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Date: Tue, 26 Nov 2013 12:56:28 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Davidlohr Bueso <davidlohr@hp.com>, Thomas Gleixner <tglx@linutronix.de>,
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        Waiman Long <Waiman.Long@hp.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket
 plist in futex_wake()
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On Tue, Nov 26, 2013 at 12:21:40PM +0100, Ingo Molnar wrote:
> I'm somewhat reluctant to chalk it up to a single mfence - maybe 
> timings/behavior changed in some substantial way?

Ah indeed! We also changed the case where an enqueueing futex sees the
uval change and bails. It is now far more expensive due to having to
both queue and unqueue, whereas before it wouldn't queue at all.

I suppose the idea was to offset that by not requiring locking on the
wake side.