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/