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

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Date:	Thu, 12 Mar 2009 11:07:35 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Darren Hart <dvhltc@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 3/6] futex: add double_unlock_hb()

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.


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