ilock() (pc/lock.c) calls splhi() and then calls lock(). If that lock were contended, how would the system not stop? And on a UP system, if you're inside an splhi() block,
why would you need to take an uncontended lock?

ilock is for locks that get used both in normal procs

and in interrupt handlers. Using splhi makes sure

that the interrupt handler does not get started on the

same cpu while the proc holds the lock, which would

cause a deadlock.

If there were no interrupts involved, the lock would be

managed with lock/unlock instead of ilock/iunlock.

On a uniprocessor you could get away with splhi/splx

instead of ilock/iunlock, but it's easier and more portable

to program as if always on a multiprocessor.

Russ