Re: [qemu-s390x] [Qemu-devel] [PATCH v3 1/6] vfio-ccw: make it safe to access channel programs

[Farhan Ali]

On 02/05/2019 09:48 AM, Eric Farman wrote:
> On 02/05/2019 07:35 AM, Cornelia Huck wrote:
> > On Tue, 5 Feb 2019 12:52:29 +0100
> > Halil Pasic <address@hidden> wrote:
> >
> > > On Mon, 4 Feb 2019 16:31:02 +0100
> > > Cornelia Huck <address@hidden> wrote:
> > >
> > > > On Thu, 31 Jan 2019 13:34:55 +0100
> > > > Halil Pasic <address@hidden> wrote:
> > > > > On Thu, 31 Jan 2019 12:52:20 +0100
> > > > > Cornelia Huck <address@hidden> wrote:
> > > > > > On Wed, 30 Jan 2019 19:51:27 +0100
> > > > > > Halil Pasic <address@hidden> wrote:
> > > > > > > On Wed, 30 Jan 2019 14:22:07 +0100
> > > > > > > Cornelia Huck <address@hidden> 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

I ran the my workloads/tests with the patches and like Eric I notice the 
errors I previously hit less frequently.


> > > 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 :)