Re: Bug in herd7 [Was: Re: Litmus test for question from Al Viro]

["Paul E. McKenney" <paulmck@kernel.org>]

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@rowland.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@rowland.harvard.edu>
> >     Signed-off-by: Paul E. McKenney <paulmck@kernel.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@rowland.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@rowland.harvard.edu>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.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)