A machine-independent spinlock implementation. It has the advantages of

A machine-independent spinlock implementation.  It has the advantages of
  1.  being written in C except for the most low-level atomic swap primitive,
        which is universally supported on current processor architectures
  2.  having a very small inlined memory footprint for spin_lock(),
        with the slow-path defered to a subroutine call
  3.  only requiring a bus-locked operation for lock acquisition,
         and not requiring a bus-locked operation for lock release
  4.  doing a non-bus-locked check first in the spin loop to
        reduce bus contention
  5.  doing exponential backoff in the uncommon contested case, which
        Sun has found to reduce bus contention by a factor of 5 or more

Reviewed by:	Matt Dillon