diff --git a/block/qcow.c b/block/qcow.c
index e155d3c002..8f2bdfda51 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -159,6 +159,8 @@ static int qcow_open(BlockDriverState *bs, int flags)
             goto fail;
         bs->backing_file[len] = '\0';
     }
+
+    qemu_co_mutex_init(&s->lock);
     return 0;
 
  fail: