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

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Date:	Fri, 24 Dec 2010 15:17:18 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Hillf Danton <dhillf@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix unsafe operation in high resolution timer

On Thu, Dec 23, 2010 at 9:29 PM, Hillf Danton <dhillf@...il.com> 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@...il.com>
> ---
>
> --- a/kernel/hrtimer.c  2010-11-01 19:54:12.000000000 +0800
> +++ b/kernel/hrtimer.c  2010-12-23 21:17:02.000000000 +0800
> @@ -1225,6 +1225,7 @@ static void __run_hrtimer(struct hrtimer
>        raw_spin_unlock(&cpu_base->lock);
>        trace_hrtimer_expire_entry(timer, now);
>        restart = fn(timer);
> +       if (restart != HRTIMER_NORESTART)
>        trace_hrtimer_expire_exit(timer);
>        raw_spin_lock(&cpu_base->lock);
>
> @@ -1236,11 +1237,8 @@ static void __run_hrtimer(struct hrtimer
>        if (restart != HRTIMER_NORESTART) {
>                BUG_ON(timer->state != HRTIMER_STATE_CALLBACK);
>                enqueue_hrtimer(timer, base);
> +               timer->state &= ~HRTIMER_STATE_CALLBACK;
>        }
> -
> -       WARN_ON_ONCE(!(timer->state & HRTIMER_STATE_CALLBACK));
> -
> -       timer->state &= ~HRTIMER_STATE_CALLBACK;
>  }
>
>  #ifdef CONFIG_HIGH_RES_TIMERS
> --
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