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 | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/blockdev.c b/blockdev.c
index 506755bef1..2e7fda6780 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1377,9 +1377,10 @@ static void external_snapshot_action(TransactionAction *action,
     const char *new_image_file;
     ExternalSnapshotState *state = g_new0(ExternalSnapshotState, 1);
     uint64_t perm, shared;
+    BlockDriverState *check_bs;
 
     /* TODO We'll eventually have to take a writer lock in this function */
-    GRAPH_RDLOCK_GUARD_MAINLOOP();
+    bdrv_graph_rdlock_main_loop();
 
     tran_add(tran, &external_snapshot_drv, state);
 
@@ -1412,11 +1413,25 @@ static void external_snapshot_action(TransactionAction *action,
 
     state->old_bs = bdrv_lookup_bs(device, node_name, errp);
     if (!state->old_bs) {
+        bdrv_graph_rdunlock_main_loop();
         return;
     }
 
+    /* Need to drain while unlocked. */
+    bdrv_graph_rdunlock_main_loop();
     /* Paired with .clean() */
     bdrv_drained_begin(state->old_bs);
+    GRAPH_RDLOCK_GUARD_MAINLOOP();
+
+    /* Make sure the associated bs did not change with the drain. */
+    check_bs = bdrv_lookup_bs(device, node_name, errp);
+    if (state->old_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_is_inserted(state->old_bs)) {
         error_setg(errp, "Device '%s' has no medium",
-- 
2.39.5