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

[Halil Pasic <pasic@linux.ibm.com>]

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.

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.

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.

Regards,
Halil