doc: Make reader aware of rcu_dereference_protected

The whatisRCU.txt document says rcu_dereference() cannot be used
outside of rcu_read_lock() protected sections.  The commit adds a
mention of rcu_dereference_protected(), so that the new reader knows
that this API can be used to avoid update-side use of rcu_read_lock()
and rcu_read_unlock().

Cc: tytso@mit.edu
Suggested-by: tytso@mit.edu
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Update wording, including further feedback from Joel. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

1 files changed, 14 insertions, 1 deletions

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 7c33445fd0e5..4a6854318b17 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -266,7 +266,7 @@ rcu_dereference()
         unnecessary overhead on Alpha CPUs.
 
         Note that the value returned by rcu_dereference() is valid
-        only within the enclosing RCU read-side critical section.
+        only within the enclosing RCU read-side critical section [1].
         For example, the following is -not- legal:
 
                 rcu_read_lock();
@@ -292,6 +292,19 @@ rcu_dereference()
         typically used indirectly, via the _rcu list-manipulation
         primitives, such as list_for_each_entry_rcu().
 
+        [1] The variant rcu_dereference_protected() can be used outside
+        of an RCU read-side critical section as long as the usage is
+        protected by locks acquired by the update-side code.  This variant
+        avoids the lockdep warning that would happen when using (for
+        example) rcu_dereference() without rcu_read_lock() protection.
+        Using rcu_dereference_protected() also has the advantage
+        of permitting compiler optimizations that rcu_dereference()
+        must prohibit.  The rcu_dereference_protected() variant takes
+        a lockdep expression to indicate which locks must be acquired
+        by the caller. If the indicated protection is not provided,
+        a lockdep splat is emitted.  See RCU/Design/Requirements.html
+        and the API's code comments for more details and example usage.
+
 The following diagram shows how each API communicates among the
 reader, updater, and reclaimer.