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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/10/15 13:34, David Scott wrote:
>
>> On 27 Oct 2015, at 17:31, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>>
>> On 27/10/15 17:28, Samuel Thibault wrote:
>>> Andrew Cooper, le Tue 27 Oct 2015 17:21:39 +0000, a écrit :
>>>> as the second attempted write could return short as well.
>>> That is fine. The second attempt will only return a short write if there
>>> was really not enough room for it, which is what we want.
>> Then surely the bug is that Xs_ring.write returns short when it shouldn’t?
>
>> On 27 Oct 2015, at 17:31, Samuel Thibault <samuel.thibault@xxxxxxxxxxxx> 
>> wrote:
>>
>> Another way would be to make ml_interface_write write both pieces
>> instead of just a contiguous one.  The caml code would then look less
>> suspicious :)
>>
> I agree with both Andrew and Samuel that it would be a better fix if 
> Xs_ring.write (== “ml_interface_write”) wrote both pieces at once.
>
> However I believe the ‘suspicious’ OCaml patch fixes the specific issue (— or 
> have I missed something?).

Actually the suspicious OCaml is still buggy in the case that there was
a genuine short write followed by a false short write from wrapping the
ring.

>
> Last time I meddled with the C-level ring reading/writing code I didn’t get 
> it quite right. Does anyone have time to prototype what a C-level fix would 
> look like? If we’re short of time I could live with the OCaml-level fix, 
> especially since Wei has done some stress-testing (assuming everyone believes 
> it fixes the issue)

There are several bugs in that function (the remaining size calculations
are plain wrong), and Samuels patch sadly doesn't address all of them. 

I will throw a different patch together addressing all the issues I can
spot.

~Andrew

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