Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 11/03/14 15:44, Ian Campbell wrote:
> On Thu, 2014-03-06 at 15:47 +0000, Zoltan Kiss wrote:
> > Hi,
> >
> > I was wondering for a while why this macro looks like this:
> >
> > #define RING_HAS_UNCONSUMED_REQUESTS(_r)                                \
> >      ({                                                                 \
> >         unsigned int req = (_r)->sring->req_prod - (_r)->req_cons;     \
> >         unsigned int rsp = RING_SIZE(_r) -                              \
> >                            ((_r)->req_cons - (_r)->rsp_prod_pvt); \
> >         req < rsp ? req : rsp;                                               \
> >      })
> >
> > I would expect to check prod - cons, like RING_HAS_UNCONSUMED_RESPONSES does:
> >
> > #define RING_HAS_UNCONSUMED_RESPONSES(_r)                               \
> >      ((_r)->sring->rsp_prod - (_r)->rsp_cons)
> >
> > By my understanding, there is no way rsp could be smaller than req, so
> > there is no point having this. Am I missing something?
> Just looking at this again. All that stuff I said about wraparound was
> misleading/irrelevant since req and rsp are not the pointers, but
> actually the number of things. Sorry.
>
> Is it the case that this macro considers a request to be unconsumed if
> the *response* to a request is outstanding as well as if the request
> itself is still on the ring?
I don't think that would make sense. I think everywhere where this macro 
is called the caller is not interested in pending request (pending means 
consumed but not responded)
> I wonder if this apparently weird construction is due to pathological
> cases when one or the other end is not picking up requests/responses?
> i.e. trying to avoid deadlocking the ring or generating an interrupt
> storm when the ring it is full of one or the other or something along
> those lines?
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