On Mon, Oct 05, 2020 at 10:44:22AM -0400, joel@xxxxxxxxxxxxxxxxx wrote:
> 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>
Applied, thank you all!
This has been pushed to my github litmus archive.
Thanx, Paul
> 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)
