linux-kernel - [PATCH tip/core/rcu 2/3] documentation: Update rcu_dereference.txt based on WG21 discussions

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Message-Id: <1431465787-32541-2-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Tue, 12 May 2015 14:23:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 2/3] documentation: Update rcu_dereference.txt based on WG21 discussions

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

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 <paulmck@...ux.vnet.ibm.com>
---
 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

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