Here's the hypothetical scenario:

1. Thread 1 sees that the lock document does not exist and tries to create it.

C1: unlocked, owned by nobody
C2: -
C3: -

2. Thread 2 comes in and sees on the first config the the lock is in unlocked state and tries to grab it by sending updates to 3 config servers.

3. However, since the update is upsert: false, it will only succeed in updating the first one.

C1: locked, owned by thread 2
C2: -
C3: -

4. Since thread 2 failed to update to all 3 config servers, it will enter the tournament round which will then fail immediately because the lock document does not exist on the other config servers.