Spinlock versus Semaphore

very simply, a semaphore is a "yielding" synchronisation object, a spinlock is a 'busywait' one. (there's a little more to semaphores in that they synchronise several threads, unlike a mutex or guard or monitor or critical section that protects a code region from a single thread)

You'd use a semaphore in more circumstances, but use a spinlock where you are going to lock for a very short time - there is a cost to locking especially if you lock a lot. In such cases it can be more efficient to spinlock for a little while waiting for the protected resource to become unlocked. Obviously there is a performance hit if you spin for too long.

typically if you spin for longer than a thread quantum, then you should use a semaphore.

Over and above what Yoav Aviram and gbjbaanb said, the other key point used to be that you would never use a spin-lock on a single-CPU machine, whereas a semaphore would make sense on such a machine. Nowadays, you are frequently hard-pressed to find a machine without multiple cores, or hyperthreading, or equivalent, but in the circumstances that you have just a single CPU, you should use semaphores. (I trust the reason is obvious. If the single CPU is busy waiting for something else to release the spin-lock, but it is running on the only CPU, the lock is unlikely to be released until the current process or thread is preempted by the O/S, which might take a while and nothing useful happens until the preemption occurs.)

Spinlock refers to an implementation of inter-thread locking using machine dependent assembly instructions (such as test-and-set). It is called a spinlock because the thread simply waits in a loop ("spins") repeatedly checking until the lock becomes available (busy wait). Spinlocks are used as a substitute for mutexes, which are a facility supplied by operating systems (not the CPU), because spinlocks perform better, if locked for a short period of time.

A Semaphor is a facility supplied by operating systems for IPC, therefor it's main purpose is inter-process-communication. Being a facility supplied by the operating system it's performance will not be as good as that of a spinlock for inter-thead locking (although possible). Semaphores are better for locking for longer periods of time.

That said - implementing splinlocks in assembly is tricky, and not portable.

spin lock can be held by only one process while semaphore can be held by one or more processes. Spin lock wait until the process releases a lock and then acquires a lock. Semaphore is sleeping lock i.e waits and goes to sleep.