>If splbar() > splfoo(), this doesn't happen, right?  Is it common practice,
>then, to simply have such an ordering?

Some hardware pretty much forces it (VAX comes to mind).

One reason I suggested totally-ordered SPLs, because thats well-known
to be deadlock-free. 

I dont know how we currently support multiple CPUs in the kernel
handling clock interrupts whilst guaranteeing no deadlocks: the
manpage I have doesnt mention it.  I'd guess splclock() through
splhigh() really are totally ordered?