On Wed, Oct 07, 2020 at 10:50:40AM -0700, Paul E. McKenney wrote:
> And here is the updated version.
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit b7cd60d4b41ad56b32b36b978488f509c4f7e228
> Author: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Date: Tue Oct 6 09:38:37 2020 -0700
>
> manual/kernel: Add LB+mb+data litmus test
Let's change this to:
manual/kernel: Add LB data dependency test with no intermediate variable
Without that extra qualification, people reading just the title would
wonder why we need a simple LB litmus test in the archive.
>
> Test whether herd7 can detect a data dependency when there is no
> intermediate local variable, as in WRITE_ONCE(*x, READ_ONCE(*y)).
> Commit 0f3f8188a326 in herdtools fixed an oversight which caused such
> dependencies to be missed.
>
> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> diff --git a/manual/kernel/C-LB+mb+data.litmus b/manual/kernel/C-LB+mb+data.litmus
> new file mode 100644
> index 0000000..0cf9a7a
> --- /dev/null
> +++ b/manual/kernel/C-LB+mb+data.litmus
> @@ -0,0 +1,27 @@
> +C LB+mb+data
> +(*
> + * Result: Never
> + *
> + * Test whether herd7 can detect a data dependency when there is no
> + * intermediate local variable, as in WRITE_ONCE(*x, READ_ONCE(*y)).
> + * Commit 0f3f8188a326 in herdtools fixed an oversight which caused such
> + * dependencies to be missed.
You changed this comment! It should have remained the way it was:
+ * Versions of herd7 prior to commit 0f3f8188a326 ("[herd] Fix dependency
+ * definition") recognize data dependencies only when they flow through
+ * an intermediate local variable. Since the dependency in P1 doesn't,
+ * those versions get the wrong answer for this test.
> + *)
> +
> +{}
> +
> +P0(int *x, int *y)
> +{
> + int r1;
> +
> + r1 = READ_ONCE(*x);
> + smp_mb();
> + WRITE_ONCE(*y, r1);
> +}
> +
> +P1(int *x, int *y)
> +{
> + WRITE_ONCE(*x, READ_ONCE(*y));
> +}
> +
> +exists (0:r1=1)
Alan
