diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index c30a5ff8de..80b7684dba 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -223,6 +223,7 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
     QDict *opts;
 
     assert(source->total_sectors == target->total_sectors);
+    GLOBAL_STATE_CODE();
 
     opts = qdict_new();
     qdict_put_str(opts, "driver", "copy-before-write");
@@ -245,6 +246,7 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
 
 void bdrv_cbw_drop(BlockDriverState *bs)
 {
+    GLOBAL_STATE_CODE();
     bdrv_drop_filter(bs, &error_abort);
     bdrv_unref(bs);
 }
diff --git a/block/copy-before-write.h b/block/copy-before-write.h
index 51847e711a..6e72bb25e9 100644
--- a/block/copy-before-write.h
+++ b/block/copy-before-write.h
@@ -29,6 +29,13 @@
 #include "block/block_int.h"
 #include "block/block-copy.h"
 
+/*
+ * Global state (GS) API. These functions run under the BQL.
+ *
+ * See include/block/block-global-state.h for more information about
+ * the GS API.
+ */
+
 BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   BlockDriverState *target,
                                   const char *filter_node_name,