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

[Boqun Feng <boqun.feng@gmail.com>]

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