diff --git a/storage/innobase/buf/buf0buf.c b/storage/innobase/buf/buf0buf.c
index b6bfcc3dbc0..4963f1c30c3 100644
--- a/storage/innobase/buf/buf0buf.c
+++ b/storage/innobase/buf/buf0buf.c
@@ -1,6 +1,6 @@
 /*****************************************************************************
 
-Copyright (c) 1995, 2014, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 1995, 2015, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
 
 Portions of this file contain modifications contributed and copyrighted by
@@ -1528,6 +1528,9 @@ buf_pool_watch_set(
 		return(NULL);
 	}
 
+	/* The maximum number of purge threads should never exceed
+	BUF_POOL_WATCH_SIZE. So there is no way for purge thread
+	instance to hold a watch when setting another watch. */
 	for (i = 0; i < BUF_POOL_WATCH_SIZE; i++) {
 		bpage = &buf_pool->watch[i];