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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	dja@axtens.net,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	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, 12 Dec 2019 12:49:52 -0800	[thread overview]
Message-ID: <CAHk-=wiMuHmWzQ7-CRQB6o+SHtA-u-Rp6VZwPcqDbjAaug80rQ@mail.gmail.com> (raw)
In-Reply-To: <20191212193401.GB19020@willie-the-truck>

On Thu, Dec 12, 2019 at 11:34 AM Will Deacon <will@kernel.org> wrote:
>
> The root of my concern in all of this, and what started me looking at it in
> the first place, is the interaction with 'typeof()'. Inheriting 'volatile'
> for a pointer means that local variables in macros declared using typeof()
> suddenly start generating *hideous* code, particularly when pointless stack
> spills get stackprotector all excited.

Yeah, removing volatile can be a bit annoying.

For the particular case of the bitops, though, it's not an issue.
Since you know the type there, you can just cast it.

And if we had the rule that READ_ONCE() was an arithmetic type, you could do

    typeof(0+(*p)) __var;

since you might as well get the integer promotion anyway (on the
non-volatile result).

But that doesn't work with structures or unions, of course.

I'm not entirely sure we have READ_ONCE() with a struct. I do know we
have it with 64-bit entities on 32-bit machines, but that's ok with
the "0+" trick.

               Linus

  parent reply	other threads:[~2019-12-12 20:50 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 [this message]
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
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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