From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933923AbbELVXY (ORCPT ); Tue, 12 May 2015 17:23:24 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:44825 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933761AbbELVXQ (ORCPT ); Tue, 12 May 2015 17:23:16 -0400 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, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 2/3] documentation: Update rcu_dereference.txt based on WG21 discussions Date: Tue, 12 May 2015 14:23:06 -0700 Message-Id: <1431465787-32541-2-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.1.5 In-Reply-To: <1431465787-32541-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20150512212234.GA32351@linux.vnet.ibm.com> <1431465787-32541-1-git-send-email-paulmck@linux.vnet.ibm.com> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15051221-0009-0000-0000-00000AC67288 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" This commit provides another caveat for the care and feeding of pointers returned by rcu_dereference() that was pointed out in discussions within the C++ standards committee. Signed-off-by: Paul E. McKenney --- Documentation/RCU/rcu_dereference.txt | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt index ceb05da5a5ac..2d05c9241a33 100644 --- a/Documentation/RCU/rcu_dereference.txt +++ b/Documentation/RCU/rcu_dereference.txt @@ -193,6 +193,11 @@ o Be very careful about comparing pointers obtained from pointer. Note that the volatile cast in rcu_dereference() will normally prevent the compiler from knowing too much. + However, please note that if the compiler knows that the + pointer takes on only one of two values, a not-equal + comparison will provide exactly the information that the + compiler needs to deduce the value of the pointer. + o Disable any value-speculation optimizations that your compiler might provide, especially if you are making use of feedback-based optimizations that take data collected from prior runs. Such -- 1.8.1.5