test-bdrv-drain: Take graph lock more selectively

If we take a reader lock, we can't call any functions that take a writer
lock internally without causing deadlocks once the reader lock is
actually enforced in the main thread, too. Take the reader lock only
where it is actually needed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230510203601.418015-6-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/unit/test-bdrv-drain.c

@@ -1004,8 +1004,6 @@ static void coroutine_fn test_co_delete_by_drain(void *opaque)
     void *buffer = g_malloc(65536);
     QEMUIOVector qiov = QEMU_IOVEC_INIT_BUF(qiov, buffer, 65536);
 
-    GRAPH_RDLOCK_GUARD();
-
     /* Pretend some internal write operation from parent to child.
      * Important: We have to read from the child, not from the parent!
      * Draining works by first propagating it all up the tree to the
@@ -1014,7 +1012,9 @@ static void coroutine_fn test_co_delete_by_drain(void *opaque)
      * everything will be drained before we go back down the tree, but
      * we do not want that.  We want to be in the middle of draining
      * when this following requests returns. */
+    bdrv_graph_co_rdlock();
     bdrv_co_preadv(tts->wait_child, 0, 65536, &qiov, 0);
+    bdrv_graph_co_rdunlock();
 
     g_assert_cmpint(bs->refcnt, ==, 1);