If multiple TiDBs are upgraded concurrently, the upgrade may block.

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Start a cluster, it has two TiDBs, and the version is v7.1.1.
Then upgrade TiDB1 and TiDB2 to master in the meantime.

• Before TiDB1 does PauseAllJobsBySystem, TiDB2 does bootstrap's DDL ops(like add column for tidb_background_subtask).
• Before TiDB2 finishes bootstrap's DDL( add column for tidb_background_subtask), TiDB1 does PauseAllJobsBySystem

2. What did you expect to see? (Required)

Upgrade successfully.

3. What did you see instead (Required)

Upgrade block( add column for tidb_background_subtask block).

4. What is your TiDB version? (Required)

master