Potential Race Condition

[h-artzi]

The following line mutex ||= (data[:mutex] ||= Mutex.new) can lead to a race condition.

The above line translates to the following:
data[:mutex] ||= Mutex.new # line 1
mutex = data[:mutex] # line 2
Imagine line 1 gets hit by 2 threads simultaneously: 2 mutexes will be created, and data[:mutex] will be set by first to one, then overwritten by the other.