linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Will Deacon <will@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Daniel Axtens <dja@axtens.net>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Christophe Leroy <christophe.leroy@c-s.fr>,
	linux-arch <linux-arch@vger.kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))
Date: Thu, 19 Dec 2019 12:11:16 +0000	[thread overview]
Message-ID: <20191219121115.GB32361@willie-the-truck> (raw)
In-Reply-To: <CAHk-=wjNLxSZZs+A0tyb-MnJ2YU-sqTAfy0-X+9vxjXfs_O4zA@mail.gmail.com>

On Tue, Dec 17, 2019 at 10:32:35AM -0800, Linus Torvalds wrote:
> On Tue, Dec 17, 2019 at 10:04 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Let me think about it.
> 
> How about we just get rid of the union entirely, and just use
> 'unsigned long' or 'unsigned long long' depending on the size.
> 
> Something like the attached patch - it still requires that it be an
> arithmetic type, but now because of the final cast.
> 
> But it might still be a cast to a volatile type, of course. Then the
> result will be volatile, but at least now READ_ONCE() won't be taking
> the address of a volatile variable on the stack - does that at least
> fix some of the horrible code generation. Hmm?

Sounds like it according to mpe, but I'll confirm too for arm64.

> This is untested, because I obviously still have the cases of
> structures (page table entries) being accessed once..
> 
>               Linus

>  include/linux/compiler.h | 33 +++++++++++++++++----------------
>  1 file changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 5e88e7e33abe..8b4282194f16 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -179,18 +179,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  
>  #include <uapi/linux/types.h>
>  
> -#define __READ_ONCE_SIZE						\
> -({									\
> -	switch (size) {							\
> -	case 1: *(__u8 *)res = *(volatile __u8 *)p; break;		\
> -	case 2: *(__u16 *)res = *(volatile __u16 *)p; break;		\
> -	case 4: *(__u32 *)res = *(volatile __u32 *)p; break;		\
> -	case 8: *(__u64 *)res = *(volatile __u64 *)p; break;		\
> -	default:							\
> -		barrier();						\
> -		__builtin_memcpy((void *)res, (const void *)p, size);	\
> -		barrier();						\
> -	}								\
> +/* "unsigned long" or "unsigned long long" - make it fit in a register if possible */
> +#define __READ_ONCE_TYPE(size) \
> +	__typeof__(__builtin_choose_expr(size > sizeof(0UL), 0ULL, 0UL))

Ha, I wondered when '__builtin_choose_expr()' would make an appearance in
this thread! Nice trick. I'll try integrating this with what I have and see
what I run into next.

Back down the rabbit hole...

Will

  parent reply	other threads:[~2019-12-19 12:11 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-06 12:46 [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops) Michael Ellerman
2019-12-06 13:16 ` Peter Zijlstra
2019-12-10  5:38   ` Michael Ellerman
2019-12-10 10:15     ` Peter Zijlstra
2019-12-11  0:29       ` Michael Ellerman
2019-12-12  5:42   ` READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops)) Michael Ellerman
2019-12-12  8:01     ` Peter Zijlstra
2019-12-12 10:07       ` Will Deacon
2019-12-12 10:46         ` Peter Zijlstra
2019-12-12 17:04           ` Will Deacon
2019-12-12 17:16             ` Will Deacon
2019-12-12 17:41           ` Linus Torvalds
2019-12-12 17:50             ` Segher Boessenkool
2019-12-12 18:06             ` Will Deacon
2019-12-12 18:29               ` Christian Borntraeger
2019-12-12 18:43               ` Linus Torvalds
2019-12-12 19:34                 ` Will Deacon
2019-12-12 20:21                   ` Peter Zijlstra
2019-12-12 20:53                     ` Peter Zijlstra
2019-12-13 10:47                       ` Luc Van Oostenryck
2019-12-13 12:56                         ` Peter Zijlstra
2019-12-13 14:28                           ` Luc Van Oostenryck
2019-12-12 20:49                   ` Linus Torvalds
2019-12-13 13:17                     ` Arnd Bergmann
2019-12-13 21:32                       ` Arnd Bergmann
2019-12-13 22:01                         ` Linus Torvalds
2019-12-16 10:28                       ` Will Deacon
2019-12-16 11:47                         ` Peter Zijlstra
2019-12-16 12:06                         ` Arnd Bergmann
2019-12-17 17:07                     ` Will Deacon
2019-12-17 18:04                       ` Linus Torvalds
2019-12-17 18:05                         ` Linus Torvalds
2019-12-17 18:31                           ` Will Deacon
2019-12-17 18:32                         ` Linus Torvalds
2019-12-18 12:17                           ` Michael Ellerman
2019-12-19 12:11                           ` Will Deacon [this message]
2019-12-18 10:22                     ` Christian Borntraeger
2019-12-18 10:35                       ` Will Deacon
2019-12-13 12:07           ` Michael Ellerman
2019-12-13 13:53             ` Segher Boessenkool
2019-12-13 21:06               ` Michael Ellerman
2019-12-12 15:10     ` Segher Boessenkool
2019-12-06 22:15 ` [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops) pr-tracker-bot

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=20191219121115.GB32361@willie-the-truck \
    --to=will@kernel.org \
    --cc=arnd@arndb.de \
    --cc=borntraeger@de.ibm.com \
    --cc=christophe.leroy@c-s.fr \
    --cc=dja@axtens.net \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=mark.rutland@arm.com \
    --cc=mpe@ellerman.id.au \
    --cc=peterz@infradead.org \
    --cc=segher@kernel.crashing.org \
    --cc=torvalds@linux-foundation.org \
    --subject='Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))' \
    /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
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).