From: "Paul E. McKenney" <paulmck@kernel.org>
To: joel@joelfernandes.org
Cc: Alan Stern <stern@rowland.harvard.edu>,
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, luc.maranget@inria.fr, dlustig@nvidia.com,
viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org,
linux-arch@vger.kernel.org
Subject: Re: [PATCH] tools: memory-model: Document that the LKMM can easily miss control dependencies
Date: Sun, 4 Oct 2020 16:12:25 -0700 [thread overview]
Message-ID: <20201004231225.GN29330@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20201004210747.GA4078883@google.com>
On Sun, Oct 04, 2020 at 05:07:47PM -0400, joel@joelfernandes.org wrote:
> On Sat, Oct 03, 2020 at 09:40:22PM -0400, Alan Stern wrote:
> > Add a small section to the litmus-tests.txt documentation file for
> > the Linux Kernel Memory Model explaining that the memory model often
> > fails to recognize certain control dependencies.
> >
> > Suggested-by: Akira Yokosawa <akiyks@gmail.com>
> > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
>
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Good addition! Applied, and thank you all!!!
Thanx, Paul
> thanks,
>
> - Joel
>
> >
> > ---
> >
> > tools/memory-model/Documentation/litmus-tests.txt | 17 +++++++++++++++++
> > 1 file changed, 17 insertions(+)
> >
> > Index: usb-devel/tools/memory-model/Documentation/litmus-tests.txt
> > ===================================================================
> > --- usb-devel.orig/tools/memory-model/Documentation/litmus-tests.txt
> > +++ usb-devel/tools/memory-model/Documentation/litmus-tests.txt
> > @@ -946,6 +946,23 @@ Limitations of the Linux-kernel memory m
> > carrying a dependency, then the compiler can break that dependency
> > by substituting a constant of that value.
> >
> > + Conversely, LKMM sometimes doesn't recognize that a particular
> > + optimization is not allowed, and as a result, thinks that a
> > + dependency is not present (because the optimization would break it).
> > + The memory model misses some pretty obvious control dependencies
> > + because of this limitation. A simple example is:
> > +
> > + r1 = READ_ONCE(x);
> > + if (r1 == 0)
> > + smp_mb();
> > + WRITE_ONCE(y, 1);
> > +
> > + There is a control dependency from the READ_ONCE to the WRITE_ONCE,
> > + even when r1 is nonzero, but LKMM doesn't realize this and thinks
> > + that the write may execute before the read if r1 != 0. (Yes, that
> > + doesn't make sense if you think about it, but the memory model's
> > + intelligence is limited.)
> > +
> > 2. Multiple access sizes for a single variable are not supported,
> > and neither are misaligned or partially overlapping accesses.
> >
next prev parent reply other threads:[~2020-10-04 23:12 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 [this message]
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
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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