Re: perf events ring buffer memory barrier on powerpc

[Peter Zijlstra]

On Wed, Oct 30, 2013 at 04:52:05PM +0200, Victor Kaplansky wrote:
> 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.

Ah indeed, my argument was flawed in that @head is the important part.
But we still do a comparison of @tail against @head before we do further
reads.

Although I suppose speculative reads are allowed -- they don't have the
destructive behaviour speculative writes have -- and thus we could in
fact get reorder issues.

But since it is still a dependent load in that we do that @tail vs @head
comparison before doing other loads, wouldn't a read_barrier_depends()
be sufficient? Or do we still need a complete rmb?

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

Agreed, the ACCESS_ONCE() around tail is superfluous since we're the one
updating tail, so there's no problem with the value changing
unexpectedly.
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