From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753529AbaBQV1J (ORCPT ); Mon, 17 Feb 2014 16:27:09 -0500 Received: from e37.co.us.ibm.com ([32.97.110.158]:41617 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753023AbaBQV1D (ORCPT ); Mon, 17 Feb 2014 16:27:03 -0500 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, 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, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 2/6] Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherence Date: Mon, 17 Feb 2014 13:26:49 -0800 Message-Id: <1392672413-5114-2-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.1.5 In-Reply-To: <1392672413-5114-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20140217212625.GA4083@linux.vnet.ibm.com> <1392672413-5114-1-git-send-email-paulmck@linux.vnet.ibm.com> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14021721-7164-0000-0000-000006266562 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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. + (*) 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