Mutexes are typically implemented over atomic instructions. So you'd do something like atomic compare/exchange to acquire the mutex and if there's no contentio

Mutexes are typically implemented over atomic instructions. So you'd do something like atomic compare/exchange to acquire the mutex and if there's no contention you got it. If there is contention you go to the OSes synchronization constructs which are typically much slower... An atomic increment should always be faster than acquiring a mutex and incrementing...