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Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness



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
- frontend writes req_prod, now its 256
- backend picks it up, and consumes those slots, req_cons become 256
- callback reads req_cons = 256
- req is UINT_MAX-3 therefore, but actually there isn't any request to consume, it should be 0 - 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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