Re: [PULL 13/14] virtio-ccw: fix virtio_set_ind_atomic

[Cornelia Huck]

On Mon, 6 Jul 2020 13:23:11 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Fri,  3 Jul 2020 12:06:49 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > From: Halil Pasic <pasic@linux.ibm.com>
> > 
> > The atomic_cmpxchg() loop is broken because we occasionally end up with
> > old and _old having different values (a legit compiler can generate code
> > that accessed *ind_addr again to pick up a value for _old instead of
> > using the value of old that was already fetched according to the
> > rules of the abstract machine). This means the underlying CS instruction
> > may use a different old (_old) than the one we intended to use if
> > atomic_cmpxchg() performed the xchg part.
> > 
> > Let us use volatile to force the rules of the abstract machine for
> > accesses to *ind_addr. Let us also rewrite the loop so, we that the  
> 
> Michael T. Has pointed out that this sentence is ungrammatical. 
> 
> s/we// would IMHO solve the problem. Can we fix this before it gets
> merged?

Unfortunately, it's already too late :(

> 
> > new old is used to compute the new desired value if the xchg part
> > is not performed.