Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxx] On Behalf Of Ian Campbell
> Sent: 12 March 2014 10:28
> To: Zoltan Kiss
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Wei Liu; Tim (Xen.org)
> Subject: Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness
> 
> 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.
> 

Huh? The act of consuming the request will create the space for the response. 
If responses and requests are not balanced then I'd argue that the protocol is 
broken.

  Paul

> (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?
> 
> 
> 
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