Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness

[Tim Deegan <tim@xxxxxxx>]

At 16:31 +0000 on 06 Mar (1394119880), Zoltan Kiss wrote:
> On 06/03/14 15:53, Ian Campbell wrote:
> > On Thu, 2014-03-06 at 15:47 +0000, Zoltan Kiss wrote:
> >> By my understanding, there is no way rsp could be smaller than req, so
> >> there is no point having this. Am I missing something?
> >
> > It happens during wraparound, i.e. after req has wrapped but rsp hasn't
> > yet.
> 
> The name of the macro suggest we are interested whether the ring has 
> unconsumed requests, and netback uses it that way. The answer to that 
> question is req_prod - req_cons. And it works if prod wrapped but cons 
> didn't.

Yes.

> rsp calculates the number of "consumed but not responded" requests (it 
> also works well if req_cons wrapped but rsp_prod_pvt didn't), then 
> subtract it from the ring size.

That is indeed an odd thing to check, since it seems like it could only
be relevant if the request producer overran the response producer.
It's been there in one form or another since the original ring.h,
and RING_REQUEST_CONS_OVERFLOW does something similar.

I can't remember the original reasoning, and so I'm reluctant to
suggest removing it without some more eyes on the code...

Tim.

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