Re: [Xen-devel] [PATCH] oxenstored: fix short-write issue

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/11/15 14:24, Ian Campbell wrote:
> On Mon, 2015-11-02 at 14:10 +0000, Andrew Cooper wrote:
>> On 02/11/15 13:44, Ian Campbell wrote:
>>> On Tue, 2015-10-27 at 17:10 +0000, Wei Liu wrote:
>>>> When oxenstored wrote to the ring, it wrote a chunk of contiguous
>>>> data.
>>>> Originally when it tried to write across ring boundary, it returned a
>>>> short-write when there is still room.  That led to stalling mini-os's
>>>> xenstore thread at times.
>>> What is a "short-write" in this context?
>>>
>>> Given data bytes 0..M I assumed it is only writing bytes 0..N and not
>>> N+1..M because the ring boundary is at N. But what is it writing to the
>>> ->prod ring pointer N or M?
>> Prod gets incremented by N in this case.
>>
>>> AIUI writing N should be allowed by the ring protocol, the client
>>> should
>>> keep looking for more data until it has a complete request.
>>>
>>> Writing M would be a server error.
>> Correct, and this is what is happening.
> The first or second? Your first comment suggests the first, but your second
> binds most closely to the second.

Oops yes.  Writing M would be an error.  Prod currently gets incremented
by N.

>
>> The server (believes) that the ring is full, when it is not.  It waits
>> for the client to make more space in the ring, while the client is
>> waiting for the server to complete its message in the ring, thus
>> stalling.
> That makes sense, thanks.
>
> I think this needs to be spelled out more fully in the commit message, in
> particular "server thinks ring is full when it is not" is the most relevant
> thing, the short write is just how we arrived there.

This patch here is buggy, and superseeded by one of mine which attempts
to fix the C stubs.

I need to post a v2.

~Andrew

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