Re: [PATCH] fix unsafe operation in high resolution timer

[Yong Zhang]

On Fri, Dec 24, 2010 at 10:28:52PM +0800, Hillf Danton wrote:
> On Fri, Dec 24, 2010 at 3:17 PM, Yong Zhang <yong.zhang0@xxxxxxxxx> wrote:
> > On Thu, Dec 23, 2010 at 9:29 PM, Hillf Danton <dhillf@xxxxxxxxx> wrote:
> >> After calling the callback function of hrtimer, the timer could become
> >> unreliable in corner cases where the timer will no longer be queued
> >> and the mm segment, in which the timer is embedded, could be reclaimed
> >> in the callback.
> >>
> >> The unreliability is fixed by checking the result of callback before
> >> operating the timer again.
> >
> > Though the patch is buggy. But it actually explores a real problem.
> >
> 
> Would you please finger out why the patch is buggy?

No problem.

Actually the patch change the behavior of current hrtimer.
See comments below :)

> >> ---
> >>
> >> --- a/kernel/hrtimer.c Â2010-11-01 19:54:12.000000000 +0800
> >> +++ b/kernel/hrtimer.c Â2010-12-23 21:17:02.000000000 +0800
> >> @@ -1225,6 +1225,7 @@ static void __run_hrtimer(struct hrtimer
> >> Â Â Â Âraw_spin_unlock(&cpu_base->lock);
> >> Â Â Â Âtrace_hrtimer_expire_entry(timer, now);
> >> Â Â Â Ârestart = fn(timer);
> >> + Â Â Â if (restart != HRTIMER_NORESTART)
> >> Â Â Â Âtrace_hrtimer_expire_exit(timer);
> >> Â Â Â Âraw_spin_lock(&cpu_base->lock);
> >>
> >> @@ -1236,11 +1237,8 @@ static void __run_hrtimer(struct hrtimer
> >> Â Â Â Âif (restart != HRTIMER_NORESTART) {
> >> Â Â Â Â Â Â Â ÂBUG_ON(timer->state != HRTIMER_STATE_CALLBACK);
> >> Â Â Â Â Â Â Â Âenqueue_hrtimer(timer, base);
> >> + Â Â Â Â Â Â Â timer->state &= ~HRTIMER_STATE_CALLBACK;

HRTIMER_STATE_CALLBACK is only cleared for RESTART hrtimer with
your modification.

> >> Â Â Â Â}
> >> -
> >> - Â Â Â WARN_ON_ONCE(!(timer->state & HRTIMER_STATE_CALLBACK));
> >> -
> >> - Â Â Â timer->state &= ~HRTIMER_STATE_CALLBACK;

But for a hrtimer which is not free in its callback, like a
static defined one. the hrtimer could be referenced at the same
time. So here you cann't just delete the two lines.

Thanks,
Yong
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