The atomic guarantee of CAS slows down processors. So some checking is done without it first:

void spin_lock(int *mutex) {
   while (1) {
      if (*mutex == 0) {
         // the mutex was at least momentarily unlocked
         if (!CAS(mutex, 0, 1)
            break;  // we have locked the mutex
         // some other thread beat us to it, so try again
      }
   }
}