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.
>
> >
> > 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:
I might get this right sooner or later. You never know.
Like this?
Thanx, Paul
------------------------------------------------------------------------
commit 5b6a4ff2c8ad25fc77f4151e71e6cbd8f3268d7b
Author: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Date: Tue Oct 6 09:38:37 2020 -0700
manual/kernel: Add LB+mb+data litmus test
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)
