Re: XDP socket rings, and LKMM litmus tests

[Björn Töpel]

On Tue, 2 Mar 2021 at 20:57, Paul E. McKenney <paulmck@xxxxxxxxxx> 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