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SubjectRe: [RFC 2/3] tools/memory-model: Add a litmus test for atomic_set()
Luc,

Could you have a look at the problem Andrea and I discuss here? It seems
that you have done a few things in herd for atomic_add_unless() in
particular, and based on the experiments of Andrea and me, seems
atomic_add_unless() works correctly. So can you confirm that herd now
can handle atomic_add_unless() or there is still something missing?

Thanks!

Regards,
Boqun

On Fri, Feb 14, 2020 at 06:40:03PM +0800, Boqun Feng wrote:
> On Fri, Feb 14, 2020 at 09:12:13AM +0100, Andrea Parri wrote:
> > > @@ -0,0 +1,24 @@
> > > +C Atomic-set-observable-to-RMW
> > > +
> > > +(*
> > > + * Result: Never
> > > + *
> > > + * Test of the result of atomic_set() must be observable to atomic RMWs.
> > > + *)
> > > +
> > > +{
> > > + atomic_t v = ATOMIC_INIT(1);
> > > +}
> > > +
> > > +P0(atomic_t *v)
> > > +{
> > > + (void)atomic_add_unless(v,1,0);
> >
> > We blacklisted this primitive some time ago, cf. section "LIMITATIONS",
> > entry (6b) in tools/memory-model/README; the discussion was here:
> >
> > https://lkml.kernel.org/r/20180829211053.20531-3-paulmck@linux.vnet.ibm.com
> >
>
> And in an email replying to that email, you just tried and seemed
> atomic_add_unless() works ;-)
>
> > but unfortunately I can't remember other details at the moment: maybe
> > it is just a matter of or the proper time to update that section.
> >
>
> I spend a few time looking into the changes in herd, the dependency
> problem seems to be as follow:
>
> For atomic_add_unless(ptr, a, u), the return value (true or false)
> depends on both *ptr and u, this is different than other atomic RMW,
> whose return value only depends on *ptr. Considering the following
> litmus test:
>
> C atomic_add_unless-dependency
>
> {
> int y = 1;
> }
>
> P0(int *x, int *y, int *z)
> {
> int r0;
> int r1;
> int r2;
>
> r0 = READ_ONCE(*x);
> if (atomic_add_unless(y, 2, r0))
> WRITE_ONCE(*z, 42);
> else
> WRITE_ONCE(*z, 1);
> }
>
> P1(int *x, int *y, int *z)
> {
> int r0;
>
> r0 = smp_load_acquire(z);
>
> WRITE_ONCE(*x, 1);
> }
>
> exists
> (1:r0 = 1 /\ 0:r0 = 1)
>
> , the exist-clause will never trigger, however if we replace
> "atomic_add_unless(y, 2, r0)" with "atomic_add_unless(y, 2, 1)", the
> write on *z and the read from *x on CPU 0 are not ordered, so we could
> observe the exist-clause triggered.
>
> I just tried with the latest herd, and herd can work out this
> dependency. So I think we are good now and can change the limitation
> section in the document. But I will wait for Luc's input for this. Luc,
> did I get this correct? Is there any other limitation on
> atomic_add_unless() now?
>
> Regards,
> Boqun
>
> > Thanks,
> > Andrea

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