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Date: Wed, 27 Nov 2013 17:58:40 +0100
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v6 4/5] MCS Lock: Barrier corrections
Message-ID: <20131127165840.GA26138@redhat.com>
References: <20131122155835.GR3866@twins.programming.kicks-ass.net> <20131122182632.GW4138@linux.vnet.ibm.com> <20131122185107.GJ4971@laptop.programming.kicks-ass.net> <20131125173540.GK3694@twins.programming.kicks-ass.net> <20131125180250.GR4138@linux.vnet.ibm.com> <20131125182715.GG10022@twins.programming.kicks-ass.net> <20131125235252.GA4138@linux.vnet.ibm.com> <20131126095945.GI10022@twins.programming.kicks-ass.net> <CA+55aFxXEbHuaKuxBDH=7a2-n_z849CdfeDtdL=_nFxu_Tx9_g@mail.gmail.com> <20131126192133.GF789@laptop.programming.kicks-ass.net>
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, 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>, 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 11/26, Peter Zijlstra wrote:
>
> On Tue, Nov 26, 2013 at 11:00:50AM -0800, Linus Torvalds wrote:
>
> > IOW, where do we really care about the "unlock+lock" is a memory
> > barrier? And could we make those places explicit, and then do
> > something similar to the above to them?
>
> So I don't know :-(
>
> I do know myself and Oleg have often talked about it, and I'm fairly
> sure we must have used it at some point.

No... I can't recall any particular place which explicitely relies
on "unlock+lock => mb().

(although I know the out-of-tree example which can be ignored ;)

I can only recall that this was mentioned in the context like
"no, the lack of mb() can't explain the problem because we have
unlock+lock".

Oleg.

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