From: "Paul E. McKenney" <paulmck@kernel.org>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
will@kernel.org, stern@rowland.harvard.edu,
parri.andrea@gmail.com, boqun.feng@gmail.com, npiggin@gmail.com,
dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
akiyks@gmail.com, linux-kernel@vger.kernel.org,
linux-toolchains@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: [RFC] LKMM: Add volatile_if()
Date: Sun, 6 Jun 2021 12:01:41 -0700 [thread overview]
Message-ID: <20210606190141.GK4397@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210606113651.GR18427@gate.crashing.org>
On Sun, Jun 06, 2021 at 06:36:51AM -0500, Segher Boessenkool wrote:
> On Fri, Jun 04, 2021 at 01:40:42PM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 04, 2021 at 02:53:01PM -0500, Segher Boessenkool wrote:
> > > On Fri, Jun 04, 2021 at 11:55:26AM -0700, Paul E. McKenney wrote:
> > > > On Fri, Jun 04, 2021 at 11:40:47AM -0500, Segher Boessenkool wrote:
> > > > > My point is that you ask compiler developers to paint themselves into a
> > > > > corner if you ask them to change such fundamental C syntax.
> > > >
> > > > Once we have some experience with a language extension, the official
> > > > syntax for a standardized version of that extension can be bikeshedded.
> > > > Committees being what they are, what we use in the meantime will
> > > > definitely not be what is chosen, so there is not a whole lot of point
> > > > in worrying about the exact syntax in the meantime. ;-)
> > >
> > > I am only saying that it is unlikely any compiler that is used in
> > > production will want to experiment with "volatile if".
> >
> > That unfortunately matches my experience over quite a few years. But if
> > something can be implemented using existing extensions, the conversations
> > often get easier. Especially given many more people are now familiar
> > with concurrency.
>
> This was about the syntax "volatile if", not about the concept, let's
> call that "volatile_if". And no, it was not me who brought this up :-)
I agree that it is likely that the syntax "volatile if" would be at best
a very reluctantly acquired taste among most of the committee. But some
might point to the evolving semantics of "auto" as a counter-example,
to say nothing of the celebrated spaceship operator. Me, I am not
all that worried about the exact syntax.
> > > > Which is exactly why these conversations are often difficult. There is
> > > > a tension between pushing the as-if rule as far as possible within the
> > > > compiler on the one hand and allowing developers to write code that does
> > > > what is needed on the other. ;-)
> > >
> > > There is a tension between what users expect from the compiler and what
> > > actually is promised. The compiler is not pushing the as-if rule any
> > > further than it always has: it just becomes better at optimising over
> > > time. The as-if rule is and always has been absolute.
> >
> > Heh! The fact that the compiler has become better at optimizing
> > over time is exactly what has been pushing the as-if rule further.
> >
> > The underlying problem is that it is often impossible to write large
> > applications (such as the Linux kernel) completely within the confines of
> > the standard. Thus, most large applications, and especially concurrent
> > applications, are vulnerable to either the compiler becoming better
> > at optimizing or compilers pushing the as-if rule, however you want to
> > say it.
>
> Oh definitely. But there is nothing the compiler can do about most
> cases of undefined behaviour: it cannot detect it, and there is no way
> it *can* be handled sanely. Take for example dereferencing a pointer
> that does not point to an object.
Almost.
The compiler's use of provenance allows detection in some cases.
For a stupid example, please see https://godbolt.org/z/z9cWvqdhE.
Less stupidly, this sort of thing can be quite annoying to people trying
to use ABA-tolerant concurrent algorithms. See for example P1726R4
[1] (update in progress) and for an even more controversial proposal,
P2188R1 [2]. The Lifo Singly Linked Push algorithm described beginning
on page 14 of [1] is a simple example of an ABA-tolerant algorithm that
was already in use when I first programmed a computer. ;-)
Thanx, Paul
[1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1726r4.pdf
[2] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2188r1.html
next prev parent reply other threads:[~2021-06-06 19:01 UTC|newest]
Thread overview: 127+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-04 10:12 [RFC] LKMM: Add volatile_if() Peter Zijlstra
2021-06-04 10:44 ` Will Deacon
2021-06-04 11:13 ` Will Deacon
2021-06-04 11:31 ` Peter Zijlstra
2021-06-04 13:44 ` Will Deacon
2021-06-04 13:56 ` Peter Zijlstra
2021-06-04 15:13 ` Will Deacon
2021-06-04 15:22 ` Peter Zijlstra
2021-06-04 15:36 ` Alan Stern
2021-06-04 15:42 ` Peter Zijlstra
2021-06-04 15:51 ` Alan Stern
2021-06-04 16:17 ` Peter Zijlstra
2021-06-04 18:27 ` Alan Stern
2021-06-04 19:09 ` Linus Torvalds
2021-06-04 19:18 ` Linus Torvalds
2021-06-04 20:56 ` Paul E. McKenney
2021-06-04 21:27 ` Linus Torvalds
2021-06-04 21:40 ` Paul E. McKenney
2021-06-04 22:19 ` Linus Torvalds
2021-06-05 14:57 ` Alan Stern
2021-06-06 0:14 ` Paul E. McKenney
2021-06-06 1:29 ` Alan Stern
2021-06-06 3:41 ` Linus Torvalds
2021-06-06 4:43 ` Paul E. McKenney
2021-06-06 13:17 ` Segher Boessenkool
2021-06-06 19:07 ` Paul E. McKenney
2021-06-06 12:59 ` Segher Boessenkool
2021-06-06 13:47 ` Alan Stern
2021-06-06 17:13 ` Segher Boessenkool
2021-06-06 18:25 ` Linus Torvalds
2021-06-06 19:19 ` Segher Boessenkool
2021-06-06 18:41 ` Alan Stern
2021-06-06 18:59 ` Jakub Jelinek
2021-06-06 19:15 ` Paul E. McKenney
2021-06-06 19:22 ` Linus Torvalds
2021-06-06 20:11 ` Segher Boessenkool
2021-06-06 21:19 ` Alexander Monakov
2021-06-06 22:38 ` Linus Torvalds
2021-06-06 23:39 ` Rasmus Villemoes
2021-06-06 23:44 ` Rasmus Villemoes
2021-06-07 8:01 ` Alexander Monakov
2021-06-07 8:27 ` Marco Elver
2021-06-07 15:28 ` Paul E. McKenney
2021-06-07 17:04 ` Marco Elver
2021-06-08 9:30 ` Marco Elver
2021-06-08 11:22 ` Peter Zijlstra
2021-06-08 15:28 ` Segher Boessenkool
2021-06-09 12:44 ` Marco Elver
2021-06-09 15:31 ` Segher Boessenkool
2021-06-09 16:13 ` Marco Elver
2021-06-09 17:14 ` Segher Boessenkool
2021-06-09 17:31 ` Nick Desaulniers
2021-06-09 20:24 ` Segher Boessenkool
2021-06-09 18:25 ` Linus Torvalds
2021-06-07 17:52 ` Segher Boessenkool
2021-06-07 18:07 ` Alexander Monakov
2021-06-07 18:18 ` Segher Boessenkool
2021-06-07 17:42 ` Segher Boessenkool
2021-06-07 20:31 ` Linus Torvalds
2021-06-07 22:54 ` Segher Boessenkool
2021-06-06 11:53 ` Segher Boessenkool
2021-06-06 13:45 ` Alan Stern
2021-06-06 18:04 ` Linus Torvalds
2021-06-06 18:22 ` Alan Stern
2021-06-06 18:43 ` Linus Torvalds
2021-06-07 10:43 ` Peter Zijlstra
2021-06-07 11:52 ` Will Deacon
2021-06-07 15:25 ` Paul E. McKenney
2021-06-07 16:02 ` Will Deacon
2021-06-07 18:08 ` Paul E. McKenney
[not found] ` <20210730172020.GA32396@knuckles.cs.ucl.ac.uk>
2021-07-30 20:35 ` Alan Stern
2021-08-02 21:18 ` Alan Stern
2021-08-02 23:31 ` Paul E. McKenney
2021-08-04 20:09 ` Alan Stern
2021-08-05 19:47 ` Alan Stern
2021-08-07 0:51 ` Alan Stern
2021-06-06 18:40 ` Segher Boessenkool
2021-06-06 18:48 ` Linus Torvalds
2021-06-06 18:53 ` Linus Torvalds
2021-06-06 19:52 ` Segher Boessenkool
2021-06-06 20:11 ` Linus Torvalds
2021-06-06 20:26 ` Segher Boessenkool
2021-06-06 23:37 ` Paul E. McKenney
2021-06-07 14:12 ` Segher Boessenkool
2021-06-07 15:27 ` Paul E. McKenney
2021-06-07 18:23 ` Segher Boessenkool
2021-06-07 19:51 ` Alan Stern
2021-06-07 20:16 ` Paul E. McKenney
2021-06-07 22:40 ` Segher Boessenkool
2021-06-07 23:26 ` Paul E. McKenney
2021-06-07 10:52 ` Peter Zijlstra
2021-06-07 14:16 ` Segher Boessenkool
2021-06-04 22:05 ` Peter Zijlstra
2021-06-05 3:14 ` Alan Stern
2021-06-05 16:24 ` Linus Torvalds
2021-06-04 15:50 ` Segher Boessenkool
2021-06-04 15:47 ` Segher Boessenkool
2021-06-04 11:44 ` Peter Zijlstra
2021-06-04 14:13 ` Paul E. McKenney
2021-06-04 15:35 ` Segher Boessenkool
2021-06-04 16:10 ` Peter Zijlstra
2021-06-04 16:40 ` Segher Boessenkool
2021-06-04 18:55 ` Paul E. McKenney
2021-06-04 19:53 ` Segher Boessenkool
2021-06-04 20:40 ` Paul E. McKenney
2021-06-06 11:36 ` Segher Boessenkool
2021-06-06 19:01 ` Paul E. McKenney [this message]
2021-06-04 14:25 ` Alan Stern
2021-06-04 16:09 ` Segher Boessenkool
2021-06-04 16:33 ` Peter Zijlstra
2021-06-04 16:30 ` Linus Torvalds
2021-06-04 16:37 ` Peter Zijlstra
2021-06-04 16:52 ` Segher Boessenkool
2021-06-04 17:10 ` Linus Torvalds
2021-06-04 17:24 ` Segher Boessenkool
2021-06-04 17:38 ` Linus Torvalds
2021-06-04 18:25 ` Segher Boessenkool
2021-06-04 19:17 ` Peter Zijlstra
2021-06-04 20:43 ` Paul E. McKenney
2021-06-04 18:23 ` Alan Stern
2021-06-08 12:48 ` David Laight
2021-09-24 18:38 ` Mathieu Desnoyers
2021-09-24 19:52 ` Alan Stern
2021-09-24 20:22 ` Mathieu Desnoyers
2021-09-24 19:55 ` Segher Boessenkool
2021-09-24 20:39 ` Mathieu Desnoyers
2021-09-24 22:07 ` Mathieu Desnoyers
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