[PATCH tip/core/rcu 15/20] doc: Make reader aware of rcu_dereference_protected

[Paul E. McKenney]

From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>

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@xxxxxxx
Suggested-by: tytso@xxxxxxx
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
[ paulmck: Update wording, including further feedback from Joel. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
 Documentation/RCU/whatisRCU.txt | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

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.
 
-- 
2.17.1