rcu: Add list_next_or_null_rcu

This is a convenience function that returns the next entry in an RCU
list or NULL if at the end of the list.
Signed-off-by: NTom Herbert <tom@herbertland.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/rculist.h

@@ -318,6 +318,27 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
 	likely(__ptr != __next) ? list_entry_rcu(__next, type, member) : NULL; \
 })
 
+/**
+ * list_next_or_null_rcu - get the first element from a list
+ * @head:	the head for the list.
+ * @ptr:        the list head to take the next element from.
+ * @type:       the type of the struct this is embedded in.
+ * @member:     the name of the list_head within the struct.
+ *
+ * Note that if the ptr is at the end of the list, NULL is returned.
+ *
+ * This primitive may safely run concurrently with the _rcu list-mutation
+ * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
+ */
+#define list_next_or_null_rcu(head, ptr, type, member) \
+({ \
+	struct list_head *__head = (head); \
+	struct list_head *__ptr = (ptr); \
+	struct list_head *__next = READ_ONCE(__ptr->next); \
+	likely(__next != __head) ? list_entry_rcu(__next, type, \
+						  member) : NULL; \
+})
+
 /**
  * list_for_each_entry_rcu	-	iterate over rcu list of given type
  * @pos:	the type * to use as a loop cursor.