On Wed, Oct 07, 2020 at 10:25:37PM -0400, Alan Stern wrote:
> On Wed, Oct 07, 2020 at 03:38:51PM -0700, Paul E. McKenney wrote:
> > On Wed, Oct 07, 2020 at 03:40:50PM -0400, Alan Stern wrote:
> > > 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.
>
> > I might get this right sooner or later. You never know.
> >
> > Like this?
> >
> > Thanx, Paul
>
> Paul, I think you must need new reading glasses. You completely missed
> the text above.
There are some distractions at the moment.
Please see below. If this is not exactly correct, I will use "git rm"
and let you submit the patch as you wish.
Thanx, Paul
------------------------------------------------------------------------
commit dc0119c24b64f9d541b94ba5d17eec0cbc265bfa
Author: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Date: Tue Oct 6 09:38:37 2020 -0700
manual/kernel: Add LB data dependency test with no intermediate variable
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..e9e24e0
--- /dev/null
+++ b/manual/kernel/C-LB+mb+data.litmus
@@ -0,0 +1,27 @@
+C LB+mb+data
+(*
+ * Result: Never
+ *
+ * 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)
