In stream_prepare(), we need to temporarily drop the AioContext lock
that job_prepare_locked() took for us while calling the graph write lock
functions which can poll.

All block nodes related to this block job are in the same AioContext, so
we can pass any of them to bdrv_graph_wrlock()/ bdrv_graph_wrunlock().
Unfortunately, the one that we picked is base, which can be NULL - and
in this case the AioContext lock is not released and deadlocks can
occur.

Fix this by passing s->target_bs, which is never NULL.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/stream.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/stream.c b/block/stream.c
index e3aa696289..01fe7c0f16 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -99,9 +99,9 @@ static int stream_prepare(Job *job)
             }
         }
 
-        bdrv_graph_wrlock(base);
+        bdrv_graph_wrlock(s->target_bs);
         bdrv_set_backing_hd_drained(unfiltered_bs, base, &local_err);
-        bdrv_graph_wrunlock(base);
+        bdrv_graph_wrunlock(s->target_bs);
 
         /*
          * This call will do I/O, so the graph can change again from here on.
-- 
2.41.0