Why Redis SETNX succeeds for two clients

[mong0520]

I am studying the locking method on Redis, and found this official documents:
https://redis.io/commands/setnx#handling-deadlocks

My understanding is that SETNX is an atomic operation and ONLY ONE client can do it successful (get 1) and the others failed (get 0)

However in the section Handling deadlocks on this document, it said:

1) C1 and C2 read lock.foo to check the timestamp, because they both received 0 after executing SETNX, as the lock is still held by C3 that crashed after holding the lock.
2) C1 sends DEL lock.foo
3) C1 sends SETNX lock.foo and it succeeds
4) C2 sends DEL lock.foo
5) C2 sends SETNX lock.foo and it succeeds
ERROR: both C1 and C2 acquired the lock because of the race condition.

My question is that why 3) and 5) can be succeed? I think only C1 or C2 can be succeed but not both.

Please correct my understanding, thank you,.

[sundb]

First, check if the timestamp, DEL lock.foo and SETNX lock.foo are executed in a lua script? or if you are executing them in 3 separate commands.
If you did it in 3 separate commands, there is a chance that both C1 and C2 will get locks at the same time, because we need to ensure that the three commands are atomic