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From: Andrea Parri <parri.andrea@gmail.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>,
	Will Deacon <will@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	bsingharora@gmail.com, Marco Elver <elver@google.com>,
	stable <stable@vger.kernel.org>,
	syzbot <syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: [PATCH v6] taskstats: fix data-race
Date: Thu, 24 Oct 2019 16:40:49 +0200	[thread overview]
Message-ID: <20191024144049.GA13747@andrea.guest.corp.microsoft.com> (raw)
In-Reply-To: <CACT4Y+ZXQyqgBvwgb6cy7NP5FTBbktq5j4ZyySp7jrbcJwFUTA@mail.gmail.com>

On Thu, Oct 24, 2019 at 03:58:40PM +0200, Dmitry Vyukov wrote:
> On Thu, Oct 24, 2019 at 3:43 PM Andrea Parri <parri.andrea@gmail.com> wrote:
> >
> > > But why? I think kernel contains lots of such cases and it seems to be
> > > officially documented by the LKMM:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt
> > > address dependencies and ppo
> >
> > Well, that same documentation also alerts about some of the pitfalls
> > developers can incur while relying on dependencies.  I'm sure you're
> > more than aware of some of the debate surrounding these issues.
> 
> I thought that LKMM is finally supposed to stop all these
> centi-threads around subtle details of ordering. And not we finally
> have it. And it says that using address-dependencies is legal. And you
> are one of the authors. And now you are arguing here that we better
> not use it :) Can we have some black/white yes/no for code correctness
> reflected in LKMM please :) If we are banning address dependencies,
> don't we need to fix all of rcu uses?

Current limitations of the LKMM are listed in tools/memory-model/README
(and I myself discussed a number of them at LPC recently); the relevant
point here seems to be:

1.	Compiler optimizations are not accurately modeled.  Of course,
	the use of READ_ONCE() and WRITE_ONCE() limits the compiler's
	ability to optimize, but under some circumstances it is possible
	for the compiler to undermine the memory model.  [...]

	Note that this limitation in turn limits LKMM's ability to
	accurately model address, control, and data dependencies.

A less elegant, but hopefully more effective, way to phrase such point
is maybe "feel free to rely on dependencies, but then do not blame the
LKMM authors please".  ;-)

Thanks,
  Andrea

  reply	other threads:[~2019-10-24 14:41 UTC|newest]

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-05  4:26 KCSAN: data-race in taskstats_exit / taskstats_exit syzbot
2019-10-05  4:29 ` Dmitry Vyukov
2019-10-05 11:29   ` Christian Brauner
2019-10-05 11:28 ` [PATCH] taskstats: fix data-race Christian Brauner
2019-10-05 13:33   ` Marco Elver
2019-10-05 14:15     ` Christian Brauner
2019-10-05 14:34       ` Marco Elver
2019-10-06 10:00   ` Dmitry Vyukov
2019-10-06 10:59     ` Christian Brauner
2019-10-06 23:52   ` Christian Brauner
2019-10-07  7:31     ` Dmitry Vyukov
2019-10-07  9:29       ` Christian Brauner
2019-10-07 10:40     ` Andrea Parri
2019-10-07 10:50       ` Christian Brauner
2019-10-07 11:01       ` [PATCH v2] " Christian Brauner
2019-10-07 13:18         ` Andrea Parri
2019-10-07 13:28           ` Christian Brauner
2019-10-07 13:50           ` Dmitry Vyukov
2019-10-07 13:55             ` Christian Brauner
2019-10-07 14:08               ` Dmitry Vyukov
2019-10-07 14:10                 ` Christian Brauner
2019-10-07 14:14             ` Andrea Parri
2019-10-07 14:18               ` Dmitry Vyukov
2019-10-08 14:20                 ` Andrea Parri
2019-10-08 14:24                   ` Christian Brauner
2019-10-08 15:26                     ` Andrea Parri
2019-10-08 15:35                       ` Christian Brauner
2019-10-08 15:44                         ` Andrea Parri
2019-10-09 11:31                           ` [PATCH] " Christian Brauner
2019-10-09 11:40                             ` Christian Brauner
2019-10-09 11:48                             ` [PATCH v5] " Christian Brauner
2019-10-09 12:08                               ` Andrea Parri
2019-10-09 13:26                               ` Christian Brauner
2019-10-21 11:33                               ` [PATCH v6] " Christian Brauner
2019-10-21 12:19                                 ` Rasmus Villemoes
2019-10-21 13:04                                   ` Christian Brauner
2019-11-29 17:56                                     ` Will Deacon
2019-11-30 15:08                                       ` Christian Brauner
2019-10-23 12:16                                 ` Andrea Parri
2019-10-23 12:39                                   ` Dmitry Vyukov
2019-10-23 13:11                                     ` Christian Brauner
2019-10-23 13:20                                       ` Dmitry Vyukov
2019-10-24 11:31                                     ` Andrea Parri
2019-10-24 11:51                                       ` Dmitry Vyukov
2019-10-24 13:05                                         ` Andrea Parri
2019-10-24 13:13                                           ` Dmitry Vyukov
2019-10-24 13:21                                             ` Christian Brauner
2019-10-24 13:34                                               ` Dmitry Vyukov
2019-10-24 13:43                                             ` Andrea Parri
2019-10-24 13:58                                               ` Dmitry Vyukov
2019-10-24 14:40                                                 ` Andrea Parri [this message]
2019-10-24 14:49                                                   ` Dmitry Vyukov
2019-11-29 17:57                                 ` Will Deacon
2019-10-09 11:48                             ` [PATCH] " Marco Elver
2019-10-09 11:53                               ` Christian Brauner
2019-11-06  0:27   ` Balbir Singh
2019-11-06  0:09 ` KCSAN: data-race in taskstats_exit / taskstats_exit Balbir Singh
2019-11-06 10:23   ` Marco Elver
2019-11-07 10:39     ` Balbir Singh
2019-11-08  0:54     ` Balbir Singh
2019-11-08  8:55       ` Dmitry Vyukov
2019-11-09  3:42         ` Balbir Singh

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