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

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Message-ID: <20201005023846.GA359428@rowland.harvard.edu>
Date:   Sun, 4 Oct 2020 22:38:46 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 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.

Alan