Re: [Qemu-devel] [PATCH v3 1/6] vfio-ccw: make it safe to access channel programs

[Eric Farman <farman@linux.ibm.com>]

On 02/05/2019 07:35 AM, Cornelia Huck wrote:
> On Tue, 5 Feb 2019 12:52:29 +0100
> Halil Pasic <pasic@linux.ibm.com> wrote:
> 
>> On Mon, 4 Feb 2019 16:31:02 +0100
>> Cornelia Huck <cohuck@redhat.com> wrote:
>>
>>> On Thu, 31 Jan 2019 13:34:55 +0100
>>> Halil Pasic <pasic@linux.ibm.com> wrote:
>>>    
>>>> On Thu, 31 Jan 2019 12:52:20 +0100
>>>> Cornelia Huck <cohuck@redhat.com> wrote:
>>>>    
>>>>> On Wed, 30 Jan 2019 19:51:27 +0100
>>>>> Halil Pasic <pasic@linux.ibm.com> wrote:
>>>>>      
>>>>>> On Wed, 30 Jan 2019 14:22:07 +0100
>>>>>> Cornelia Huck <cohuck@redhat.com> wrote:
>>>>>>      
>>>>>>> When we get a solicited interrupt, the start function may have
>>>>>>> been cleared by a csch, but we still have a channel program
>>>>>>> structure allocated. Make it safe to call the cp accessors in
>>>>>>> any case, so we can call them unconditionally.
>>>>>>
>>>>>> I read this like it is supposed to be safe regardless of
>>>>>> parallelism and threads. However I don't see any explicit
>>>>>> synchronization done for cp->initialized.
>>>>>>
>>>>>> I've managed to figure out how is that supposed to be safe
>>>>>> for the cp_free() (which is probably our main concern) in
>>>>>> vfio_ccw_sch_io_todo(), but if fail when it comes to the one
>>>>>> in vfio_ccw_mdev_notifier().
>>>>>>
>>>>>> Can you explain us how does the synchronization work?
>>>>>
>>>>> You read that wrong, I don't add synchronization, I just add a check.
>>>>>      
>>>>
>>>> Now I'm confused. Does that mean we don't need synchronization for this?
>>>
>>> If we lack synchronization (that is not provided by the current state
>>> machine handling, or the rework here), we should do a patch on top
>>> (preferably on top of the whole series, so this does not get even more
>>> tangled up.) This is really just about the extra check.
>>>    
>>
>> I'm not a huge fan of keeping or introducing races -- it makes things
>> difficult to reason about, but I do have some understanging your
>> position.
> 
> The only thing I want to avoid is knowingly making things worse than
> before, and I don't think this patch does that.
> 
>>
>> This patch-series is AFAICT a big improvement over what we have. I would
>> like Farhan confirming that it makes these hick-ups when he used to hit
>> BUSY with another ssch request disappear. If it does (I hope it does)
>> it's definitely a good thing for anybody who wants to use vfio-ccw.
> 
> Yep. There remains a lot to be done, but it's a first step.

s/a first step/an excellent first step/  :)

Can't speak for Farhan, but this makes things somewhat better for me. 
I'm still getting some periodic errors, but they happen infrequently 
enough now that debugging them is frustrating.  ;-)

  - Eric

> 
>>
>> Yet I find it difficult to slap my r-b over racy code, or partial
>> solutions. In the latter case, when I lack conceptual clarity, I find it
>> difficult to tell if we are heading into the right direction, or is what
>> we build today going to turn against us tomorrow. Sorry for being a drag.
> 
> As long as we don't introduce bad user space interfaces we have to drag
> around forever, I think anything is fair game if we think it's a good
> idea at that moment. We can rewrite things if it turned out to be a bad
> idea (although I'm not arguing for doing random crap, of course :)
>