Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1756909AbaJXPZ1 (ORCPT <rfc822;w@1wt.eu>);
	Fri, 24 Oct 2014 11:25:27 -0400
Received: from www.linutronix.de ([62.245.132.108]:48392 "EHLO
	Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1755416AbaJXPZV (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 24 Oct 2014 11:25:21 -0400
Date: Fri, 24 Oct 2014 17:25:16 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
cc: Linus Torvalds <torvalds@linux-foundation.org>,
        "Steinar H. Gunderson" <sgunderson@bigfoot.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: bisected: futex regression >= 3.14 - was - Slowdown due to
 threads bouncing between HT cores
In-Reply-To: <1412791184.5173.11.camel@marge.simpson.net>
Message-ID: <alpine.DEB.2.11.1410241724370.5308@nanos>
References: <20141003194428.GA27084@sesse.net>  <1412782664.5179.75.camel@marge.simpson.net>  <CA+55aFyuH07zzD6dD4k0pdyvJ7fUhFdFrmPYJi9W=FtHihgndA@mail.gmail.com> <1412791184.5173.11.camel@marge.simpson.net>
User-Agent: Alpine 2.11 (DEB 23 2013-08-11)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
X-Linutronix-Spam-Score: -1.0
X-Linutronix-Spam-Level: -
X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required,  ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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@gmail.com> wrote:
> > >
> > > 11d4616bd07f38d496bd489ed8fad1dc4d928823 is the first bad commit
> > > commit 11d4616bd07f38d496bd489ed8fad1dc4d928823
> > > Author: Linus Torvalds <torvalds@linux-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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/