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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Luc Maranget <luc.maranget@inria.fr>,
	Akira Yokosawa <akiyks@gmail.com>,
	parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org,
	boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com,
	j.alglave@ucl.ac.uk, dlustig@nvidia.com, joel@joelfernandes.org,
	viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org
Subject: Re: Bug in herd7 [Was: Re: Litmus test for question from Al Viro]
Date: Wed, 7 Oct 2020 19:50:25 -0700	[thread overview]
Message-ID: <20201008025025.GX29330@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20201008022537.GA480405@rowland.harvard.edu>

On Wed, Oct 07, 2020 at 10:25:37PM -0400, Alan Stern wrote:
> On Wed, Oct 07, 2020 at 03:38:51PM -0700, Paul E. McKenney wrote:
> > On Wed, Oct 07, 2020 at 03:40:50PM -0400, Alan Stern wrote:
> > > 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@rowland.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.
> 
> > I might get this right sooner or later.  You never know.
> > 
> > Like this?
> > 
> > 							Thanx, Paul
> 
> Paul, I think you must need new reading glasses.  You completely missed 
> the text above.

There are some distractions at the moment.

Please see below.  If this is not exactly correct, I will use "git rm"
and let you submit the patch as you wish.

						Thanx, Paul

------------------------------------------------------------------------

commit dc0119c24b64f9d541b94ba5d17eec0cbc265bfa
Author: Alan Stern <stern@rowland.harvard.edu>
Date:   Tue Oct 6 09:38:37 2020 -0700

    manual/kernel: Add LB data dependency test with no intermediate variable
    
    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@rowland.harvard.edu>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.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..e9e24e0
--- /dev/null
+++ b/manual/kernel/C-LB+mb+data.litmus
@@ -0,0 +1,27 @@
+C LB+mb+data
+(*
+ * Result: Never
+ *
+ * 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)

  reply	other threads:[~2020-10-08  2:50 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 [this message]
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
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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