linux-kernel - Re: Litmus test for question from Al Viro

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Date:   Mon, 5 Oct 2020 07:03:53 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     parri.andrea@...il.com, will@...nel.org, peterz@...radead.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com,
        dlustig@...dia.com, joel@...lfernandes.org,
        viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: Litmus test for question from Al Viro

On Sun, Oct 04, 2020 at 10:38:46PM -0400, Alan Stern wrote:
> On Sun, Oct 04, 2020 at 04:31:46PM -0700, Paul E. McKenney wrote:
> > Nice simple example!  How about like this?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit c964f404eabe4d8ce294e59dda713d8c19d340cf
> > Author: Alan Stern <stern@...land.harvard.edu>
> > Date:   Sun Oct 4 16:27:03 2020 -0700
> > 
> >     manual/kernel: Add a litmus test with a hidden dependency
> >     
> >     This commit adds a litmus test that has a data dependency that can be
> >     hidden by control flow.  In this test, both the taken and the not-taken
> >     branches of an "if" statement must be accounted for in order to properly
> >     analyze the litmus test.  But herd7 looks only at individual executions
> >     in isolation, so fails to see the dependency.
> >     
> >     Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> >     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > 
> > diff --git a/manual/kernel/crypto-control-data.litmus b/manual/kernel/crypto-control-data.litmus
> > new file mode 100644
> > index 0000000..6baecf9
> > --- /dev/null
> > +++ b/manual/kernel/crypto-control-data.litmus
> > @@ -0,0 +1,31 @@
> > +C crypto-control-data
> > +(*
> > + * LB plus crypto-control-data plus data
> > + *
> > + * Result: Sometimes
> > + *
> > + * This is an example of OOTA and we would like it to be forbidden.
> > + * The WRITE_ONCE in P0 is both data-dependent and (at the hardware level)
> > + * control-dependent on the preceding READ_ONCE.  But the dependencies are
> > + * hidden by the form of the conditional control construct, hence the 
> > + * name "crypto-control-data".  The memory model doesn't recognize them.
> > + *)
> > +
> > +{}
> > +
> > +P0(int *x, int *y)
> > +{
> > +	int r1;
> > +
> > +	r1 = 1;
> > +	if (READ_ONCE(*x) == 0)
> > +		r1 = 0;
> > +	WRITE_ONCE(*y, r1);
> > +}
> > +
> > +P1(int *x, int *y)
> > +{
> > +	WRITE_ONCE(*x, READ_ONCE(*y));
> > +}
> > +
> > +exists (0:r1=1)
> 
> Considering the bug in herd7 pointed out by Akira, we should rewrite P1 as:
> 
> P1(int *x, int *y)
> {
> 	int r2;
> 
> 	r = READ_ONCE(*y);
> 	WRITE_ONCE(*x, r2);
> }
> 
> Other than that, this is fine.

Updated as suggested by Will, like this?

							Thanx, Paul

------------------------------------------------------------------------

commit adf43667b702582331d68acdf3732a6a017a182c
Author: Alan Stern <stern@...land.harvard.edu>
Date:   Sun Oct 4 16:27:03 2020 -0700

    manual/kernel: Add a litmus test with a hidden dependency
    
    This commit adds a litmus test that has a data dependency that can be
    hidden by control flow.  In this test, both the taken and the not-taken
    branches of an "if" statement must be accounted for in order to properly
    analyze the litmus test.  But herd7 looks only at individual executions
    in isolation, so fails to see the dependency.
    
    Signed-off-by: Alan Stern <stern@...land.harvard.edu>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/manual/kernel/crypto-control-data.litmus b/manual/kernel/crypto-control-data.litmus
new file mode 100644
index 0000000..cdcdec9
--- /dev/null
+++ b/manual/kernel/crypto-control-data.litmus
@@ -0,0 +1,34 @@
+C crypto-control-data
+(*
+ * LB plus crypto-control-data plus data
+ *
+ * Result: Sometimes
+ *
+ * This is an example of OOTA and we would like it to be forbidden.
+ * The WRITE_ONCE in P0 is both data-dependent and (at the hardware level)
+ * control-dependent on the preceding READ_ONCE.  But the dependencies are
+ * hidden by the form of the conditional control construct, hence the 
+ * name "crypto-control-data".  The memory model doesn't recognize them.
+ *)
+
+{}
+
+P0(int *x, int *y)
+{
+	int r1;
+
+	r1 = 1;
+	if (READ_ONCE(*x) == 0)
+		r1 = 0;
+	WRITE_ONCE(*y, r1);
+}
+
+P1(int *x, int *y)
+{
+	int r2;
+
+	r2 = READ_ONCE(*y);
+	WRITE_ONCE(*x, r2);
+}
+
+exists (0:r1=1)