soon.

-- 
tejun

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