Messages in this thread | | | Date | Tue, 2 Sep 2003 16:54:24 +0900 | From | Tejun Huh <> | Subject | Re: [PATCH] Race condition in del_timer_sync (2.5) |
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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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