Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 12/03/14 10:28, Ian Campbell wrote:
> On Tue, 2014-03-11 at 23:24 +0000, Zoltan Kiss wrote:
> > On 11/03/14 15:44, Ian Campbell wrote:
> > > 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)
> It might be interested in such pending requests in some of the
> pathological cases I allude to in the next paragraph though?
>
> For example if the ring has unconsumed requests but there are no slots
> free for a response, it would be better to treat it as no unconsumed
> requests until space opens up for a response, otherwise something else
> just has to abort the processing of the request when it notices the lack
> of space.
>
> (I'm totally speculating here BTW, I don't have any concrete idea why
> things are done this way...)
>
>
> > > 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?
Also, let me quote again my example about when rsp makes sense:

"To clarify what does this do, let me show an example:
req_prod = 253
req_cons = 256
rsp_prod_pvt = 0

req will be UINT_MAX-2, as the values changed in the meantime, and rsp 
is 0. It's reasonable to return 0 here, as the backend hasn't replied 
anything yet, so we clearly shouldn't have any unconsumed request in the 
ring."
Zoli

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