From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754521AbaBQVlh (ORCPT ); Mon, 17 Feb 2014 16:41:37 -0500 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:33238 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753317AbaBQVlL (ORCPT ); Mon, 17 Feb 2014 16:41:11 -0500 X-Originating-IP: 50.43.14.201 Date: Mon, 17 Feb 2014 13:40:56 -0800 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, oleg@redhat.com, sbw@mit.edu Subject: Re: [PATCH tip/core/rcu 2/6] Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherence Message-ID: <20140217214056.GB7941@thin> References: <20140217212625.GA4083@linux.vnet.ibm.com> <1392672413-5114-1-git-send-email-paulmck@linux.vnet.ibm.com> <1392672413-5114-2-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1392672413-5114-2-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 17, 2014 at 01:26:49PM -0800, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > The ACCESS_ONCE() primitive provides cache coherence, but the > documentation does not clearly state this. This commit therefore upgrades > the documentation. > > Signed-off-by: Paul E. McKenney Punctuation nit below; otherwise: Reviewed-by: Josh Triplett > Documentation/memory-barriers.txt | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > index 102dc19c4119..ad6db1d48f1f 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -1249,6 +1249,23 @@ The ACCESS_ONCE() function can prevent any number of optimizations that, > while perfectly safe in single-threaded code, can be fatal in concurrent > code. Here are some examples of these sorts of optimizations: > > + (*) The compiler is within its rights to reorder loads and stores > + to the same variable, and in some cases, the CPU is within its > + rights to reorder loads to the same variable. This means that > + the following code: > + > + a[0] = x; > + a[1] = x; > + > + Might result in an older value of x stored in a[1] than in a[0]. > + Prevent both the compiler and the CPU from doing this as follows: > + > + a[0] = ACCESS_ONCE(x); > + a[1] = ACCESS_ONCE(x); > + > + In short, ACCESS_ONCE() provides "cache coherence" for accesses from > + multiple CPUs to a single variable. You don't need to "quote" the well-established term "cache coherence". > (*) The compiler is within its rights to merge successive loads from > the same variable. Such merging can cause the compiler to "optimize" > the following code: > -- > 1.8.1.5 >