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Message-ID: <20201007175040.GQ29330@paulmck-ThinkPad-P72>
Date: Wed, 7 Oct 2020 10:50:40 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Luc Maranget <luc.maranget@...ia.fr>,
Akira Yokosawa <akiyks@...il.com>, parri.andrea@...il.com,
will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
dlustig@...dia.com, joel@...lfernandes.org,
viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
linux-arch@...r.kernel.org
Subject: Re: Bug in herd7 [Was: Re: Litmus test for question from Al Viro]
On Tue, Oct 06, 2020 at 01:05:25PM -0400, Alan Stern wrote:
> On Tue, Oct 06, 2020 at 09:39:54AM -0700, Paul E. McKenney wrote:
> > On Mon, Oct 05, 2020 at 03:48:34PM -0400, Alan Stern wrote:
> > > On Mon, Oct 05, 2020 at 12:18:01PM -0700, Paul E. McKenney wrote:
> > > > Aside from naming and comment, how about my adding the following?
> > > >
> > > > Thanx, Paul
> > > >
> > > > ------------------------------------------------------------------------
> > > >
> > > > C crypto-control-data-1
> > >
> > > Let's call it something more along the lines of
> > > dependencies-in-nested-expressions. Maybe you can think of something a
> > > little more succinct, but that's the general idea of the test.
> > >
> > > > (*
> > > > * LB plus crypto-mb-data plus data.
> > >
> > > The actual pattern is LB+mb+data.
> > >
> > > > *
> > > > * Result: Never
> > > > *
> > > > * This is an example of OOTA and we would like it to be forbidden.
> > > > * If you want herd7 to get the right answer, you must use herdtools
> > > > * 0f3f8188a326 (" [herd] Fix dependency definition") or later.
> > >
> > > 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)
> > > > {
> > > > int r2;
> > >
> > > No need for r2.
> >
> > Thank you for looking this over!
> >
> > Like this, then?
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit 51898676302d8ebc93856209f7c587f1ac0fdd11
> > Author: Alan Stern <stern@...land.harvard.edu>
> > Date: Tue Oct 6 09:38:37 2020 -0700
> >
> > manual/kernel: Add LB+mb+data litmus test
> >
> > 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.
>
> Shouldn't the commit message be different from the actual contents of
> the update? It's supposed to explain why the update was made, not just
> say what it does. How about this:
>
> 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 fixed an oversight which caused such dependencies
> to be missed.
Much better, thank you! I added "in herdtools" just in case someone was
confused enough to look for this commit in the Linux kernel or some such.
Which I should have done more explicitly in the original, to be sure.
> > Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> >
> > diff --git a/manual/kernel/C-LB+mb+data.litmus b/manual/kernel/C-LB+mb+data.litmus
> > new file mode 100644
> > index 0000000..673eec9
> > --- /dev/null
> > +++ b/manual/kernel/C-LB+mb+data.litmus
> > @@ -0,0 +1,29 @@
> > +C LB+mb+data.litmus
>
> Do you normally put ".litmus" at the end of test names? I leave it out,
> including it only in the filename.
No, I don't, and thank you for catching this.
> > +(*
> > + * LB plus crypto-mb-data plus data.
>
> As I said earlier, the actual pattern is LB+mb+data. There's nothing
> "crypto" about this litmus test (for example, no control dependencies).
>
> Besides, it hardly seems worthwhile making the first comment line a
> repeat of the test name immediately above it. Just leave it out.
Done! ;-)
> > + *
> > + * 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)
>
> Otherwise okay.
And here is the updated version.
Thanx, Paul
------------------------------------------------------------------------
commit b7cd60d4b41ad56b32b36b978488f509c4f7e228
Author: Alan Stern <stern@...land.harvard.edu>
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@...land.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
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.
+ *)
+
+{}
+
+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)
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