Spin locks are a low-level synchronization mechanism suitable primarily for use on shared memory multiprocessors. When the calling thread requests a spin lock that is already held by another thread, the second thread spins in a loop to test if the lock has become available. When the lock is obtained, it should be held only for a short time, as the spinning wastes processor cycles. Callers should unlock spin locks before calling sleep operations to enable other threads to obtain the lock.
Spin locks can be implemented using mutexes and conditional variables, but the pthread_spin_* functions are a standardized way to practice spin locking. The pthread_spin_* functions require much lower overhead for locks of short duration.
When performing any lock, a trade-off is made between the processor resources consumed while setting up to block the thread and the processor resources consumed by the thread while it is blocked. Spin locks require few resources to set up the blocking of a thread and then do a simple loop, repeating the atomic locking operation until the lock is available. The thread continues to consume processor resources while it is waiting.
Compared to spin locks, mutexes consume a larger amount of processor resources to block the thread. When a mutex lock is not available, the thread changes its scheduling state and adds itself to the queue of waiting threads. When the lock becomes available, these steps must be reversed before the thread obtains the lock. While the thread is blocked, it consumes no processor resources.
Therefore, spin locks and mutexes can be useful for different purposes. Spin locks might have lower overall overhead for very short-term blocking, and mutexes might have lower overall overhead when a thread will be blocked for longer periods of time.