should ia64_spinlock_contention do backoff?

Has anyone studied the impact of doing exponential backoff in
ia64_spinlock_contention.  My theory is that it wouldn't buy much _if_
spinlocks always were in their own cachelines, but since they're not,
not using backoff could cause extra cache-line bouncing.  To be
honest, I'd rather not spend time on this myself, since I don't have
convenient access to large machines, but me thinks this is a question
that's long overdue to have a proper answer.

	--david
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