Re: [PATCH 0/2] two atomic_cmpxchg() related fixes

[Cornelia Huck]

On Fri, 3 Jul 2020 15:37:11 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Wed, 1 Jul 2020 14:06:11 +0200
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> 
> > 
> > 
> > On 01.07.20 14:01, Cornelia Huck wrote:  
> > > On Tue, 16 Jun 2020 06:50:33 +0200
> > > Halil Pasic <pasic@linux.ibm.com> wrote:
> > >   
> > >> The story short: compiler can generate code that does two
> > >> distinct fetches of *ind_addr for old and _old. If that happens we can
> > >> not figure out if we had the desired xchg or not.
> > >>
> > >> Halil Pasic (2):
> > >>   virtio-ccw: fix virtio_set_ind_atomic
> > >>   s390x/pci: fix set_ind_atomic
> > >>
> > >>  hw/s390x/s390-pci-bus.c | 16 +++++++++-------
> > >>  hw/s390x/virtio-ccw.c   | 18 ++++++++++--------
> > >>  2 files changed, 19 insertions(+), 15 deletions(-)
> > >>
> > >>
> > >> base-commit: 7d3660e79830a069f1848bb4fa1cdf8f666424fb  
> > > 
> > > Have we managed to reach any kind of agreement on this? (A v2?)
> > > 
> > > We can still get in fixes post-softfreeze, of course.  
> > 
> > Unless Halil has a v2 ready, 
> > I think the current patch is fine as is as a fix. I would suggest
> > to go with that and we can then do more beautification later when
> > necessary.
> > 
> >   
> 
> I think we were waiting for Paolo to chime in regarding the barrier().
> The thing I could beautify is using atomic_read(), but frankly I'm not
> sure about it, especially considering multi-proccess. My understanding of
> volatile is better than my understanding of atomic_read(). On the other
> hand, same multi-process question can be asked about atomic_cmpxchg()
> and __atomic_compare_exchange_n()...

We can just improve the code on top later. Having something that
works now beats something not yet developed that might be nicer :)

(But yes, it also might be good to look at non-s390 examples of this
pattern and check if something can be done in a more central place.)