Re: XDP socket rings, and LKMM litmus tests

Björn Töpel
2021-03-02T22:37:51Z

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, 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