Operating Systemsmedium

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

Answer

A mutex provides exclusive access, a semaphore controls access to a limited number of resources, and a read-write lock allows multiple readers or one writer. Use them based on contention and access patterns.

Advanced answer

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?