Mutex vs semaphore vs read-write lock — when do you use each?

Deep dive

Pick the right primitive for the pattern:

• Mutex: single owner, simplest, good for short critical sections.
• Semaphore: throttle concurrency (e.g., DB connections).
• RW lock: many readers, few writers; watch for writer starvation.

Examples

Use a semaphore for connection pooling:

max 20 DB connections -> semaphore(20)

Common pitfalls

• Using RW locks when writes are frequent (worse than mutex).
• Forgetting to release semaphores on errors.
• Holding locks too long (contention).

Interview follow-ups

• How do you avoid writer starvation?
• When is a mutex faster than an RW lock?
• What is a spinlock and when is it useful?