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

[Cornelia Huck]

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.

> 
> 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 :)