Peter Zijlstra [off-list ref] wrote on 10/30/2013 01:25:26 PM:

Also, I'm not entirely sure on C, that too seems like a dependency, we
simply cannot read the buffer @tail before we've read the tail itself,
now can we? Similarly we cannot compare tail to head without having the
head read completed.

No, this one we cannot omit, because our problem on consumer side is not
with @tail, which is written exclusively by consumer, but with @head.

BTW, it is why you also don't need ACCESS_ONCE() around @tail, but only
around
@head read.

-- Victor