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

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Date:	Thu, 12 Mar 2009 11:10:28 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Darren Hart <dvhltc@...ibm.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 3/6] futex: add double_unlock_hb()


* 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.

The patch is still nice (as you mention), it factors out the 
unlock sequence. I'll change the commit message accordingy.

	Ingo
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