mutex

If you want to have fun with threads, you should share mutable data between them. In order to get no data race and, therefore, undefined behaviour, you have to think about the synchronisation of your threads.

There are a lot of issues with the singleton pattern. I'm totally aware of that. But the singleton pattern is an ideal use case for a variable, which has only to be initialized in a thread safe way. From that point on you can use it without synchronization. So in this post I discuss different ways to initialize a singleton in a multithreading environment. You get the performance numbers and can reason about your uses cases for the thread safe initialization of a variable.

If the previous post showed something, it's, that you should use mutexes with great care. That's why you should wrap them in a lock.

Usage of mutexes seems extremely simple. There is a critical section in the code, which can only be accessed by a single thread at any point  of time. It's ensured by a mutex m. The calls m.lock() and m.unlock() guarantee this exclusivity. But, the devil is in the details.