Threads

Spin locks

A spin lock is also used for mutually exclusive access to some resource. The _spin_lock(S) function differs from mutex_lock(S) in implementation. If a spin lock is not available, the calling thread is not blocked, instead the caller busy waits (or ``spin''s) until the lock becomes available.

   _spin_lock(  spin_t *lock);
   _spin_unlock(spin_t *lock);

Considerations for the use of spin locks:

The busy waiting prevents the LWP from being used by another thread.
This facility is intended for use when the delay is expected to be smaller than the time to context switch to another thread and back.
Use of this facility is not recommended on uniprocessor machines or if only one processor of a multiprocessor machine might be available. In those circumstances the spinning thread prevents the possible execution of the thread that is holding the lock, thereby delaying, possibly deadlocking, itself.

NOTE: Extreme care should be exercised in using spin locks. The minimally safe environment for using spin locks may be bound threads running on a system with more than one processor. However, deadlocks are always possible.