Re: perf events ring buffer memory barrier on powerpc

[Victor Kaplansky]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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

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