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>

3 files changed, 21 insertions, 9 deletions

diff --git a/block.c b/block.c
index 351344e..598624f 100644
--- a/block.c
+++ b/block.c
@@ -1155,24 +1155,33 @@ static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs)
     BlockDriverState *old_bs = child->bs;
 
     if (old_bs) {
+        if (old_bs->quiesce_counter && child->role->drained_end) {
+            child->role->drained_end(child);
+        }
         QLIST_REMOVE(child, next_parent);
     }
+
+    child->bs = new_bs;
+
     if (new_bs) {
         QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
+        if (new_bs->quiesce_counter && child->role->drained_begin) {
+            child->role->drained_begin(child);
+        }
     }
-
-    child->bs = new_bs;
 }
 
 BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
                                   const char *child_name,
-                                  const BdrvChildRole *child_role)
+                                  const BdrvChildRole *child_role,
+                                  void *opaque)
 {
     BdrvChild *child = g_new(BdrvChild, 1);
     *child = (BdrvChild) {
         .bs     = NULL,
         .name   = g_strdup(child_name),
         .role   = child_role,
+        .opaque = opaque,
     };
 
     bdrv_replace_child(child, child_bs);
@@ -1185,7 +1194,8 @@ BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
                              const char *child_name,
                              const BdrvChildRole *child_role)
 {
-    BdrvChild *child = bdrv_root_attach_child(child_bs, child_name, child_role);
+    BdrvChild *child = bdrv_root_attach_child(child_bs, child_name, child_role,
+                                              NULL);
     QLIST_INSERT_HEAD(&parent_bs->children, child, next);
     return child;
 }
diff --git a/block/block-backend.c b/block/block-backend.c
index 4e8298b..14e528e 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);
     }
diff --git a/include/block/block_int.h b/include/block/block_int.h
index b6f4755..30a9717 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -719,7 +719,8 @@ void hmp_drive_add_node(Monitor *mon, const char *optstr);
 
 BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
                                   const char *child_name,
-                                  const BdrvChildRole *child_role);
+                                  const BdrvChildRole *child_role,
+                                  void *opaque);
 void bdrv_root_unref_child(BdrvChild *child);
 
 const char *bdrv_get_parent_name(const BlockDriverState *bs);