Date: Thu, 12 Mar 2009 11:10:28 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <dvhltc@us.ibm.com>, linux-kernel@vger.kernel.org,
       Thomas Gleixner <tglx@linutronix.de>,
       Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH 3/6] futex: add double_unlock_hb()
Message-ID: <20090312101028.GA7157@elte.hu>
References: <20090312075349.9856.83687.stgit@Aeon> <20090312075552.9856.48021.stgit@Aeon> <1236852455.5090.105.camel@laptop>
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* 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.

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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