On Mon, Oct 05, 2020 at 07:03:53AM -0700, Paul E. McKenney wrote:
> On Sun, Oct 04, 2020 at 10:38:46PM -0400, Alan Stern wrote:
> > On Sun, Oct 04, 2020 at 04:31:46PM -0700, Paul E. McKenney wrote:
> > > Nice simple example! How about like this?
> > >
> > > Thanx, Paul
> > >
> > > ------------------------------------------------------------------------
> > >
> > > commit c964f404eabe4d8ce294e59dda713d8c19d340cf
> > > Author: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > > Date: Sun Oct 4 16:27:03 2020 -0700
> > >
> > > manual/kernel: Add a litmus test with a hidden dependency
> > >
> > > This commit adds a litmus test that has a data dependency that can be
> > > hidden by control flow. In this test, both the taken and the not-taken
> > > branches of an "if" statement must be accounted for in order to properly
> > > analyze the litmus test. But herd7 looks only at individual executions
> > > in isolation, so fails to see the dependency.
> > >
> > > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > >
> > > diff --git a/manual/kernel/crypto-control-data.litmus b/manual/kernel/crypto-control-data.litmus
> > > new file mode 100644
> > > index 0000000..6baecf9
> > > --- /dev/null
> > > +++ b/manual/kernel/crypto-control-data.litmus
> > > @@ -0,0 +1,31 @@
> > > +C crypto-control-data
> > > +(*
> > > + * LB plus crypto-control-data plus data
> > > + *
> > > + * Result: Sometimes
> > > + *
> > > + * This is an example of OOTA and we would like it to be forbidden.
> > > + * The WRITE_ONCE in P0 is both data-dependent and (at the hardware level)
> > > + * control-dependent on the preceding READ_ONCE. But the dependencies are
> > > + * hidden by the form of the conditional control construct, hence the
> > > + * name "crypto-control-data". The memory model doesn't recognize them.
> > > + *)
> > > +
> > > +{}
> > > +
> > > +P0(int *x, int *y)
> > > +{
> > > + int r1;
> > > +
> > > + r1 = 1;
> > > + if (READ_ONCE(*x) == 0)
> > > + r1 = 0;
> > > + WRITE_ONCE(*y, r1);
> > > +}
> > > +
> > > +P1(int *x, int *y)
> > > +{
> > > + WRITE_ONCE(*x, READ_ONCE(*y));
> > > +}
> > > +
> > > +exists (0:r1=1)
> >
> > Considering the bug in herd7 pointed out by Akira, we should rewrite P1 as:
> >
> > P1(int *x, int *y)
> > {
> > int r2;
> >
> > r = READ_ONCE(*y);
> > WRITE_ONCE(*x, r2);
> > }
> >
> > Other than that, this is fine.
>
> Updated as suggested by Will, like this?
LGTM as well,
FWIW:
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
thanks,
- Joel
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit adf43667b702582331d68acdf3732a6a017a182c
> Author: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Date: Sun Oct 4 16:27:03 2020 -0700
>
> manual/kernel: Add a litmus test with a hidden dependency
>
> This commit adds a litmus test that has a data dependency that can be
> hidden by control flow. In this test, both the taken and the not-taken
> branches of an "if" statement must be accounted for in order to properly
> analyze the litmus test. But herd7 looks only at individual executions
> in isolation, so fails to see the dependency.
>
> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> diff --git a/manual/kernel/crypto-control-data.litmus b/manual/kernel/crypto-control-data.litmus
> new file mode 100644
> index 0000000..cdcdec9
> --- /dev/null
> +++ b/manual/kernel/crypto-control-data.litmus
> @@ -0,0 +1,34 @@
> +C crypto-control-data
> +(*
> + * LB plus crypto-control-data plus data
> + *
> + * Result: Sometimes
> + *
> + * This is an example of OOTA and we would like it to be forbidden.
> + * The WRITE_ONCE in P0 is both data-dependent and (at the hardware level)
> + * control-dependent on the preceding READ_ONCE. But the dependencies are
> + * hidden by the form of the conditional control construct, hence the
> + * name "crypto-control-data". The memory model doesn't recognize them.
> + *)
> +
> +{}
> +
> +P0(int *x, int *y)
> +{
> + int r1;
> +
> + r1 = 1;
> + if (READ_ONCE(*x) == 0)
> + r1 = 0;
> + WRITE_ONCE(*y, r1);
> +}
> +
> +P1(int *x, int *y)
> +{
> + int r2;
> +
> + r2 = READ_ONCE(*y);
> + WRITE_ONCE(*x, r2);
> +}
> +
> +exists (0:r1=1)
