1 files changed, 16 insertions, 10 deletions

diff --git a/block/stream.c b/block/stream.c
index 694709bd25..8744ad103f 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -64,13 +64,16 @@ static int stream_prepare(Job *job)
     bdrv_cor_filter_drop(s->cor_filter_bs);
     s->cor_filter_bs = NULL;
 
-    bdrv_subtree_drained_begin(s->above_base);
+    /*
+     * bdrv_set_backing_hd() requires that unfiltered_bs is drained. Drain
+     * already here and use bdrv_set_backing_hd_drained() instead because
+     * the polling during drained_begin() might change the graph, and if we do
+     * this only later, we may end up working with the wrong base node (or it
+     * might even have gone away by the time we want to use it).
+     */
+    bdrv_drained_begin(unfiltered_bs);
 
     base = bdrv_filter_or_cow_bs(s->above_base);
-    if (base) {
-        bdrv_ref(base);
-    }
-
     unfiltered_base = bdrv_skip_filters(base);
 
     if (bdrv_cow_child(unfiltered_bs)) {
@@ -82,7 +85,13 @@ static int stream_prepare(Job *job)
             }
         }
 
-        bdrv_set_backing_hd(unfiltered_bs, base, &local_err);
+        bdrv_set_backing_hd_drained(unfiltered_bs, base, &local_err);
+
+        /*
+         * This call will do I/O, so the graph can change again from here on.
+         * We have already completed the graph change, so we are not in danger
+         * of operating on the wrong node any more if this happens.
+         */
         ret = bdrv_change_backing_file(unfiltered_bs, base_id, base_fmt, false);
         if (local_err) {
             error_report_err(local_err);
@@ -92,10 +101,7 @@ static int stream_prepare(Job *job)
     }
 
 out:
-    if (base) {
-        bdrv_unref(base);
-    }
-    bdrv_subtree_drained_end(s->above_base);
+    bdrv_drained_end(unfiltered_bs);
     return ret;
 }