diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index ed96474..7a6b6e2 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -75,6 +75,10 @@
 	struct blk_mq_bitmap_tags *bt;
 	int i, wake_index;
 
+	/*
+	 * Make sure all changes prior to this are visible from other CPUs.
+	 */
+	smp_mb();
 	bt = &tags->bitmap_tags;
 	wake_index = atomic_read(&bt->wake_index);
 	for (i = 0; i < BT_WAIT_QUEUES; i++) {