RE: SMP spin-locks

David Schwartz
2001-06-15T10:36:09Z

> Spinlocks are machine dependent. A simple increment of a byte
> memory variable, spinning if it's not 1 will do fine. Decrementing
> this variable will release the lock. A `lock` prefix is not necessary
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> because all Intel byte operations are atomic anyway. This assumes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> that the lock was initialized to 0. It doesn't have to be. It
> could be initialized to 0xaa (anything) and spin if it's not
> 0xab (or anything + 1).

If this is true, atomicity isn't enough to do it. Atomicity means that
there's a single instruction (and so it can't be interrupted mid-modify).
Atomicity (at least as the term is normally used) doesn't prevent the
cache-coherency logic from ping-ponging the memory location between two
processor's caches during the atomic operation.

DS