nywhere from around 15 cycles to 50, which may not be totally
insignificant in performance critical paths (code size will go down
too).

However the primary motivation for this is to have the canonical barrier
implementation for x86 architecture.

smp_rmb on buggy pentium pros remains a locked op, which is apparently
required.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
`