From: Segher Boessenkool <segher@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: linux-arch@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
Christian Borntraeger <borntraeger@de.ibm.com>,
Mark Rutland <mark.rutland@arm.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
dja@axtens.net
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
next prev parent reply other threads:[~2019-12-13 21:07 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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