Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucketplist in futex_wake()

[Davidlohr Bueso]

On Tue, 2013-11-26 at 12:56 +0100, Peter Zijlstra wrote:
> 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?

That would be nthread mfence calls.

> 
> 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.

Right, but those particular numbers do not measure that path.

Thanks,
Davidlohr

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