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Date: Sat, 23 Nov 2013 22:29:29 +0100
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v6 4/5] MCS Lock: Barrier corrections
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>, 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>, 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 Sat, Nov 23, 2013 at 12:21:13PM -0800, Linus Torvalds wrote:
> *SOME* assumption of mine must be wrong. But I don't see which one.

I haven't read your email in full detail yet, but one thing I did miss
was cache-snoops.

One of the reasons for failing transitive / multi-copy atomicity is that
CPUs might have different views of the memory state depending on from
which caches they can get snoops.

Eg. if CPU0 and CPU1 share a cache level but CPU2 does not, CPU1 might
observe a write before CPU2 can.

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