block: move drain outside of quorum_add_child()

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>
Message-ID: <20250530151125.955508-15-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h
index 168f703fa1..f9e742f812 100644
--- a/include/block/block_int-common.h
+++ b/include/block/block_int-common.h
@@ -396,6 +396,13 @@ struct BlockDriver {
     int GRAPH_RDLOCK_PTR (*bdrv_probe_geometry)(
         BlockDriverState *bs, HDGeometry *geo);
 
+    /**
+     * Hot add a BDS's child. Used in combination with bdrv_del_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_add_child)(
         BlockDriverState *parent, BlockDriverState *child, Error **errp);