Lock / Mutex / Semaphore
A lock allows only one thread to enter the part that's locked and the lock is not shared with any other processes.
A mutex is the same as a lock but it can be system wide (shared by multiple processes).
A semaphore does the same as a mutex but allows x number of threads to enter, this can be used for example to limit the number of cpu, io or ram intensive tasks running at the same time.
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