doc: Present the role of READ_ONCE()
This commit adds an explanation of the special cases where READ_ONCE()
may be used in place of a member of the rcu_dereference() family.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
index 2efed99..bb7128e 100644
--- a/Documentation/RCU/checklist.rst
+++ b/Documentation/RCU/checklist.rst
@@ -314,6 +314,13 @@
shared between readers and updaters. Additional primitives
are provided for this case, as discussed in lockdep.txt.
+ One exception to this rule is when data is only ever added to
+ the linked data structure, and is never removed during any
+ time that readers might be accessing that structure. In such
+ cases, READ_ONCE() may be used in place of rcu_dereference()
+ and the read-side markers (rcu_read_lock() and rcu_read_unlock(),
+ for example) may be omitted.
+
10. Conversely, if you are in an RCU read-side critical section,
and you don't hold the appropriate update-side lock, you -must-
use the "_rcu()" variants of the list macros. Failing to do so