Spinlocks

For N processes, you would use a semaphore initialised to N, not a spinlock.  If you wanted N
cpus to be able to do something, then I guess we could introduce a counting spinlock, similar
to a semaphore.  But we've not needed one yet, and I doubt we will.

We have many things implemented on top of spinlocks -- the BKL, semaphores, mutexes, rwlocks,
rwsems and the late and unlamented brwlock (obsoleted by RCU).  Then there's specialised
beasts such as bitlocks and seqlocks.