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Date: Fri, 22 Nov 2013 12:09:31 -0800
Message-ID: <CA+55aFz0nP1_O8jO2UkX1DmDzcBm53-fFejvz=oY=x3cGNBJSQ@mail.gmail.com>
Subject: Re: [PATCH v6 4/5] MCS Lock: Barrier corrections
From: Linus Torvalds <torvalds@linux-foundation.org>
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To: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>, Tim Chen <tim.c.chen@linux.intel.com>, Will Deacon <will.deacon@arm.com>, Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>, Thomas Gleixner <tglx@linutronix.de>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>, Waiman Long <waiman.long@hp.com>, Andrea Arcangeli <aarcange@redhat.com>, Alex Shi <alex.shi@linaro.org>, Andi Kleen <andi@firstfloor.org>, Michel Lespinasse <walken@google.com>, Davidlohr Bueso <davidlohr.bueso@hp.com>, Matthew R Wilcox <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@intel.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Rik van Riel <riel@redhat.com>, Peter Hurley <peter@hurleysoftware.com>, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, George Spelvin <linux@horizon.com>, "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>, Aswin Chandramouleeswaran <aswin@hp.com>, Scott J Norton <scott.norton@hp.com>, "Figo.zhang" <figo1802@gmail.com>

On Fri, Nov 22, 2013 at 12:06 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>
> I am sorry, but that is not always correct.  For example, in the contended
> case for Tim Chen's MCS queued locks, the x86 acquisition-side handoff
> code does -not- contain any stores or memory-barrier instructions.

So? In order to get *into* that contention code, you will have to go
through the fast-case code. Which will contain a locked instruction.

So I repeat: a "lock" sequence will always be a memory barrier on x86.

                   Linus

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