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

The bdrv_del_child() function is called by qmp_x_blockdev_change().
A drained section was already introduced there by commit "block: move
drain out of quorum_add_child()".

This finally finishes moving out the drain to places that are not
under the graph lock started in "block: move draining out of
bdrv_change_aio_context() and mark GRAPH_RDLOCK".

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

Changes in v4:
* Document requirement that all nodes need to be drained for
  bdrv_del_child() wrapper and callback.
* Add function comment for bdrv_del_child() callback.

 block.c                          | 2 ++
 block/quorum.c                   | 2 --
 include/block/block_int-common.h | 7 +++++++
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index 15a8ccb822..bfd4340b24 100644
--- a/block.c
+++ b/block.c
@@ -8276,6 +8276,8 @@ void bdrv_add_child(BlockDriverState *parent_bs, BlockDriverState *child_bs,
 /*
  * Hot remove a BDS's child. Used in combination with bdrv_add_child, so the
  * user can take a child offline when it is broken and take a new child online.
+ *
+ * All block nodes must be drained.
  */
 void bdrv_del_child(BlockDriverState *parent_bs, BdrvChild *child, Error **errp)
 {
diff --git a/block/quorum.c b/block/quorum.c
index 81407a38ee..cc3bc5f4e7 100644
--- a/block/quorum.c
+++ b/block/quorum.c
@@ -1147,9 +1147,7 @@ quorum_del_child(BlockDriverState *bs, BdrvChild *child, Error **errp)
             (s->num_children - i - 1) * sizeof(BdrvChild *));
     s->children = g_renew(BdrvChild *, s->children, --s->num_children);
 
-    bdrv_drain_all_begin();
     bdrv_unref_child(bs, child);
-    bdrv_drain_all_end();
 
     quorum_refresh_flags(bs);
 }
diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h
index f9e742f812..925a3e7353 100644
--- a/include/block/block_int-common.h
+++ b/include/block/block_int-common.h
@@ -406,6 +406,13 @@ struct BlockDriver {
     void GRAPH_WRLOCK_PTR (*bdrv_add_child)(
         BlockDriverState *parent, BlockDriverState *child, Error **errp);
 
+    /**
+     * Hot remove a BDS's child. Used in combination with bdrv_add_child, so the
+     * user can take a child offline when it is broken and take a new child
+     * online.
+     *
+     * All block nodes must be drained.
+     */
     void GRAPH_WRLOCK_PTR (*bdrv_del_child)(
         BlockDriverState *parent, BdrvChild *child, Error **errp);
 
-- 
2.39.5