Re: [PATCH 3/6] futex: add double_unlock_hb()

Darren Hart
2009-03-12T15:13:35Z

Thomas Gleixner wrote:
> On Thu, 12 Mar 2009, Ingo Molnar wrote:
>> * Peter Zijlstra <peterz@infradead.org> wrote:
>>
>>> On Thu, 2009-03-12 at 00:55 -0700, Darren Hart wrote:
>>>> The futex code uses double_lock_hb() which locks the hb->lock's in pointer
>>>> value order. There is no parallel unlock routine, and the code unlocks them
>>>> in name order, ignoring pointer value. This opens up a window for an ABBA
>>>> deadlock. This patch adds double_unlock_hb() to remove the window as well
>>>> as refactor the duplicated code segments.
>>> While I don't mind the patch per-se, I'm hard pressed to see
>>> any deadlock potential in the unordered unlock.
>>>
>>> All sites (at least those in the patch) always release both
>>> locks without taking another in between, therefore one would
>>> think there's no deadlock possible.
>> yeah.
>
> I can't see a deadlock either.
>

Right, sorry, it's the double_lock that requires the test. Duh. I need
to find a way to do some of this work during more regular hours I guess
;-) Thanks for the catch everyone.

Ingo shall I resubmit? Or did you already clean it up?

Thanks,

Darren

>> The patch is still nice (as you mention), it factors out the
>> unlock sequence. I'll change the commit message accordingy.
>
> We do not need the comparison magic. Can we just put the code into
> double_unlock_hb() which gets replaced ?
>
> i.e:
>
> spin_unlock(&hb1->lock);
> if (hb1 != hb2)
> spin_unlock(&hb2->lock);
>
> This code is confusing enough.
>
> Thanks,
>
> tglx

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team