[PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT

[Davidlohr Bueso <dave@stgolabs.net>]

With d72da4a4d97 (rbtree: Make lockless searches non-fatal) our rbtrees
provide weak guarantees that allows us to do lockless (and very speculative)
reads of the tree. Such readers cannot see partial stores on nodes, ie
left/right as well as root. As such, similar to the WRITE_ONCE semantics when
doing rotations, use READ_ONCE when checking the root node in RB_EMPTY_ROOT.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 include/linux/rbtree.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/rbtree.h b/include/linux/rbtree.h
index a5aa7ae..b690009 100644
--- a/include/linux/rbtree.h
+++ b/include/linux/rbtree.h
@@ -50,7 +50,7 @@ struct rb_root {
 #define RB_ROOT	(struct rb_root) { NULL, }
 #define	rb_entry(ptr, type, member) container_of(ptr, type, member)
 
-#define RB_EMPTY_ROOT(root)  ((root)->rb_node == NULL)
+#define RB_EMPTY_ROOT(root)  (READ_ONCE((root)->rb_node) == NULL)
 
 /* 'empty' nodes are nodes that are known not to be inserted in an rbtree */
 #define RB_EMPTY_NODE(node)  \
-- 
2.1.4