能否用原子指令实现Mutex？关于基于Compare-Exchange实现互斥锁的理解是否正确？

Hey there! Awesome questions—let’s unpack them one by one.

1. Can atomic instructions be used to implement a Mutex? #

Absolutely! Atomic instructions are the bedrock of building mutexes (and most low-level synchronization primitives). Without atomic operations, coordinating thread access to shared state would be impossible—non-atomic operations can be interrupted mid-execution, leading to messy race conditions that break your code’s correctness. So yes, atomic ops are not just usable here; they’re essential.

2. Is your understanding about using Compare-Exchange (CAS) for mutexes correct? #

You’ve got it exactly right! Let’s expand on that to confirm:

• The core idea centers on a shared flag variable (say, lock_state, where 0 = unlocked, 1 = locked). When a thread wants the mutex, it runs a CAS operation on this flag.
• The CAS checks if lock_state is 0 (unlocked). If it is, it atomically swaps it to 1—and the thread gains access to the critical section. If the flag is already 1, the thread will typically retry (usually in a loop, sometimes with small delays to avoid hogging CPU) until it succeeds.
• When releasing the mutex, the owning thread can safely do an atomic store to set lock_state back to 0—since only the lock holder should ever execute this step, no race condition arises here.

Just a quick extra note: Real-world mutexes often add bells and whistles like fairness (to stop one thread from hogging the lock forever) or integration with OS sleep/wake calls (so threads don’t spin endlessly if the lock is held for a long time). But at their core, they all rely on CAS or similar atomic instructions like test-and-set to safely claim the lock.

内容的提问来源于stack exchange，提问作者user855