Q. Give the reasons why Windows XP, Solaris and Linux implement multiple locking mechanisms. Explain the circumstances under which they use mutexes, spinlocks, semaphores, adaptive mutexes and condition variables. In every case explain why the mechanism is needed.

Answer: These operating systems offer different locking mechanisms depending on the application developers' needs. Spinlocks are helpful for multiprocessor systems where a thread can run in a busy-loop (for a short period of time) rather than incurring the overhead of being put in a sleep queue. Mutexes are valuable for locking resources. Solaris 2 utilizes adaptive mutexes meaning that the mutex is implemented with a spin lock on multiprocessor machines. Semaphores as well as condition variables are more appropriate tools for synchronization when a resource should be held for a long period of time, since spinning is inefficient for a long duration.