diff --git a/xen/lib/rbtree.c b/xen/lib/rbtree.c
index 85a4f20313..eb418baabb 100644
--- a/xen/lib/rbtree.c
+++ b/xen/lib/rbtree.c
@@ -46,7 +46,7 @@
 #define		RB_RED		0
 #define		RB_BLACK	1
 
-#define __rb_parent(pc)    ((struct rb_node *)(pc & ~3))
+#define __rb_parent(pc)    ((struct rb_node *)((pc) & ~3))
 
 #define __rb_color(pc)     ((pc) & 1)
 #define __rb_is_black(pc)  __rb_color(pc)