When would you use a spinlock vs a mutex?

Spinlock busy-waits (spins) checking lock status, consuming CPU but avoiding context switch overhead. Mutex blocks the thread, triggering context switch. Use spinlock when: critical section is very short, multiprocessor system, lock hold time < context switch time. Use mutex when: longer critical sections, single processor (spinning wastes cycles), or when sleeping is acceptable. Hybrid adaptive mutexes spin briefly then sleep. Spinlocks shouldn't be used in user space on single core.