Hi,
As far as I can remember I have implemented atomic_add_unless in herd7.
As to your test, I have first run a slightly modified version of your test
as a kernel module (using klitmus7).
C atomic_add_unless-dependency
{
atomic_t y = ATOMIC_INIT(1);
}
P0(int *x, atomic_t *y, int *z)
{
int r0;
r0 = READ_ONCE(*x);
if (atomic_add_unless((atomic_t *)y, 2, r0))
WRITE_ONCE(*z, 42);
else
WRITE_ONCE(*z, 1);
}
P1(int *x, int *z)
{
int r0;
r0 = smp_load_acquire(z);
WRITE_ONCE(*x, 1);
}
locations [y]
exists
(1:r0 = 1 /\ 0:r0 = 1)
The test is also accepted by herd7, here producing teh same final values
as actual run on a raspberry PI4B.
--Luc
> 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@xxxxxxxxxxxxxxxxxx
> > >
> >
> > 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
