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Date: Mon, 25 Nov 2013 13:09:02 +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 12:39:53PM -0800, Linus Torvalds wrote:
> On Sat, Nov 23, 2013 at 12:21 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > And as far as I can tell, the above gives you: A < B < C < D < E < F <
> > A. Which doesn't look possible.
> 
> Hmm.. I guess technically all of those cases aren't "strictly
> precedes" as much as "cannot have happened in the opposite order". So
> the "<" might be "<=". Which I guess *is* possible: "it all happened
> at the same time". And then the difference between your suggested
> "lwsync" and "sync" in the unlock path on CPU0 basically approximating
> the difference between "A <= B" and "A < B"..
> 
> Ho humm.

But remember, there's an actual full proper barrier between E and F, so
at best you'd end up with something like:

  A <= B <= C <= D <= E < F <= A

Which is still an impossibility.

I'm hoping others will explain things, as I'm very much on shaky ground
myself wrt transitivity.

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