block: Add backing_blocker in BlockDriverState

This makes use of op_blocker and blocks all the operations except for
commit target, on each BlockDriverState->backing_hd.

The asserts for op_blocker in bdrv_swap are removed because with this
change, the target of block commit has at least the backing blocker of
its child, so the assertion is not true. Callers should do their check.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/block/mirror.c b/block/mirror.c
index 1c38aa8f77..94c8661777 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -498,7 +498,7 @@ immediate_exit:
             /* drop the bs loop chain formed by the swap: break the loop then
              * trigger the unref from the top one */
             BlockDriverState *p = s->base->backing_hd;
-            s->base->backing_hd = NULL;
+            bdrv_set_backing_hd(s->base, NULL);
             bdrv_unref(p);
         }
     }