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 <st...@rowland.harvard.edu>
> > 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 <st...@rowland.harvard.edu>
> > Signed-off-by: Paul E. McKenney <paul...@kernel.org>
> >
> > 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);
(r2?)
> WRITE_ONCE(*x, r2);
> }
>
> Other than that, this is fine.
But yes, module the typo, I agree that this rewrite is much better than the
proposal above. The definition of control dependencies on arm64 (per the Arm
ARM [1]) isn't entirely clear that it provides order if the WRITE is
executed on both paths of the branch, and I believe there are ongoing
efforts to try to tighten that up. I'd rather keep _that_ topic separate
from the "bug in herd" topic to avoid extra confusion.
Will
[1] https://developer.arm.com/documentation/ddi0487/fc/
