linux-arch.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will@kernel.org>,
	Andrea Parri <parri.andrea@gmail.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Nick Piggin <npiggin@gmail.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	Akira Yokosawa <akiyks@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-toolchains@vger.kernel.org,
	linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()
Date: Sun, 6 Jun 2021 14:19:09 -0500	[thread overview]
Message-ID: <20210606191909.GZ18427@gate.crashing.org> (raw)
In-Reply-To: <CAHk-=wgPUaj7tv-JYzKQ34ukP3LEuGf82g+Nyp96pTnxN=xOtA@mail.gmail.com>

On Sun, Jun 06, 2021 at 11:25:46AM -0700, Linus Torvalds wrote:
> On Sun, Jun 6, 2021 at 6:03 AM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> >
> > On Sat, Jun 05, 2021 at 08:41:00PM -0700, Linus Torvalds wrote:
> > >
> > > I think it's something of a bug when it comes to "asm volatile", but
> > > the documentation isn't exactly super-specific.
> >
> > Why would that be?  "asm volatile" does not prevent optimisation.
> 
> Sure it does.
> 
> That's the whole and only *POINT* of the "volatile".
> 
> It's the same as a vol;atile memory access. That very much prevents
> certain optimizations. You can't just join two volatile reads or
> writes, because they have side effects.

You can though.  In exactly this same way:

volatile int x;
void g(int);
void f(int n) { if (n) g(x); else g(x); }

==>

f:
        movl    x(%rip), %edi
        jmp     g

You can do whatever you want with code with side effects.  The only
thing required is that the side effects are executed as often as before
and in the same order.  Merging identical sides of a diamond is just
fine.

> And the exact same thing is true of inline asm. Even when they are
> *identical*, inline asms have side effects that gcc simply doesn't
> understand.

Only volatile asm does (including all asm without outputs).  But that
still does not mean GCC cannot manipulate the asm!

> And yes, those side effects can - and do - include "you can't just merge these".

They do not.  That is not what a side effect is.

> > It says this code has some unspecified side effect, and that is all!
> 
> And that should be sufficient. But gcc then violates it, because gcc
> doesn't understand the side effects.
> 
> Now, the side effects may be *subtle*, but they are very very real.
> Just placement of code wrt a branch will actually affect memory
> ordering, as that one example was.

You have a different definition of "side effect" than C does apparently.

5.1.2.3/2:
  Accessing a volatile object, modifying an object, modifying a file, or
  calling a function that does any of those operations are all side
  effects, which are changes in the state of the execution environment.
  Evaluation of an expression in general includes both value
  computations and initiation of side effects.  Value computation for an
  lvalue expression includes determining the identity of the designated
  object.

> But that's what we need a compiler barrier for in the first place -
> the compiler certainly doesn't understand about this very subtle
> memory ordering issue, and we want to make sure that the code sequence
> *remains* that "if A then write B".

The compiler doesn't magically understand your intention, no.  Some real
work will need to be done to make this work.


Segher

  reply	other threads:[~2021-06-06 19:23 UTC|newest]

Thread overview: 121+ 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 [this message]
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
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

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=20210606191909.GZ18427@gate.crashing.org \
    --to=segher@kernel.crashing.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=paulmck@kernel.org \
    --cc=peterz@infradead.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