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From:   =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@gmail.com>
Date:   Tue, 2 Mar 2021 21:24:04 +0100
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Subject: Re: XDP socket rings, and LKMM litmus tests
To:     paulmck@kernel.org
Cc:     bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        stern@rowland.harvard.edu, parri.andrea@gmail.com,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, boqun.feng@gmail.com,
        npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk,
        luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com,
        joel@joelfernandes.org,
        =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>,
        "Karlsson, Magnus" <magnus.karlsson@intel.com>
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On Tue, 2 Mar 2021 at 20:57, Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Tue, Mar 02, 2021 at 07:46:27PM +0100, Bj=C3=B6rn T=C3=B6pel wrote:

[...]

>
> Before digging in too deeply, does the following simplification
> still capture your intent?
>

Thanks for having a look, Paul!

> P0(int *prod, int *cons, int *data)
> {
>     int p;
>     int cond =3D 0;
>
>     p =3D READ_ONCE(*prod);
>     if (p =3D=3D READ_ONCE(*cons))
>             cond =3D 1;

With this, yes!

>     if (cond) {
>         smp_mb();
>         WRITE_ONCE(*data, 1);
>         smp_wmb();
>         WRITE_ONCE(*prod, p ^ 1);
>     }
> }
>
> P1(int *prod, int *cons, int *data)
> {
>     int c;
>     int d =3D -1;
>     int cond =3D 0;
>
>     c =3D READ_ONCE(*cons);
>     if (READ_ONCE(*prod) =3D=3D c)
>             cond =3D 1;

Hmm, this would not be the correct state transition.

c=3D=3D1 && p=3D=3D1 would set cond to 1, right?

I would agree with:
  c =3D READ_ONCE(*cons);
  if (READ_ONCE(*prod) !=3D c)


>
>     if (cond =3D=3D 1) {
>         smp_rmb();
>         d =3D READ_ONCE(*data);
>         smp_mb();
>         WRITE_ONCE(*cons, c ^ 1);
>     }
> }
>
>                                                         Thanx, Paul
>

[...]

Bj=C3=B6rn