block: Make sure throttled BDSes always have a BB

It was already true in principle that a throttled BDS always has a BB
attached, except that the order of operations while attaching or
detaching a BDS to/from a BB wasn't careful enough.

This commit breaks graph manipulations while I/O throttling is enabled.
It would have been possible to keep things working with some temporary
hacks, but quite cumbersome, so it's not worth the hassle. We'll fix
things again in a minute.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/block.c b/block.c
index 18a497f69d..a7898449ac 100644
--- a/block.c
+++ b/block.c
@@ -2259,8 +2259,22 @@ static void swap_feature_fields(BlockDriverState *bs_top,
 
     assert(!bs_new->throttle_state);
     if (bs_top->throttle_state) {
+        /*
+         * FIXME Need to break I/O throttling with graph manipulations
+         * temporarily because of conflicting invariants (3. will go away when
+         * throttling is fully converted to work on BlockBackends):
+         *
+         * 1. Every BlockBackend has a single root BDS
+         * 2. I/O throttling functions require an attached BlockBackend
+         * 3. We need to first enable throttling on the new BDS and then
+         *    disable it on the old one (because of throttle group refcounts)
+         */
+#if 0
         bdrv_io_limits_enable(bs_new, throttle_group_get_name(bs_top));
         bdrv_io_limits_disable(bs_top);
+#else
+        abort();
+#endif
     }
 }