Taking a thread-oblivious program and making it multithreaded can be very hard. It requires global transitive knowledge of the function call graph and all share

Taking a thread-oblivious program and making it multithreaded can be very hard. It requires global transitive knowledge of the function call graph and all shared data structures. How was this achieved? Was it by a dev who was intimately familiar with the code base? Was it just applying the race detector repeatedly until it stopped squawking? Something else?