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
next prev 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
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