From: Michael Ellerman <mpe@ellerman.id.au>
To: Segher Boessenkool <segher@kernel.crashing.org>
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: Sat, 14 Dec 2019 08:06:49 +1100 [thread overview]
Message-ID: <87mubwndee.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <20191213135353.GN3152@gate.crashing.org>
Segher Boessenkool <segher@kernel.crashing.org> writes:
> 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.
Argh, yuck. Thanks, I shouldn't try to read asm listings at 11pm.
So that just confirms what Will was saying further up the thread about
the volatile pointer, rather than READ_ONCE() per se.
cheers
next prev parent reply other threads:[~2019-12-13 21:06 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
2019-12-13 21:06 ` Michael Ellerman [this message]
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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