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: Alexander Monakov <amonakov@ispras.ru>,
	Jakub Jelinek <jakub@redhat.com>,
	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: Mon, 7 Jun 2021 12:42:06 -0500	[thread overview]
Message-ID: <20210607174206.GF18427@gate.crashing.org> (raw)
In-Reply-To: <CAHk-=wjwXs5+SOZGTaZ0bP9nsoA+PymAcGE4CBDVX3edGUcVRg@mail.gmail.com>

On Sun, Jun 06, 2021 at 03:38:06PM -0700, Linus Torvalds wrote:
> In the case of "volatile_if()", we actually would like to have not a
> memory clobber, but a "memory read". IOW, it would be a barrier for
> any writes taking place, but reads can move around it.
> 
> I don't know of any way to express that to the compiler. We've used
> hacks for it before (in gcc, BLKmode reads turn into that kind of
> barrier in practice, so you can do something like make the memory
> input to the asm be a big array). But that turned out to be fairly
> unreliable, so now we use memory clobbers even if we just mean "reads
> random memory".
> 
> Example: variable_test_bit(), which generates a "bt" instruction, does
> 
>                      : "m" (*(unsigned long *)addr), "Ir" (nr) : "memory");
> 
> and the memory clobber is obviously wrong: 'bt' only *reads* memory,
> but since the whole reason we use it is that it's not just that word
> at address 'addr', in order to make sure that any previous writes are
> actually stable in memory, we use that "memory" clobber.

You can split the "I" version from the "r" version, it does not need
the memory clobber.  If you know the actual maximum bit offset used you
don't need the clobber for "r" either.  Or you could even write
  "m"(((unsigned long *)addr)[nr/32])
That should work for all cases.

> Anybody have ideas or suggestions for something like that?

Is it useful in general for the kernel to have separate "read" and
"write" clobbers in asm expressions?  And for other applications?


Segher

  parent reply	other threads:[~2021-06-07 17:46 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
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 [this message]
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=20210607174206.GF18427@gate.crashing.org \
    --to=segher@kernel.crashing.org \
    --cc=akiyks@gmail.com \
    --cc=amonakov@ispras.ru \
    --cc=boqun.feng@gmail.com \
    --cc=dhowells@redhat.com \
    --cc=j.alglave@ucl.ac.uk \
    --cc=jakub@redhat.com \
    --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