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

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Message-ID: <AANLkTimc0FwBhj5MsnibTTq94AgaQgqCKEXNSOvM_XjE@mail.gmail.com>
Date:	Fri, 24 Dec 2010 22:28:52 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix unsafe operation in high resolution timer

On Fri, Dec 24, 2010 at 3:17 PM, Yong Zhang <yong.zhang0@...il.com> wrote:
> 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.
>

Would you please finger out why the patch is buggy?

Hillf

> 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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>
> --
> Only stand for myself.
>
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