block: Fix reconfiguring graph with drained nodes

When changing the BlockDriverState that a BdrvChild points to while the
node is currently drained, we must call the .drained_end() parent
callback. Conversely, when this means attaching a new node that is
already drained, we need to call .drained_begin().

bdrv_root_attach_child() takes now an opaque parameter, which is needed
because the callbacks must also be called if we're attaching a new child
to the BlockBackend when the root node is already drained, and they need
a way to identify the BlockBackend. Previously, child->opaque was set
too late and the callbacks would still see it as NULL.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/block/block-backend.c b/block/block-backend.c
index 4e8298b291..14e528ea82 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -161,8 +161,7 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
     }
 
     blk_set_enable_write_cache(blk, true);
-    blk->root = bdrv_root_attach_child(bs, "root", &child_root);
-    blk->root->opaque = blk;
+    blk->root = bdrv_root_attach_child(bs, "root", &child_root, blk);
 
     return blk;
 }
@@ -481,8 +480,7 @@ void blk_remove_bs(BlockBackend *blk)
 void blk_insert_bs(BlockBackend *blk, BlockDriverState *bs)
 {
     bdrv_ref(bs);
-    blk->root = bdrv_root_attach_child(bs, "root", &child_root);
-    blk->root->opaque = blk;
+    blk->root = bdrv_root_attach_child(bs, "root", &child_root, blk);
 
     notifier_list_notify(&blk->insert_bs_notifiers, blk);
     if (blk->public.throttle_state) {
@@ -1676,6 +1674,9 @@ static void blk_root_drained_begin(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
 
+    /* Note that blk->root may not be accessible here yet if we are just
+     * attaching to a BlockDriverState that is drained. Use child instead. */
+
     if (blk->public.io_limits_disabled++ == 0) {
         throttle_group_restart_blk(blk);
     }