From mboxrd@z Thu Jan  1 00:00:00 1970
From: Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v6 4/5] MCS Lock: Barrier corrections
Date: Fri, 29 Nov 2013 18:18:17 +0000
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>, "Figo. zhang" <figo1802@gmail.com>, Aswin Chandramouleeswaran <aswin@hp.com>, Rik van Riel <riel@redhat.com>, Waiman Long <waiman.long@hp.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>, Andi Kleen <andi@firstfloor.org>, George Spelvin <linux@horizon.com>, Tim Chen <tim.c.chen@linux.intel.com>, Michel Lespinasse <walken@google.com>, Ingo Molnar <mingo@elte.hu>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Peter Hurley <peter@hurleysoftware.com>, "H. Peter Anvin" <hpa@zytor.com>, Andrew Morton <akpm@linux-foundation.org>, linux-mm <linux-mm@kvack.org>, Alex Shi <alex.shi@linaro.org>, Andrea Arcangeli <aarcange@redhat.com>, Scott J Norton <scott.norton@hp.com>, Thomas Gleixner <tglx@>
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On Fri, Nov 29, 2013 at 04:44:41PM +0000, Linus Torvalds wrote:
> 
> On Nov 29, 2013 8:18 AM, "Will Deacon"
> <will.deacon@arm.com<mailto:will.deacon@arm.com>> wrote:
> >
> >  To get some sort of idea, I tried adding a dmb to the start of
> >  spin_unlock on ARMv7 and I saw a 3% performance hit in hackbench on my
> >  dual-cluster board.
> 
> Don't do a dmb. Just do a dummy release. You just said that on arm64 a
> unlock+lock is a memory barrier, so just make the mb__before_spinlock() be
> a dummy store with release to the stack..

Good idea! That should work quite nicely (I don't have anything sane I can
benchmark it on), so I think that solves the issue I was moaning about.

Will

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