User-Level Spin Locks - Threads, Processes & IPC

Spin locks are used by SMP OS kernels for kernel thread (control path) synchonization. It is a busy-waiting style meaning that the thread trying to acquire the spin lock will constantly poll for the availability of the lock. Therefore thread holding a spin lock should not do a lot of things.

Here is an interesting post
The Code Project - User-Level Spin Locks - Threads, Processes & IPC about user level spin lock.