linux-kernel - Re: Unlock-lock questions and the Linux Kernel Memory Model

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Message-Id: <20171130163909.GQ3624@linux.vnet.ibm.com>
Date:   Thu, 30 Nov 2017 08:39:09 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Daniel Lustig <dlustig@...dia.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Jade Alglave <j.alglave@....ac.uk>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Unlock-lock questions and the Linux Kernel Memory Model

On Thu, Nov 30, 2017 at 05:25:01PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 30, 2017 at 08:14:01AM -0800, Paul E. McKenney wrote:
> > > (Also, technically speaking, the litmus test doesn't have any release 
> > > operations, so no release sequence...)
> > 
> > True!  But if you translated it into C11, you would probably turn the
> > smp_wmb() followed by write into a store release, which would get you
> > a release sequence.
> 
> 	smp_wmb()
> 	WRITE_ONCE(*y, 1);
> 
> does not a RELEASE make.

Agreed, but it also does not C11 make.  There is no pure write barrier
in C11.

							Thanx, Paul