[PATCH] list.h: add list_for_each_entry_continue_rcu

[Johannes Berg <johannes@sipsolutions.net>]

To implement the multicast list callback in mac80211 we need to
do partial list iteration. Since I want to convert the interface
list to an RCU list, I need a new list walking primitive:
list_for_each_entry_continue_rcu().

Additional help text was provided by Paul McKenney.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: linux-kernel@vger.kernel.org
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

---
John, I don't know if you want to merge this or not. There's no clear
list.h maintainer and this patch is requisite for the bugfix I'm going
to send in a minute so it'd probably be best if this goes into the tree
together.
If you don't want to take it, however, I have no idea how to handle the
situation :)

 include/linux/list.h |   26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

--- wireless-dev.orig/include/linux/list.h	2007-09-07 22:00:41.234425862 +0200
+++ wireless-dev/include/linux/list.h	2007-09-07 22:05:08.184425862 +0200
@@ -665,6 +665,32 @@ static inline void list_splice_init_rcu(
 		prefetch(rcu_dereference((pos))->next), (pos) != (head); \
         	(pos) = (pos)->next)
 
+
+/**
+ * list_for_each_entry_continue_rcu - continue iteration over rcu list
+ * @pos:	the type * to use as a loop cursor.
+ * @head:	the head for your list.
+ * @member:	the name of the list_struct within the struct.
+ *
+ * Continue to iterate over rcu list of given type, continuing after
+ * the current position.
+ *
+ * This list-traversal primitive may safely run concurrently with
+ * the _rcu list-mutation primitives such as list_add_rcu()
+ * as long as the traversal is guarded by rcu_read_lock().
+ *
+ * Note that the caller is responsible for making sure that
+ * the element remains in place between the earlier iterator
+ * and this one.  One way to do this is to ensure that
+ * both iterators are covered by the same rcu_read_lock(),
+ * while others involve reference counts, flags, or mutexes.
+ */
+#define list_for_each_entry_continue_rcu(pos, head, member) \
+	for ((pos) = list_entry((pos)->member.next, typeof(*pos), member); \
+		prefetch(rcu_dereference((pos))->member.next), \
+			 &pos->member != (head); \
+        	(pos) = list_entry(pos->member.next, typeof(*pos), member))
+
 /*
  * Double linked lists with a single pointer list head.
  * Mostly useful for hash tables where the two pointer list head is