linux-kernel - Re: XDP socket rings, and LKMM litmus tests

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Date:   Tue, 2 Mar 2021 21:24:04 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     paulmck@...nel.org
Cc:     bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        stern@...land.harvard.edu, parri.andrea@...il.com,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com, dlustig@...dia.com,
        joel@...lfernandes.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: XDP socket rings, and LKMM litmus tests

On Tue, 2 Mar 2021 at 20:57, Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Tue, Mar 02, 2021 at 07:46:27PM +0100, Björn Töpel 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 = 0;
>
>     p = READ_ONCE(*prod);
>     if (p == READ_ONCE(*cons))
>             cond = 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 = -1;
>     int cond = 0;
>
>     c = READ_ONCE(*cons);
>     if (READ_ONCE(*prod) == c)
>             cond = 1;

Hmm, this would not be the correct state transition.

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

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


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

[...]

Björn