This is in preparation to mark bdrv_drained_begin() as GRAPH_UNLOCKED.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

No changes in v3.

 blockdev.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/blockdev.c b/blockdev.c
index bd5ca77619..506755bef1 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1208,7 +1208,7 @@ static void internal_snapshot_action(BlockdevSnapshotInternal *internal,
     Error *local_err = NULL;
     const char *device;
     const char *name;
-    BlockDriverState *bs;
+    BlockDriverState *bs, *check_bs;
     QEMUSnapshotInfo old_sn, *sn;
     bool ret;
     int64_t rt;
@@ -1216,7 +1216,7 @@ static void internal_snapshot_action(BlockdevSnapshotInternal *internal,
     int ret1;
 
     GLOBAL_STATE_CODE();
-    GRAPH_RDLOCK_GUARD_MAINLOOP();
+    bdrv_graph_rdlock_main_loop();
 
     tran_add(tran, &internal_snapshot_drv, state);
 
@@ -1225,14 +1225,29 @@ static void internal_snapshot_action(BlockdevSnapshotInternal *internal,
 
     bs = qmp_get_root_bs(device, errp);
     if (!bs) {
+        bdrv_graph_rdunlock_main_loop();
         return;
     }
 
     state->bs = bs;
 
+    /* Need to drain while unlocked. */
+    bdrv_graph_rdunlock_main_loop();
     /* Paired with .clean() */
     bdrv_drained_begin(bs);
 
+    GRAPH_RDLOCK_GUARD_MAINLOOP();
+
+    /* Make sure the root bs did not change with the drain. */
+    check_bs = qmp_get_root_bs(device, errp);
+    if (bs != check_bs) {
+        if (check_bs) {
+            error_setg(errp, "Block node of device '%s' unexpectedly changed",
+                       device);
+        } /* else errp is already set */
+        return;
+    }
+
     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_INTERNAL_SNAPSHOT, errp)) {
         return;
     }
-- 
2.39.5