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Date: Mon, 25 Nov 2013 19:27:15 +0100
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v6 4/5] MCS Lock: Barrier corrections
Message-ID: <20131125182715.GG10022@twins.programming.kicks-ass.net>
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To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Will Deacon <will.deacon@arm.com>, Tim Chen <tim.c.chen@linux.intel.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>, Linus Torvalds <torvalds@linux-foundation.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 Mon, Nov 25, 2013 at 10:02:50AM -0800, Paul E. McKenney wrote:
> And if the two locks are different, then the guarantee applies only
> when the unlock and lock are on the same CPU, in which case, as Linus
> noted, the xchg() on entry to the slow path does the job for use.

But in that case we rely on the fact that the thing is part of a
composite and we should no longer call it load_acquire, because frankly
it doesn't have acquire semantics anymore because the read can escape
out.

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