From: "Paul E. McKenney" <paulmck@kernel.org>
To: Andrea Parri <parri.andrea@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>,
Jonas Oberhauser <jonas.oberhauser@huawei.com>,
Peter Zijlstra <peterz@infradead.org>, will <will@kernel.org>,
"boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
dhowells <dhowells@redhat.com>, "j.alglave" <j.alglave@ucl.ac.uk>,
"luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
urezki <urezki@gmail.com>,
quic_neeraju <quic_neeraju@quicinc.com>,
frederic <frederic@kernel.org>,
Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)
Date: Tue, 17 Jan 2023 07:14:16 -0800 [thread overview]
Message-ID: <20230117151416.GI2948950@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <Y8aKlNY4Z0z2Yqs0@andrea>
On Tue, Jan 17, 2023 at 12:46:28PM +0100, Andrea Parri wrote:
> On Mon, Jan 16, 2023 at 02:13:57PM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 16, 2023 at 02:20:57PM -0500, Alan Stern wrote:
> > > On Mon, Jan 16, 2023 at 11:06:52AM -0800, Paul E. McKenney wrote:
> > > > On Mon, Jan 16, 2023 at 01:11:41PM -0500, Alan Stern wrote:
> > >
> > > > > Why do you want to prohibit nesting? Why would that be a better
> > > > > approximation?
> > > >
> > > > Because the current LKMM gives wrong answers for nested critical
> > > > sections.
> > >
> > > I don't agree. Or at least, it depends on whose definition of "nested
> > > critical sections" you adopt.
> >
> > Fair point, and I have therefore updated the test's header comment
> > to read as follows:
> >
> > (*
> > * Result: Sometimes
> > *
> > * This demonstrates non-nested overlapping of SRCU read-side critical
> > * sections. Unlike RCU, SRCU critical sections do not unconditionally
> > * nest.
> > *)
> >
> > > > For example, for the litmus test shown below, mainline
> > > > LKMM will incorrectly report "Never". The two SRCU read-side critical
> > > > sections are independent, so the fact that P1()'s synchronize_srcu() is
> > > > guaranteed to wait for the first on to complete says nothing about the
> > > > second having completed. Therefore, in Linux-kernel SRCU, the "exists"
> > > > clause could be satisfied.
> > > >
> > > > In contrast, the proposed change flags this as having nesting.
> > >
> > > In fact, this litmus test has overlapping critical sections, not nested
> > > ones. But the current LKML incorrectly _thinks_ they are nested,
> > > because it matches each lock with the first unmatched unlock.
> > >
> > > If you write a litmus test that has properly nested (not overlapping!)
> > > read-side critical sections, the current LKMM will match the locks and
> > > unlocks correctly and will give the right answer.
> > >
> > > So what you really want to do is rule out overlapping, not nesting. But
> > > I guess there's no way to do one without the other.
> >
> > None that I could see!
>
> This was reminiscent of old discussions, in fact, we do have:
>
> [tools/memory-model/Documentation/litmus-tests.txt]
>
> e. Although sleepable RCU (SRCU) is now modeled, there
> are some subtle differences between its semantics and
> those in the Linux kernel. For example, the kernel
> might interpret the following sequence as two partially
> overlapping SRCU read-side critical sections:
>
> 1 r1 = srcu_read_lock(&my_srcu);
> 2 do_something_1();
> 3 r2 = srcu_read_lock(&my_srcu);
> 4 do_something_2();
> 5 srcu_read_unlock(&my_srcu, r1);
> 6 do_something_3();
> 7 srcu_read_unlock(&my_srcu, r2);
>
> In contrast, LKMM will interpret this as a nested pair of
> SRCU read-side critical sections, with the outer critical
> section spanning lines 1-7 and the inner critical section
> spanning lines 3-5.
>
> This difference would be more of a concern had anyone
> identified a reasonable use case for partially overlapping
> SRCU read-side critical sections. For more information
> on the trickiness of such overlapping, please see:
> https://paulmck.livejournal.com/40593.html
Good point, if we do change the definition, we also need to update
this documentation.
> More recently/related,
>
> https://lore.kernel.org/lkml/20220421230848.GA194034@paulmck-ThinkPad-P17-Gen-1/T/#m2a8701c7c377ccb27190a6679e58b0929b0b0ad9
It would not be a bad thing for LKMM to be able to show people the
error of their ways when they try non-nested partially overlapping SRCU
read-side critical sections. Or, should they find some valid use case,
to help them prove their point. ;-)
Thanx, Paul
> Thanks,
> Andrea
>
>
> >
> > Thanx, Paul
> >
> > > Alan
> > >
> > > > Thaxn, Paul
> > > >
> > > > ------------------------------------------------------------------------
> > > >
> > > > C C-srcu-nest-5
> > > >
> > > > (*
> > > > * Result: Sometimes
> > > > *
> > > > * This demonstrates non-nesting of SRCU read-side critical sections.
> > > > * Unlike RCU, SRCU critical sections do not nest.
> > > > *)
> > > >
> > > > {}
> > > >
> > > > P0(int *x, int *y, struct srcu_struct *s1)
> > > > {
> > > > int r1;
> > > > int r2;
> > > > int r3;
> > > > int r4;
> > > >
> > > > r3 = srcu_read_lock(s1);
> > > > r2 = READ_ONCE(*y);
> > > > r4 = srcu_read_lock(s1);
> > > > srcu_read_unlock(s1, r3);
> > > > r1 = READ_ONCE(*x);
> > > > srcu_read_unlock(s1, r4);
> > > > }
> > > >
> > > > P1(int *x, int *y, struct srcu_struct *s1)
> > > > {
> > > > WRITE_ONCE(*y, 1);
> > > > synchronize_srcu(s1);
> > > > WRITE_ONCE(*x, 1);
> > > > }
> > > >
> > > > locations [0:r1]
> > > > exists (0:r1=1 /\ 0:r2=0)
next prev parent reply other threads:[~2023-01-17 15:14 UTC|newest]
Thread overview: 161+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20220921173109.GA1214281@paulmck-ThinkPad-P17-Gen-1>
[not found] ` <YytfFiMT2Xsdwowf@rowland.harvard.edu>
[not found] ` <YywXuzZ/922LHfjI@hirez.programming.kicks-ass.net>
[not found] ` <114ECED5-FED1-4361-94F7-8D9BC02449B7>
[not found] ` <YzSAnclenTz7KQyt@rowland.harvard.edu>
[not found] ` <f763bd5ff835458d8750b61da47fe316@huawei.com>
2023-01-03 18:56 ` Internal vs. external barriers (was: Re: Interesting LKMM litmus test) Alan Stern
2023-01-04 15:37 ` Andrea Parri
2023-01-04 20:58 ` Alan Stern
[not found] ` <ee186bc17a5e48298a5373f688496dce@huawei.com>
2023-01-05 17:32 ` Paul E. McKenney
[not found] ` <bea712c82e6346f8973399a5711ff78a@huawei.com>
2023-01-11 15:06 ` Alan Stern
[not found] ` <768ffe7edc7f4ddfacd5b0a8e844ed84@huawei.com>
2023-01-11 17:01 ` Alan Stern
[not found] ` <07579baee4b84532a76ea8b0b33052bb@huawei.com>
2023-01-12 21:57 ` Paul E. McKenney
2023-01-13 16:38 ` Alan Stern
2023-01-13 19:54 ` Paul E. McKenney
[not found] ` <06a8aef7eb8d46bca34521a80880dae3@huawei.com>
2023-01-14 17:42 ` Paul E. McKenney
[not found] ` <e51c82a113484b6bb62354a49376f248@huawei.com>
2023-01-14 16:42 ` Alan Stern
2023-01-17 17:48 ` Jonas Oberhauser
2023-01-17 21:19 ` Alan Stern
2023-01-18 11:25 ` Jonas Oberhauser
2023-01-19 2:28 ` Alan Stern
2023-01-19 11:22 ` Jonas Oberhauser
2023-01-19 16:41 ` Alan Stern
2023-01-19 18:43 ` Paul E. McKenney
2023-01-23 16:16 ` Jonas Oberhauser
2023-01-23 19:58 ` Alan Stern
2023-01-23 20:06 ` Jonas Oberhauser
2023-01-23 20:41 ` Alan Stern
2023-01-24 13:21 ` Jonas Oberhauser
2023-01-24 15:54 ` Jonas Oberhauser
2023-01-24 17:22 ` Alan Stern
[not found] ` <4c1abc7733794519ad7c5153ae8b58f9@huawei.com>
2023-01-13 16:28 ` Alan Stern
2023-01-13 20:07 ` Paul E. McKenney
2023-01-13 20:32 ` Paul E. McKenney
2023-01-14 17:40 ` Alan Stern
2023-01-14 17:48 ` Paul E. McKenney
[not found] ` <136d019d8c8049f6b737627df830e66f@huawei.com>
2023-01-14 17:53 ` Paul E. McKenney
2023-01-14 18:15 ` Paul E. McKenney
2023-01-14 19:58 ` Alan Stern
2023-01-15 5:19 ` Paul E. McKenney
2023-01-14 20:19 ` Alan Stern
2023-01-15 5:15 ` Paul E. McKenney
2023-01-15 16:23 ` Alan Stern
2023-01-15 18:10 ` Paul E. McKenney
2023-01-15 20:46 ` Alan Stern
2023-01-16 4:23 ` Paul E. McKenney
2023-01-16 18:11 ` Alan Stern
2023-01-16 19:06 ` Paul E. McKenney
2023-01-16 19:20 ` Alan Stern
2023-01-16 22:13 ` Paul E. McKenney
2023-01-17 11:46 ` Andrea Parri
2023-01-17 15:14 ` Paul E. McKenney [this message]
2023-01-17 15:56 ` Alan Stern
2023-01-17 17:43 ` Paul E. McKenney
2023-01-17 18:27 ` Jonas Oberhauser
2023-01-17 18:55 ` Paul E. McKenney
2023-01-17 20:20 ` Jonas Oberhauser
2023-01-17 20:15 ` Alan Stern
2023-01-18 3:50 ` Paul E. McKenney
2023-01-18 16:50 ` Alan Stern
2023-01-18 19:42 ` Jonas Oberhauser
2023-01-18 20:19 ` Paul E. McKenney
2023-01-18 20:30 ` Jonas Oberhauser
2023-01-18 21:12 ` Paul E. McKenney
2023-01-18 21:24 ` Jonas Oberhauser
2023-01-19 0:11 ` Paul E. McKenney
2023-01-19 13:39 ` Jonas Oberhauser
2023-01-19 18:41 ` Paul E. McKenney
2023-01-19 19:51 ` Alan Stern
2023-01-19 21:53 ` Paul E. McKenney
2023-01-19 22:04 ` Alan Stern
2023-01-19 23:03 ` Paul E. McKenney
2023-01-20 9:43 ` Jonas Oberhauser
2023-01-20 15:39 ` Paul E. McKenney
2023-01-20 20:46 ` Jonas Oberhauser
2023-01-20 21:37 ` Paul E. McKenney
2023-01-20 22:36 ` Jonas Oberhauser
2023-01-20 23:19 ` Paul E. McKenney
2023-01-21 0:03 ` Jonas Oberhauser
2023-01-21 0:34 ` Paul E. McKenney
2023-01-20 3:55 ` Paul E. McKenney
2023-01-20 9:20 ` Jonas Oberhauser
2023-01-20 12:34 ` Jonas Oberhauser
2023-01-20 12:51 ` Jonas Oberhauser
2023-01-20 15:32 ` Paul E. McKenney
2023-01-20 20:56 ` Jonas Oberhauser
2023-01-20 21:40 ` Paul E. McKenney
2023-01-20 16:14 ` Alan Stern
2023-01-20 17:30 ` Paul E. McKenney
2023-01-20 18:15 ` Alan Stern
2023-01-20 18:59 ` Paul E. McKenney
2023-01-20 10:13 ` Jonas Oberhauser
2023-01-20 15:47 ` Paul E. McKenney
2023-01-20 22:21 ` Jonas Oberhauser
2023-01-20 16:18 ` Alan Stern
2023-01-20 21:41 ` Jonas Oberhauser
2023-01-21 4:38 ` Paul E. McKenney
2023-01-21 17:36 ` Alan Stern
2023-01-21 18:40 ` Paul E. McKenney
2023-01-21 19:56 ` Alan Stern
2023-01-21 20:10 ` Paul E. McKenney
2023-01-21 21:03 ` Alan Stern
2023-01-21 23:49 ` Paul E. McKenney
2023-01-23 11:48 ` Jonas Oberhauser
2023-01-23 15:55 ` Alan Stern
2023-01-23 19:40 ` Jonas Oberhauser
2023-01-23 20:34 ` Alan Stern
2023-01-18 20:06 ` Paul E. McKenney
2023-01-18 20:54 ` Alan Stern
2023-01-18 21:05 ` Jonas Oberhauser
2023-01-19 0:02 ` Paul E. McKenney
2023-01-19 2:19 ` Alan Stern
2023-01-19 11:23 ` Paul E. McKenney
2023-01-20 16:01 ` Alan Stern
2023-01-20 17:58 ` Paul E. McKenney
2023-01-20 18:37 ` Alan Stern
2023-01-20 19:20 ` Paul E. McKenney
2023-01-20 20:36 ` Alan Stern
2023-01-20 21:20 ` Paul E. McKenney
2023-01-22 20:32 ` Alan Stern
2023-01-23 20:16 ` Paul E. McKenney
2023-01-24 2:18 ` Alan Stern
2023-01-24 4:06 ` Paul E. McKenney
2023-01-24 11:09 ` Andrea Parri
2023-01-24 14:54 ` Paul E. McKenney
2023-01-24 15:11 ` Jonas Oberhauser
2023-01-24 16:22 ` Paul E. McKenney
2023-01-24 16:39 ` Jonas Oberhauser
2023-01-24 17:26 ` Paul E. McKenney
2023-01-24 19:30 ` Jonas Oberhauser
2023-01-24 22:15 ` Paul E. McKenney
2023-01-24 22:35 ` Alan Stern
2023-01-24 22:54 ` Paul E. McKenney
2023-01-25 1:54 ` Alan Stern
2023-01-25 2:20 ` Paul E. McKenney
2023-01-25 13:10 ` Jonas Oberhauser
2023-01-25 15:05 ` Paul E. McKenney
2023-01-25 15:34 ` Alan Stern
2023-01-25 17:18 ` Paul E. McKenney
2023-01-25 17:42 ` Jonas Oberhauser
2023-01-25 19:08 ` Alan Stern
2023-01-25 19:46 ` Paul E. McKenney
2023-01-25 20:36 ` Andrea Parri
2023-01-25 21:10 ` Jonas Oberhauser
2023-01-25 21:23 ` Paul E. McKenney
2023-01-25 20:46 ` Alan Stern
2023-01-25 21:38 ` Paul E. McKenney
2023-01-25 23:33 ` Paul E. McKenney
2023-01-26 1:45 ` Alan Stern
2023-01-26 1:53 ` Paul E. McKenney
2023-01-26 12:17 ` Jonas Oberhauser
2023-01-26 18:48 ` Paul E. McKenney
2023-01-27 15:03 ` Jonas Oberhauser
2023-01-27 16:50 ` Paul E. McKenney
2023-01-27 16:54 ` Paul E. McKenney
2023-01-18 19:57 ` Jonas Oberhauser
2023-01-18 21:06 ` Paul E. McKenney
2023-01-18 2:15 ` Alan Stern
2023-01-18 5:17 ` Paul E. McKenney
2023-01-18 16:03 ` Alan Stern
2023-01-18 16:59 ` Boqun Feng
2023-01-18 17:08 ` Alan Stern
2023-01-18 17:41 ` Paul E. McKenney
2023-01-19 19:07 ` Paul E. McKenney
2023-01-14 16:55 ` Alan Stern
2023-01-14 17:35 ` Paul E. McKenney
[not found] ` <17078dd97cb6480f9c51e27058af3197@huawei.com>
2023-01-14 17:27 ` Alan Stern
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