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

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Date:   Mon, 5 Oct 2020 08:54:42 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Luc Maranget <luc.maranget@...ia.fr>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        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 05:15:57PM +0200, Luc Maranget wrote:
> > On Sun, Oct 04, 2020 at 12:16:31AM +0900, Akira Yokosawa wrote:
> > > Hi Alan,
> > > 
> > > Just a minor nit in the litmus test.
> > > 
> > > On Sat, 3 Oct 2020 09:22:12 -0400, Alan Stern wrote:
> > > > To expand on my statement about the LKMM's weakness regarding control 
> > > > constructs, here is a litmus test to illustrate the issue.  You might 
> > > > want to add this to one of the archives.
> > > > 
> > > > Alan
> > > > 
> > > > C crypto-control-data
> > > > (*
> > > >  * LB plus crypto-control-data plus data
> > > >  *
> > > >  * Expected result: allowed
> > > >  *
> > > >  * 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));
> > > 
> > > Looks like this one-liner doesn't provide data-dependency of y -> x on herd7.
> > 
> > You're right.  This is definitely a bug in herd7.
> > 
> > Luc, were you aware of this?
> 
> Hi Alan,
> 
> No I was not aware of it. Now I am, the bug is normally fixed in the master branch of herd git deposit.
> <https://github.com/herd/herdtools7/commit/0f3f8188a326d5816a82fb9970fcd209a2678859>
> 
> Thanks for the report.

Thank you very much, Luc!  I will rebuild and give it a try.

							Thanx, Paul