linux-kernel - Re: bisected: futex regression >= 3.14 - was - Slowdown due to threads bouncing between HT cores

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Date:	Fri, 24 Oct 2014 17:25:16 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Steinar H. Gunderson" <sgunderson@...foot.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: bisected: futex regression >= 3.14 - was - Slowdown due to
 threads bouncing between HT cores

On Wed, 8 Oct 2014, Mike Galbraith wrote:
> On Wed, 2014-10-08 at 13:04 -0400, Linus Torvalds wrote: 
> > On Wed, Oct 8, 2014 at 11:37 AM, Mike Galbraith
> > <umgwanakikbuti@...il.com> wrote:
> > >
> > > 11d4616bd07f38d496bd489ed8fad1dc4d928823 is the first bad commit
> > > commit 11d4616bd07f38d496bd489ed8fad1dc4d928823
> > > Author: Linus Torvalds <torvalds@...ux-foundation.org>
> > > Date:   Thu Mar 20 22:11:17 2014 -0700
> > >
> > >     futex: revert back to the explicit waiter counting code
> > 
> > While that revert might make things a tiny bit slower (I hated doing
> > it, but the clever approach sadly didn't work on powerpc and depended
> > on x86 locking semantics), I seriously doubt it's really relevant.
> > It's more likely that the *real* problem itself is very
> > timing-dependent, and the subtle synchronization changes here then
> > expose it or hide it, rather than really fixing anything.
> > 
> > So like Thomas, I would suspect a race condition in the futex use, and
> > then the exact futex implementation details are just exposing it
> > incidentally.
> 
> Whew, good, futex.c is hard.  Heads up chess guys <punt>. 

I wonder whether the barrier fix which got into 3.17 late fixes that
issue as well.

Thanks,

	tglx
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