Re: XDP socket rings, and LKMM litmus tests

Paul E. McKenney
2021-03-02T22:37:54Z

On Tue, Mar 02, 2021 at 09:24:04PM +0100, Björn Töpel wrote:
> 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)

Right you are!

With that, it looks to me like LKMM is OK with removing the smp_mb().
My guess is that the issue is that LKMM confines the effect of control
dependencies to a single "if" statement, hence my reworking of your
original.

Thanx, Paul

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