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

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Date:   Mon, 5 Oct 2020 15:48:34 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>, parri.andrea@...il.com,
        will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        dlustig@...dia.com, joel@...lfernandes.org,
        viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: Bug in herd7 [Was: Re: Litmus test for question from Al Viro]

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.

> 
> 	WRITE_ONCE(*x, READ_ONCE(*y));
> }
> 
> exists (0:r1=1)

Alan