Spinlock versus Semaphore

Question

What are the basic differences between a semaphore & spin-lock?

When would we use a semaphore over a spin-lock?

Answer 1

Spinlock is used if and only if you are pretty certain that your expected result will happen very shortly, before your thread's execution slice time expires.

Example: In device driver module, The driver writes "0" in hardware Register R0 and now it needs to wait for that R0 register to become 1. The H/W reads the R0 and does some work and writes "1" in R0. This is generally quick(in micro seconds). Now spinning is much better than going to sleep and interrupted by the H/W. Of course, while spinning, H/W failure condition needs to be taken care!

There is absolutely no reason for a user application to spin. It doesn't make sense. You are going to spin for some event to happen and that event needs to be completed by another user level application which is never guaranteed to happen within quick time frame. So, I will not spin at all in user mode. I better to sleep() or mutexlock() or semaphore lock() in user mode.