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From: Segher Boessenkool <segher@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	dja@axtens.net, linux-kernel@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, christophe.leroy@c-s.fr,
	linux-arch@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
	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: Fri, 13 Dec 2019 07:53:53 -0600	[thread overview]
Message-ID: <20191213135353.GN3152@gate.crashing.org> (raw)
In-Reply-To: <87pngso2ck.fsf@mpe.ellerman.id.au>

Hi!

On Fri, Dec 13, 2019 at 11:07:55PM +1100, Michael Ellerman wrote:
> I tried this:
> 
> > @@ -295,6 +296,23 @@ void __write_once_size(volatile void *p, void *res, int size)
> >   */
> >  #define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
> >  
> > +#else /* GCC_VERSION < 40800 */
> > +
> > +#define READ_ONCE_NOCHECK(x)						\
> > +({									\
> > +	typeof(x) __x = *(volatile typeof(x))&(x);			\
> 
> Didn't compile, needed:
> 
> 	typeof(x) __x = *(volatile typeof(&x))&(x);			\
> 
> 
> > +	smp_read_barrier_depends();					\
> > +	__x;
> > +})
> 
> 
> And that works for me. No extra stack check stuff.
> 
> I guess the question is does that version of READ_ONCE() implement the
> read once semantics. Do we have a good way to test that?
> 
> The only differences are because of the early return in the generic
> test_and_set_bit_lock():

No, there is another difference:

>   30         ld      r10,560(r9)
>   31         std     r10,104(r1)
>   32         ld      r10,104(r1)
>   33         andi.   r10,r10,1
>   34         bne     <ext4_resize_begin_generic+0xd0>       29         bne     <ext4_resize_begin_ppc+0xd0>

The stack var is volatile, so it is read back immediately after writing
it, here.  This is a bad idea for performance, in general.


Segher

  reply	other threads:[~2019-12-13 20:37 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
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 [this message]
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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