block: bdrv_set_backing_hd(): use drained section

Graph modifications should be done in drained section. stream_prepare()
handler of block stream job call bdrv_set_backing_hd() without using
drained section and it's theoretically possible that some IO request
will interleave with graph modification and will use outdated pointers
to removed block nodes.

Some other callers use bdrv_set_backing_hd() not caring about drained
sections too. So it seems good to make a drained section exactly in
bdrv_set_backing_hd().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20220124173741.2984056-1-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/block.c b/block.c
index 7b3ce415d8..b54d59d1fa 100644
--- a/block.c
+++ b/block.c
@@ -3341,6 +3341,8 @@ int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
     int ret;
     Transaction *tran = tran_new();
 
+    bdrv_drained_begin(bs);
+
     ret = bdrv_set_backing_noperm(bs, backing_hd, tran, errp);
     if (ret < 0) {
         goto out;
@@ -3350,6 +3352,8 @@ int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
 out:
     tran_finalize(tran, ret);
 
+    bdrv_drained_end(bs);
+
     return ret;
 }