Re: Litmus test for question from Al Viro

[Will Deacon]

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);

(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/