RE: Litmus test for question from Al Viro

[David Laight <David.Laight@ACULAB.COM>]

From: Paul E. McKenney
> Sent: 05 October 2020 00:32
...
>     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@rowland.harvard.edu>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.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);
> +}

Hmmm.... the compiler will semi-randomly transform that to/from:
	if (READ_ONCE(*x) == 0)
		r1 = 0;
	else
		r1 = 1;
and
	r1 = READ_ONCE(*x) != 0;

Both of which (probably) get correctly detected as a write to *y
that is dependant on *x - so is 'problematic' with P1() which
does the opposite assignment.

Which does rather imply that hurd is a bit broken.

	David

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