diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 7a8d251..935293a 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -335,6 +335,9 @@ static inline void radix_tree_preload_end(void)
 	preempt_enable();
 }
 
+int radix_tree_join(struct radix_tree_root *, unsigned long index,
+			unsigned new_order, void *);
+
 #define RADIX_TREE_ITER_TAG_MASK	0x00FF	/* tag index in lower byte */
 #define RADIX_TREE_ITER_TAGGED		0x0100	/* lookup tagged slots */
 #define RADIX_TREE_ITER_CONTIG		0x0200	/* stop at first hole */