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

[Yong Zhang]

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.

We can't deference a maybe freed object. I'm not sure how
to fix it. Or we just prevent the hrtimer->fn free the object which
contains hrtimer?

Thomas, what's your comment?

Thanks,
Yong
>
> Signed-off-by: Hillf Danton <dhillf@xxxxxxxxx>
> ---
>
> --- 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;
> Â Â Â Â}
> -
> - Â Â Â WARN_ON_ONCE(!(timer->state & HRTIMER_STATE_CALLBACK));
> -
> - Â Â Â timer->state &= ~HRTIMER_STATE_CALLBACK;
> Â}
>
> Â#ifdef CONFIG_HIGH_RES_TIMERS
> --
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