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

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Message-ID: <20201007194050.GC468921@rowland.harvard.edu>
Date:   Wed, 7 Oct 2020 15:40:50 -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 Wed, Oct 07, 2020 at 10:50:40AM -0700, Paul E. McKenney wrote:
> And here is the updated version.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit b7cd60d4b41ad56b32b36b978488f509c4f7e228
> Author: Alan Stern <stern@...land.harvard.edu>
> Date:   Tue Oct 6 09:38:37 2020 -0700
> 
>     manual/kernel: Add LB+mb+data litmus test

Let's change this to:

      manual/kernel: Add LB data dependency test with no intermediate variable

Without that extra qualification, people reading just the title would
wonder why we need a simple LB litmus test in the archive.

>     
>     Test whether herd7 can detect a data dependency when there is no
>     intermediate local variable, as in WRITE_ONCE(*x, READ_ONCE(*y)).
>     Commit 0f3f8188a326 in herdtools fixed an oversight which caused such
>     dependencies to be missed.
>     
>     Signed-off-by: Alan Stern <stern@...land.harvard.edu>
>     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> diff --git a/manual/kernel/C-LB+mb+data.litmus b/manual/kernel/C-LB+mb+data.litmus
> new file mode 100644
> index 0000000..0cf9a7a
> --- /dev/null
> +++ b/manual/kernel/C-LB+mb+data.litmus
> @@ -0,0 +1,27 @@
> +C LB+mb+data
> +(*
> + * Result: Never
> + *
> + * Test whether herd7 can detect a data dependency when there is no
> + * intermediate local variable, as in WRITE_ONCE(*x, READ_ONCE(*y)).
> + * Commit 0f3f8188a326 in herdtools fixed an oversight which caused such
> + * dependencies to be missed.

You changed this comment!  It should have remained the way it was:

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

Alan