Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.12.2020 15:44, Julien Grall wrote:
> On 23/12/2020 13:41, Jan Beulich wrote:
>> On 23.12.2020 14:33, Julien Grall wrote:
>>> On 23/12/2020 13:12, Jan Beulich wrote:
>>>>  From the input by both of you I still can't
>>>> conclude whether this patch should remain as is in v4, or revert
>>>> back to its v2 version. Please can we get this settled so I can get
>>>> v4 out?
>>>
>>> I haven't had time to investigate the rest of the VM event code to find
>>> other cases where this may happen. I still think there is a bigger
>>> problem in the VM event code, but the maintainer disagrees here.
>>>
>>> At which point, I see limited reason to try to paper over in the common
>>> code. So I would rather ack/merge v2 rather than v3.
>>
>> Since I expect Tamas and/or the Bitdefender folks to be of the
>> opposite opinion, there's still no way out, at least if "rather
>> ack" implies a nak for v3.
> 
> The only way out here is for someone to justify why this patch is 
> sufficient for the VM event race.

I think this is too much you demand. Moving in the right direction
without arriving at the final destination is still a worthwhile
thing to do imo.

>> Personally, if this expectation of
>> mine is correct, I'd prefer to keep the accounting but make it
>> optional (as suggested in a post-commit-message remark).
>> That'll eliminate the overhead you appear to be concerned of,
>> but of course it'll further complicate the logic (albeit just
>> slightly).
> 
> I am more concerned about adding over complex code that would just 
> papering over a bigger problem. I also can't see use of it outside of 
> the VM event discussion.

I think it is a generally appropriate thing to do to wait for
callbacks to complete before tearing down their origin control
structure. There may be cases where code structure makes this
unnecessary, but I don't think this can be an expectation to
all the users of the functionality. Hence my suggestion to
possibly make this optional (driven directly or indirectly by
the user of the registration function).

Jan