Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-03-12 at 15:14 +0000, Zoltan Kiss wrote:
> On 12/03/14 14:30, Ian Campbell wrote:
> > On Wed, 2014-03-12 at 14:27 +0000, Zoltan Kiss wrote:
> >> 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
> >
> > I think to make sense of this I need to see the sequence of reads/writes
> > from both parties in a sensible ordering which would result in reads
> > showing the above. i.e. a demonstration of the race not just an
> > assertion that if the values are read as is things makes sense.
> 
> Let me extend it:
> 
> - callback reads req_prod = 253

callback == backend? Which context is this code running in? Which part
of the system is the callback logically part of?

> - frontend writes req_prod, now its 256
> - backend picks it up, and consumes those slots, req_cons become 256

"it"? Do you mean req_prod? Please be precise.

> - callback reads req_cons = 256

But the backend has also seen req_prod at 256 at this point, hasn't it?
You said so above but said "it" so I'm not sure. If the callback is part
of the backend then why hasn't it also seen this?

> - req is UINT_MAX-3 therefore, but actually there isn't any request to 
> consume, it should be 0

Only if something is ignoring the fact that it has seen req_prod == 256.

If callback is some separate entity to backend within dom0 then what you
have here is an internal inconsistency in dom0 AFAICT. IOW it seems like
you are missing some synchronisation and/or have two different entities
acting as backend.

> - callback reads rsp_prod_pvt = 0, because backend haven't responded to 
> any requests
> - rsp is therefore 256 - (256 -0) = 0
> - the macro returns rsp, as it is smaller. And that's good, because 
> despite the macro failed to determine the number of unconsumed requests, 
> at least it detected that the ring is full with consumed but not replied 
> requests, so there shouldn't be any unconsumed req
> 
> And I call this best effort because if rsp_prod_pvt is e.g. 10, rsp will 
> be then 10 as well, we return it, and the caller thinks there are 
> unconsumed requests, despite there isn't any.
> 
> Zoli



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