From: "Paul E. McKenney" <paulmck@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.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, akiyks@gmail.com,
dlustig@nvidia.com, joel@joelfernandes.org, urezki@gmail.com,
quic_neeraju@quicinc.com, frederic@kernel.org,
linux-kernel@vger.kernel.org, lkmm@lists.linux.dev,
hernan.poncedeleon@huaweicloud.com
Subject: Re: [RFC] tools/memory-model: Rule out OOTA
Date: Wed, 23 Jul 2025 12:57:13 -0700 [thread overview]
Message-ID: <9963f6a6-9da9-4120-b1fe-e4a1df9edda1@paulmck-laptop> (raw)
In-Reply-To: <b9c250b2-e4c0-4e8f-b37c-b51d93b980f0@rowland.harvard.edu>
On Wed, Jul 23, 2025 at 03:25:13PM -0400, Alan Stern wrote:
> On Tue, Jul 22, 2025 at 05:43:16PM -0700, Paul E. McKenney wrote:
> > Also, C-JO-OOTA-7.litmus includes a "*r2 = a" statement that makes herd7
> > very unhappy. On the other hand, initializing registers to the address
> > of a variable is straight forward, as shown in the resulting litmus test.
>
> ...
>
> > diff --git a/manual/oota/C-JO-OOTA-7.litmus b/manual/oota/C-JO-OOTA-7.litmus
> > new file mode 100644
> > index 00000000..31c0b8ae
> > --- /dev/null
> > +++ b/manual/oota/C-JO-OOTA-7.litmus
> > @@ -0,0 +1,47 @@
> > +C C-JO-OOTA-7
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * But LKMM finds the all-ones result, due to OOTA on r2.
> > + *
> > + * https://lore.kernel.org/all/1147ad3e-e3ad-4fa1-9a63-772ba136ea9a@huaweicloud.com/
> > + *)
> > +
> > +{
> > + 0:r2=a;
> > + 1:r2=b;
> > +}
>
> In this litmus test a and b are never assigned any values, so they
> always contain 0.
>
> > +
> > +P0(int *a, int *b, int *x, int *y)
> > +{
> > + int r1;
> > + int r2;
> > +
> > + r1 = READ_ONCE(*x);
> > + smp_rmb();
> > + if (r1 == 1) {
> > + r2 = READ_ONCE(*a);
>
> If this executes then r2 now contains 0.
>
> > + }
> > + *r2 = a;
>
> And so what is supposed to happen here? No wonder herd7 is unhappy!
Nothing good, I will admit! Good eyes, and thank you!
> > + smp_wmb();
> > + WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y)
> > +{
> > + int r1;
> > + int r2;
> > +
> > + r1 = READ_ONCE(*y);
> > + smp_rmb();
> > + if (r1 == 1) {
> > + r2 = READ_ONCE(*b);
> > + }
> > + *r2 = b;
>
> Same here.
>
> > + smp_wmb();
> > + WRITE_ONCE(*x, 1);
> > +}
> > +
> > +locations [0:r2;1:r2]
> > +exists (0:r1=1 /\ 1:r1=1)
Yes, I did misinterpret Jonas's initialization advice, which reads
as follows: "unless you know how to initialize *a and *b to valid
addresses, you may need to add something like `if (r2 == 0) r2 = a`
to run this in herd7".
Given that there are two instances of r2, there are a number of
possible combinations of initialization. I picked the one shown
in the patch below, and got this:
$ herd7 -conf linux-kernel.cfg ~/paper/scalability/LWNLinuxMM/litmus/manual/oota/C-JO-OOTA-7.litmus
Test C-JO-OOTA-7 Allowed
States 3
0:r1=0; 0:r2=a; 1:r1=0; 1:r2=b;
0:r1=0; 0:r2=a; 1:r1=1; 1:r2=b;
0:r1=1; 0:r2=a; 1:r1=0; 1:r2=b;
No
Witnesses
Positive: 0 Negative: 3
Flag mixed-accesses
Condition exists (0:r1=1 /\ 1:r1=1)
Observation C-JO-OOTA-7 Never 0 3
Time C-JO-OOTA-7 0.01
Hash=d9bb35335e45b31b1a39bab88eca837c
I get something very similar if I cross-initialize them, that is
a=b;b=a.
Thoughts?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/manual/oota/C-JO-OOTA-7.litmus b/manual/oota/C-JO-OOTA-7.litmus
index 31c0b8ae..d7fe0f94 100644
--- a/manual/oota/C-JO-OOTA-7.litmus
+++ b/manual/oota/C-JO-OOTA-7.litmus
@@ -11,6 +11,8 @@ C C-JO-OOTA-7
{
0:r2=a;
1:r2=b;
+ a=a;
+ b=b;
}
P0(int *a, int *b, int *x, int *y)
prev parent reply other threads:[~2025-07-23 19:57 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-01-06 21:40 [RFC] tools/memory-model: Rule out OOTA Jonas Oberhauser
2025-01-07 10:06 ` Peter Zijlstra
2025-01-07 11:02 ` Jonas Oberhauser
2025-01-07 15:46 ` Jonas Oberhauser
2025-01-07 16:09 ` Alan Stern
2025-01-07 18:47 ` Paul E. McKenney
2025-01-08 17:39 ` Jonas Oberhauser
2025-01-08 18:09 ` Paul E. McKenney
2025-01-08 19:17 ` Jonas Oberhauser
2025-01-09 17:54 ` Paul E. McKenney
2025-01-09 18:35 ` Jonas Oberhauser
2025-01-10 14:54 ` Paul E. McKenney
2025-01-10 16:21 ` Jonas Oberhauser
2025-01-13 22:04 ` Paul E. McKenney
2025-01-16 18:40 ` Paul E. McKenney
2025-01-16 19:13 ` Jonas Oberhauser
2025-01-16 19:31 ` Paul E. McKenney
2025-01-16 20:21 ` Jonas Oberhauser
2025-01-16 19:28 ` Jonas Oberhauser
2025-01-16 19:39 ` Paul E. McKenney
2025-01-17 12:08 ` Jonas Oberhauser
2025-01-16 19:08 ` Jonas Oberhauser
2025-01-16 23:02 ` Alan Stern
2025-01-17 8:34 ` Hernan Ponce de Leon
2025-01-17 11:29 ` Jonas Oberhauser
2025-01-17 20:01 ` Alan Stern
2025-01-21 10:36 ` Jonas Oberhauser
2025-01-21 16:39 ` Alan Stern
2025-01-22 3:46 ` Jonas Oberhauser
2025-01-22 19:11 ` Alan Stern
2025-01-17 15:52 ` Alan Stern
2025-01-17 16:45 ` Jonas Oberhauser
2025-01-17 19:02 ` Alan Stern
2025-01-09 20:37 ` Peter Zijlstra
2025-01-09 21:13 ` Paul E. McKenney
2025-01-08 17:33 ` Jonas Oberhauser
2025-01-08 18:47 ` Alan Stern
2025-01-08 19:22 ` Jonas Oberhauser
2025-01-09 16:17 ` Alan Stern
2025-01-09 16:44 ` Jonas Oberhauser
2025-01-09 19:27 ` Alan Stern
2025-01-09 20:09 ` Jonas Oberhauser
2025-01-10 3:12 ` Alan Stern
2025-01-10 12:21 ` Jonas Oberhauser
2025-01-10 21:51 ` Alan Stern
2025-01-11 12:46 ` Jonas Oberhauser
2025-01-11 21:19 ` Alan Stern
2025-01-12 15:55 ` Jonas Oberhauser
2025-01-13 19:43 ` Alan Stern
2025-07-23 0:43 ` Paul E. McKenney
2025-07-23 7:26 ` Hernan Ponce de Leon
2025-07-23 16:39 ` Paul E. McKenney
2025-07-24 14:14 ` Paul E. McKenney
2025-07-25 5:23 ` Hernan Ponce de Leon
2025-07-29 20:34 ` Paul E. McKenney
2025-07-23 17:13 ` Alan Stern
2025-07-23 17:27 ` Paul E. McKenney
2025-07-23 19:25 ` Alan Stern
2025-07-23 19:57 ` Paul E. McKenney [this message]
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