Re: XDP socket rings, and LKMM litmus tests

Björn Töpel
2021-03-02T22:37:55Z

On Tue, 2 Mar 2021 at 21:41, Paul E. McKenney <paulmck@kernel.org> wrote:
>
> 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.
>

Interesting!

I tried the acquire/release version:

P0(int *prod, int *cons, int *data)
{
int p;
int cond = 0;

p = READ_ONCE(*prod);
if (p == READ_ONCE(*cons)) {
WRITE_ONCE(*data, 1);
smp_store_release(prod, p ^ 1);
}
}

P1(int *prod, int *cons, int *data)
{
int c;
int d = -1;

c = READ_ONCE(*cons);
if (smp_load_acquire(prod) != c) {
d = READ_ONCE(*data);
smp_store_release(cons, c ^ 1);
}
}

and as with the previous example, restructuring the if-statement makes
"if (p == READ_ONCE(*cons)) {" sufficient, instead of "if (p ==
smp_load_acquire(cons)) {".

Yay!

Björn

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