linux-kernel - Re: [PATCH 3/6] futex: add double_unlock

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Date:	Thu, 12 Mar 2009 08:13:21 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 3/6] futex: add double_unlock_hb()

Thomas Gleixner wrote:
> On Thu, 12 Mar 2009, Ingo Molnar wrote:
>> * Peter Zijlstra <peterz@...radead.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
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