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

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Date:   Mon, 5 Oct 2020 10:44:22 -0400
From:   joel@...lfernandes.org
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Alan Stern <stern@...land.harvard.edu>, 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,
        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 Mon, Oct 05, 2020 at 07:03:53AM -0700, Paul E. McKenney wrote:
> 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?

LGTM as well,

FWIW:
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel

> 
> 							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)