documentation: Update rcu_dereference.txt based on WG21 discussions

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@linux.vnet.ibm.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt
index ceb05da..2d05c92 100644 (file)
--- 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