This is part of resolving the deadlock mentioned in commit "block:
move draining out of bdrv_change_aio_context() and mark GRAPH_RDLOCK".

The quorum_add_child() callback runs under the graph lock, so it is
not allowed to drain. It is only called as the .bdrv_add_child()
callback, which is only called in the bdrv_add_child() function, which
also runs under the graph lock.

The bdrv_add_child() function is called by qmp_x_blockdev_change(),
where a drained section is introduced.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

Changes in v2:
* Split up into smaller pieces, flesh out commit messages.

 block/quorum.c | 2 --
 blockdev.c     | 2 ++
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/quorum.c b/block/quorum.c
index ea17b0ec13..ed8ce801ee 100644
--- a/block/quorum.c
+++ b/block/quorum.c
@@ -1096,10 +1096,8 @@ quorum_add_child(BlockDriverState *bs, BlockDriverState *child_bs, Error **errp)
     /* We can safely add the child now */
     bdrv_ref(child_bs);
 
-    bdrv_drain_all_begin();
     child = bdrv_attach_child(bs, child_bs, indexstr, &child_of_bds,
                               BDRV_CHILD_DATA, errp);
-    bdrv_drain_all_end();
     if (child == NULL) {
         s->next_child_index--;
         return;
diff --git a/blockdev.c b/blockdev.c
index 750beba41f..bd5ca77619 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3531,6 +3531,7 @@ void qmp_x_blockdev_change(const char *parent, const char *child,
     BlockDriverState *parent_bs, *new_bs = NULL;
     BdrvChild *p_child;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
 
     parent_bs = bdrv_lookup_bs(parent, parent, errp);
@@ -3568,6 +3569,7 @@ void qmp_x_blockdev_change(const char *parent, const char *child,
 
 out:
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 }
 
 BlockJobInfoList *qmp_query_block_jobs(Error **errp)
-- 
2.39.5