How is spin lock implemented under the hood?

Question

This is a lock that can be held by only one thread of execution at a time. An attempt to acquire the lock by another thread of execution makes the latter loop until the lock is released.

How does it handle the case when two threads try to acquire the lock exactly the same time?

I think this question also applies to various of other mutex implementation.

Answer 1

Depends on the processor and the threading implementation. Most processors have instructions that can be executed atomically, on top of which you can build things like spin locks. For example IA-32 has an xchg instruction that does an atomic swap. You can then implement a naive spinlock like:

  eax = 1;
  while( xchg(eax, lock_address) != 0 );
  // now I have the lock
  ... code ...
  *lock_address = 0; // release the lock

Answer 2

http://en.wikipedia.org/wiki/Spinlock