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Date: Wed, 8 Oct 2014 13:04:01 -0400
Message-ID: <CA+55aFyuH07zzD6dD4k0pdyvJ7fUhFdFrmPYJi9W=FtHihgndA@mail.gmail.com>
Subject: Re: bisected: futex regression >= 3.14 - was - Slowdown due to
 threads bouncing between HT cores
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: "Steinar H. Gunderson" <sgunderson@bigfoot.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
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Sender: linux-kernel-owner@vger.kernel.org

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.

               Linus
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