One thing I was surprised to learn about spinlocks is that not all spinlocks are spinlocks! At at least one company I have worked for, spinlocks are actually just locks that are biased to spin, but still fall back to kernel-based locking and scheduling eventually. Likewise, that company's main mutex class spins for a while before locking.<p>When I asked why the rationale was something like, "If a spinlock is ever held more than such and so number of microseconds, it's an error that it's a spinlock and actually it should be a mutex." Instead of doing degenerate behavior in this case, they just made the spinlock actually act like a mutex.
One thing I don’t get is how blocking is implemented once you have basic mutual exclusion working? Lets say you try to take the lock, but you spin, waiting for it. But say in your OS now you want the thread to go to sleep until it’s available.. how is actually implemented without spinning and using CPU?