linux-arch.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
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: Fri, 4 Jun 2021 11:55:26 -0700	[thread overview]
Message-ID: <20210604185526.GW4397@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210604164047.GH18427@gate.crashing.org>

On Fri, Jun 04, 2021 at 11:40:47AM -0500, Segher Boessenkool wrote:
> On Fri, Jun 04, 2021 at 06:10:55PM +0200, Peter Zijlstra wrote:
> > On Fri, Jun 04, 2021 at 10:35:18AM -0500, Segher Boessenkool wrote:
> > > On Fri, Jun 04, 2021 at 01:44:37PM +0200, Peter Zijlstra wrote:
> > > > On naming (sorry Paul for forgetting that in the initial mail); while I
> > > > think using the volatile qualifier for the language feature (can we haz
> > > > plz, kthxbai) makes perfect sense, Paul felt that we might use a
> > > > 'better' name for the kernel use, ctrl_dep_if() was proposed.
> > > 
> > > In standard C statements do not have qualifiers.  Unless you can
> > > convince the ISO C committee to have them on "if", you will have a very
> > > hard time convincing any serious compiler to do this.
> > 
> > While some people like talking to the Committee, I would much rather
> > explore language extensions with the compiler communities. Such
> > extensions can then make their way into the Committee once they show
> > their usefulness.
> 
> 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.  ;-)

> > If you have another proposal on how to express this; one you'd rather
> > see implemented, I'm all ears.
> 
> I would love to see something that meshes well with the rest of C.  But
> there is no 1-1 translation from C code to machine code (not in either
> direction), so anything that more or less depends on that will always
> be awkward.  If you can actually express the dependency in your source
> code that will get us 95% to where we want to be.
> 
> > Data dependencies, control dependencies and address dependencies, C
> > doesn't really like them, we rely on them. It would be awesome if we can
> > fix this.
> 
> Yes.  The problem is that C is a high-level language.  All C semantics
> are expressed on a an "as-if" level, never as "do this, then that" --
> well, of course that *is* what it says, it's an imperative language just
> like most, but that is just how you *think* about things on a conceptual
> level, there is nothing that says the machine code has to do the same
> thing in the same order as you wrote!

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.  ;-)

							Thanx, Paul

  reply	other threads:[~2021-06-04 18:55 UTC|newest]

Thread overview: 125+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-04 10:12 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 [this message]
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
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210604185526.GW4397@paulmck-ThinkPad-P17-Gen-1 \
    --to=paulmck@kernel.org \
    --cc=akiyks@gmail.com \
    --cc=boqun.feng@gmail.com \
    --cc=dhowells@redhat.com \
    --cc=j.alglave@ucl.ac.uk \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-toolchains@vger.kernel.org \
    --cc=luc.maranget@inria.fr \
    --cc=npiggin@gmail.com \
    --cc=parri.andrea@gmail.com \
    --cc=peterz@infradead.org \
    --cc=segher@kernel.crashing.org \
    --cc=stern@rowland.harvard.edu \
    --cc=torvalds@linux-foundation.org \
    --cc=will@kernel.org \
    --subject='Re: [RFC] LKMM: Add volatile_if()' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
on how to clone and mirror all data and code used for this inbox