rculist.h: Add list_tail_rcu()

This patch adds the macro list_tail_rcu() and documents it.
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
[ paulmck: Reword a bit. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/rculist.h

@@ -40,6 +40,16 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
  */
 #define list_next_rcu(list)	(*((struct list_head __rcu **)(&(list)->next)))
 
+/**
+ * list_tail_rcu - returns the prev pointer of the head of the list
+ * @head: the head of the list
+ *
+ * Note: This should only be used with the list header, and even then
+ * only if list_del() and similar primitives are not also used on the
+ * list header.
+ */
+#define list_tail_rcu(head)	(*((struct list_head __rcu **)(&(head)->prev)))
+
 /*
  * Check during list traversal that we are within an RCU reader
  */