block: move drain outside of bdrv_root_unref_child()

This is part of resolving the deadlock mentioned in commit "block:
move draining out of bdrv_change_aio_context() and mark GRAPH_RDLOCK".

bdrv_root_unref_child() is called by:
1. blk_remove_bs(), where a drained section is introduced.
2. bdrv_unref_child(), which runs under the graph lock, so the drain
   will be moved further up to its callers.
3. block_job_remove_all_bdrv(), where a drained section is introduced.

For all callers of bdrv_unref_child() and its generated
bdrv_co_unref_child() coroutine variant, a drained section is
introduced, they are not explicilty listed here. The caller
quorum_del_child() holds the graph lock, so it is not actually allowed
to drain. This will be addressed in the next commit.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-16-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/tests/unit/test-bdrv-drain.c b/tests/unit/test-bdrv-drain.c
index ac76525e5a..59c2793725 100644
--- a/tests/unit/test-bdrv-drain.c
+++ b/tests/unit/test-bdrv-drain.c
@@ -955,11 +955,13 @@ static void bdrv_test_top_close(BlockDriverState *bs)
 {
     BdrvChild *c, *next_c;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     QLIST_FOREACH_SAFE(c, &bs->children, next, next_c) {
         bdrv_unref_child(bs, c);
     }
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 }
 
 static int coroutine_fn GRAPH_RDLOCK
@@ -1016,7 +1018,9 @@ static void coroutine_fn test_co_delete_by_drain(void *opaque)
         bdrv_graph_co_rdlock();
         QLIST_FOREACH_SAFE(c, &bs->children, next, next_c) {
             bdrv_graph_co_rdunlock();
+            bdrv_drain_all_begin();
             bdrv_co_unref_child(bs, c);
+            bdrv_drain_all_end();
             bdrv_graph_co_rdlock();
         }
         bdrv_graph_co_rdunlock();