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
WARNING: multiple messages have this Message-ID (diff)
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 20:37 UTC|newest] Thread overview: 86+ 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 12:46 ` Michael Ellerman 2019-12-06 13:16 ` Peter Zijlstra 2019-12-06 13:16 ` Peter Zijlstra 2019-12-10 5:38 ` Michael Ellerman 2019-12-10 5:38 ` Michael Ellerman 2019-12-10 10:15 ` Peter Zijlstra 2019-12-10 10:15 ` Peter Zijlstra 2019-12-11 0:29 ` Michael Ellerman 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 5:42 ` Michael Ellerman 2019-12-12 8:01 ` Peter Zijlstra 2019-12-12 8:01 ` Peter Zijlstra 2019-12-12 10:07 ` Will Deacon 2019-12-12 10:07 ` Will Deacon 2019-12-12 10:46 ` Peter Zijlstra 2019-12-12 10:46 ` Peter Zijlstra 2019-12-12 17:04 ` Will Deacon 2019-12-12 17:04 ` Will Deacon 2019-12-12 17:16 ` Will Deacon 2019-12-12 17:16 ` Will Deacon 2019-12-12 17:41 ` Linus Torvalds 2019-12-12 17:41 ` Linus Torvalds 2019-12-12 17:50 ` Segher Boessenkool 2019-12-12 17:50 ` Segher Boessenkool 2019-12-12 18:06 ` Will Deacon 2019-12-12 18:06 ` Will Deacon 2019-12-12 18:29 ` Christian Borntraeger 2019-12-12 18:29 ` Christian Borntraeger 2019-12-12 18:43 ` Linus Torvalds 2019-12-12 18:43 ` Linus Torvalds 2019-12-12 19:34 ` Will Deacon 2019-12-12 19:34 ` Will Deacon 2019-12-12 20:21 ` Peter Zijlstra 2019-12-12 20:21 ` Peter Zijlstra 2019-12-12 20:53 ` Peter Zijlstra 2019-12-12 20:53 ` Peter Zijlstra 2019-12-13 10:47 ` Luc Van Oostenryck 2019-12-13 10:47 ` Luc Van Oostenryck 2019-12-13 12:56 ` Peter Zijlstra 2019-12-13 12:56 ` Peter Zijlstra 2019-12-13 14:28 ` Luc Van Oostenryck 2019-12-13 14:28 ` Luc Van Oostenryck 2019-12-12 20:49 ` Linus Torvalds 2019-12-12 20:49 ` Linus Torvalds 2019-12-13 13:17 ` Arnd Bergmann 2019-12-13 13:17 ` Arnd Bergmann 2019-12-13 21:32 ` Arnd Bergmann 2019-12-13 21:32 ` Arnd Bergmann 2019-12-13 22:01 ` Linus Torvalds 2019-12-13 22:01 ` Linus Torvalds 2019-12-16 10:28 ` Will Deacon 2019-12-16 10:28 ` Will Deacon 2019-12-16 11:47 ` Peter Zijlstra 2019-12-16 11:47 ` Peter Zijlstra 2019-12-16 12:06 ` Arnd Bergmann 2019-12-16 12:06 ` Arnd Bergmann 2019-12-17 17:07 ` Will Deacon 2019-12-17 17:07 ` Will Deacon 2019-12-17 18:04 ` Linus Torvalds 2019-12-17 18:04 ` Linus Torvalds 2019-12-17 18:05 ` Linus Torvalds 2019-12-17 18:05 ` Linus Torvalds 2019-12-17 18:31 ` Will Deacon 2019-12-17 18:31 ` Will Deacon 2019-12-17 18:32 ` Linus Torvalds 2019-12-17 18:32 ` Linus Torvalds 2019-12-18 12:17 ` Michael Ellerman 2019-12-18 12:17 ` Michael Ellerman 2019-12-19 12:11 ` Will Deacon 2019-12-19 12:11 ` Will Deacon 2019-12-18 10:22 ` Christian Borntraeger 2019-12-18 10:22 ` Christian Borntraeger 2019-12-18 10:35 ` Will Deacon 2019-12-18 10:35 ` Will Deacon 2019-12-13 12:07 ` Michael Ellerman 2019-12-13 12:07 ` Michael Ellerman 2019-12-13 13:53 ` Segher Boessenkool [this message] 2019-12-13 13:53 ` Segher Boessenkool 2019-12-13 21:06 ` Michael Ellerman 2019-12-13 21:06 ` Michael Ellerman 2019-12-12 15:10 ` Segher Boessenkool 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 2019-12-06 22:15 ` pr-tracker-bot
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