[PATCH v2 6/6] rcu: Document rcu_access_pointer() feeding into cmpxchg()

["Paul E. McKenney" <paulmck@kernel.org>]

This commit documents the rcu_access_pointer() use case for fetching the
old value of an RCU-protected pointer within a lockless updater for use
by an atomic cmpxchg() operation.

Reported-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 include/linux/rcupdate.h | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index bfa765132de858..5e95acc33989b6 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -592,11 +592,13 @@ context_unsafe(							      \
  * lockdep checks for being in an RCU read-side critical section.  This is
  * useful when the value of this pointer is accessed, but the pointer is
  * not dereferenced, for example, when testing an RCU-protected pointer
- * against NULL.  Although rcu_access_pointer() may also be used in cases
- * where update-side locks prevent the value of the pointer from changing,
- * you should instead use rcu_dereference_protected() for this use case.
- * Within an RCU read-side critical section, there is little reason to
- * use rcu_access_pointer().
+ * against NULL.  Within an RCU read-side critical section, there is little
+ * reason to use rcu_access_pointer().  Although rcu_access_pointer() may
+ * also be used in cases where update-side locks prevent the value of the
+ * pointer from changing, you should instead use rcu_dereference_protected()
+ * for this use case.  It is also permissible to use rcu_access_pointer()
+ * within lockless updaters to obtain the old value for an atomic operation,
+ * for example, for cmpxchg().
  *
  * It is usually best to test the rcu_access_pointer() return value
  * directly in order to avoid accidental dereferences being introduced
-- 
2.40.1