From: Will Deacon <will@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
parri.andrea@gmail.com, peterz@infradead.org,
boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com,
j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com,
dlustig@nvidia.com, joel@joelfernandes.org,
viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org,
linux-arch@vger.kernel.org
Subject: Re: Litmus test for question from Al Viro
Date: Mon, 5 Oct 2020 09:20:03 +0100 [thread overview]
Message-ID: <20201005082002.GA23216@willie-the-truck> (raw)
In-Reply-To: <20201005023846.GA359428@rowland.harvard.edu>
On Sun, Oct 04, 2020 at 10:38:46PM -0400, Alan Stern wrote:
> 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@rowland.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@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);
> > +}
> > +
> > +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);
(r2?)
> WRITE_ONCE(*x, r2);
> }
>
> Other than that, this is fine.
But yes, module the typo, I agree that this rewrite is much better than the
proposal above. The definition of control dependencies on arm64 (per the Arm
ARM [1]) isn't entirely clear that it provides order if the WRITE is
executed on both paths of the branch, and I believe there are ongoing
efforts to try to tighten that up. I'd rather keep _that_ topic separate
from the "bug in herd" topic to avoid extra confusion.
Will
[1] https://developer.arm.com/documentation/ddi0487/fc/
next prev parent reply other threads:[~2020-10-05 8:20 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-01 4:51 Litmus test for question from Al Viro Paul E. McKenney
2020-10-01 16:15 ` Alan Stern
2020-10-01 16:36 ` Al Viro
2020-10-01 18:39 ` Alan Stern
2020-10-01 19:29 ` Al Viro
2020-10-01 21:30 ` Paul E. McKenney
2020-10-03 2:01 ` Alan Stern
2020-10-03 13:22 ` Alan Stern
2020-10-03 15:16 ` Akira Yokosawa
2020-10-03 17:13 ` Bug in herd7 [Was: Re: Litmus test for question from Al Viro] Alan Stern
2020-10-03 22:50 ` Akira Yokosawa
2020-10-04 1:40 ` [PATCH] tools: memory-model: Document that the LKMM can easily miss control dependencies Alan Stern
2020-10-04 21:07 ` joel
2020-10-04 23:12 ` Paul E. McKenney
2020-10-05 15:15 ` Bug in herd7 [Was: Re: Litmus test for question from Al Viro] Luc Maranget
2020-10-05 15:53 ` Alan Stern
2020-10-05 16:52 ` Paul E. McKenney
2020-10-05 18:19 ` Alan Stern
2020-10-05 19:18 ` Paul E. McKenney
2020-10-05 19:48 ` Alan Stern
2020-10-06 16:39 ` Paul E. McKenney
2020-10-06 17:05 ` Alan Stern
2020-10-07 17:50 ` Paul E. McKenney
2020-10-07 19:40 ` Alan Stern
2020-10-07 22:38 ` Paul E. McKenney
2020-10-08 2:25 ` Alan Stern
2020-10-08 2:50 ` Paul E. McKenney
2020-10-08 14:01 ` Alan Stern
2020-10-08 18:32 ` Paul E. McKenney
2020-10-05 15:54 ` Paul E. McKenney
2020-10-04 23:31 ` Litmus test for question from Al Viro Paul E. McKenney
2020-10-05 2:38 ` Alan Stern
2020-10-05 8:20 ` Will Deacon [this message]
2020-10-05 9:12 ` Will Deacon
2020-10-05 14:01 ` Paul E. McKenney
2020-10-05 14:23 ` Alan Stern
2020-10-05 15:13 ` Will Deacon
2020-10-05 15:16 ` Alan Stern
2020-10-05 15:35 ` Peter Zijlstra
2020-10-05 15:49 ` Paul E. McKenney
2020-10-05 14:16 ` Alan Stern
2020-10-05 14:03 ` Paul E. McKenney
2020-10-05 14:24 ` Alan Stern
2020-10-05 14:44 ` joel
2020-10-05 15:55 ` Paul E. McKenney
2020-10-05 8:36 ` David Laight
2020-10-05 13:59 ` Paul E. McKenney
2020-10-03 16:08 ` joel
2020-10-03 16:11 ` joel
2020-10-04 23:13 ` Paul E. McKenney
2020-10-03 2:35 ` Jon Masters
2020-10-04 23:32 ` Paul E. McKenney
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