Mutexes (locks) ensure only one thread executes critical code sections at a time. This guide implements several mutex variants in Go from scratch: a naive spin lock with race conditions, a working spin lock using atomic operations, and a futex-based lock leveraging OS primitives for efficiency. Each implementation explores performance tradeoffs—spin locks waste CPU cycles busy-waiting, while futex locks use syscalls to sleep threads. An adaptive approach combines spinning briefly before sleeping, balancing latency and CPU usage. Benchmarks compare custom implementations against Go's sync.Mutex across different contention scenarios, revealing how design choices affect real-world performance.