From: Arnd Bergmann <arnd@arndb.de>
To: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
Marco Elver <elver@google.com>, Kees Cook <keescook@chromium.org>,
"Paul E. McKenney" <paulmck@kernel.org>,
"Michael S. Tsirkin" <mst@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Jason Wang <jasowang@redhat.com>,
Nick Desaulniers <ndesaulniers@google.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
Sami Tolvanen <samitolvanen@google.com>,
alpha <linux-alpha@vger.kernel.org>,
Alan Stern <stern@rowland.harvard.edu>,
Matt Turner <mattst88@gmail.com>,
virtualization@lists.linux-foundation.org,
Android Kernel Team <kernel-team@android.com>,
Boqun Feng <boqun.feng@gmail.com>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation
Date: Thu, 2 Jul 2020 12:08:41 +0200 [thread overview]
Message-ID: <CAK8P3a07G1dLC+RUBDkzDbCRCP_gwZAaVK+k1UxvXT+7Kj=o+g@mail.gmail.com> (raw)
In-Reply-To: <20200702094833.GA16248@willie-the-truck>
On Thu, Jul 2, 2020 at 11:48 AM Will Deacon <will@kernel.org> wrote:
> On Thu, Jul 02, 2020 at 10:32:39AM +0100, Mark Rutland wrote:
> > On Tue, Jun 30, 2020 at 06:37:20PM +0100, Will Deacon wrote:
> > > -#define read_barrier_depends() __asm__ __volatile__("mb": : :"memory")
> > > +#define __smp_load_acquire(p) \
> > > +({ \
> > > + __unqual_scalar_typeof(*p) ___p1 = \
> > > + (*(volatile typeof(___p1) *)(p)); \
> > > + compiletime_assert_atomic_type(*p); \
> > > + ___p1; \
> > > +})
> >
> > Sorry if I'm being thick, but doesn't this need a barrier after the
> > volatile access to provide the acquire semantic?
> >
> > IIUC prior to this commit alpha would have used the asm-generic
> > __smp_load_acquire, i.e.
> >
> > | #ifndef __smp_load_acquire
> > | #define __smp_load_acquire(p) \
> > | ({ \
> > | __unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p); \
> > | compiletime_assert_atomic_type(*p); \
> > | __smp_mb(); \
> > | (typeof(*p))___p1; \
> > | })
> > | #endif
I also have a question that I didn't dare ask when the same
code came up before (I guess it's also what's in the kernel today):
With the cast to 'typeof(*p)' at the end, doesn't that mean we
lose the effect of __unqual_scalar_typeof() again, so any "volatile"
pointer passed into __READ_ONCE_SCALAR() or
__smp_load_acquire() still leads to a volatile load of the original
variable, plus another volatile access to ___p1 after
spilling it to the stack as a non-volatile variable?
I hope I'm missing something obvious here.
Arnd
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next prev parent reply other threads:[~2020-07-02 10:10 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-30 17:37 [PATCH 00/18] Allow architectures to override __READ_ONCE() Will Deacon
2020-06-30 17:37 ` [PATCH 01/18] tools: bpf: Use local copy of headers including uapi/linux/filter.h Will Deacon
2020-07-01 16:38 ` Alexei Starovoitov
2020-06-30 17:37 ` [PATCH 02/18] compiler.h: Split {READ, WRITE}_ONCE definitions out into rwonce.h Will Deacon
2020-06-30 19:11 ` Arnd Bergmann
2020-07-01 10:16 ` [PATCH 02/18] compiler.h: Split {READ,WRITE}_ONCE " Will Deacon
2020-07-01 11:33 ` [PATCH 02/18] compiler.h: Split {READ, WRITE}_ONCE " Arnd Bergmann
2020-06-30 17:37 ` [PATCH 03/18] asm/rwonce: Allow __READ_ONCE to be overridden by the architecture Will Deacon
2020-06-30 17:37 ` [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation Will Deacon
2020-07-02 9:32 ` Mark Rutland
2020-07-02 9:48 ` Will Deacon
2020-07-02 10:08 ` Arnd Bergmann [this message]
2020-07-02 11:18 ` Will Deacon
2020-07-02 11:39 ` Arnd Bergmann
2020-07-02 14:43 ` Joel Fernandes
2020-07-02 14:55 ` Will Deacon
2020-07-02 15:07 ` Joel Fernandes
2020-06-30 17:37 ` [PATCH 05/18] asm/rwonce: Remove smp_read_barrier_depends() invocation Will Deacon
2020-06-30 17:37 ` [PATCH 06/18] vhost: Remove redundant use of read_barrier_depends() barrier Will Deacon
2020-06-30 17:37 ` [PATCH 07/18] alpha: Replace smp_read_barrier_depends() usage with smp_[r]mb() Will Deacon
2020-06-30 17:37 ` [PATCH 08/18] locking/barriers: Remove definitions for [smp_]read_barrier_depends() Will Deacon
2020-06-30 17:37 ` [PATCH 09/18] Documentation/barriers: Remove references to [smp_]read_barrier_depends() Will Deacon
2020-06-30 17:37 ` [PATCH 10/18] Documentation/barriers/kokr: " Will Deacon
2020-06-30 17:37 ` [PATCH 11/18] tools/memory-model: Remove smp_read_barrier_depends() from informal doc Will Deacon
2020-06-30 17:37 ` [PATCH 12/18] include/linux: Remove smp_read_barrier_depends() from comments Will Deacon
2020-06-30 17:37 ` [PATCH 13/18] checkpatch: Remove checks relating to [smp_]read_barrier_depends() Will Deacon
2020-06-30 17:37 ` [PATCH 14/18] arm64: Reduce the number of header files pulled into vmlinux.lds.S Will Deacon
2020-06-30 17:37 ` [PATCH 15/18] arm64: alternatives: Split up alternative.h Will Deacon
2020-06-30 17:37 ` [PATCH 16/18] arm64: cpufeatures: Add capability for LDAPR instruction Will Deacon
2020-06-30 17:37 ` [PATCH 17/18] arm64: alternatives: Remove READ_ONCE() usage during patch operation Will Deacon
2020-06-30 17:37 ` [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y Will Deacon
2020-06-30 19:25 ` Arnd Bergmann
2020-07-01 10:19 ` Will Deacon
2020-07-01 10:59 ` Arnd Bergmann
2020-06-30 19:47 ` Marco Elver
2020-06-30 20:20 ` Peter Zijlstra
2020-06-30 22:57 ` Sami Tolvanen
2020-07-01 10:25 ` Will Deacon
2020-07-01 10:24 ` Will Deacon
2020-07-01 17:07 ` Dave P Martin
2020-07-02 7:23 ` Will Deacon
2020-07-06 16:00 ` Dave Martin
2020-07-06 16:34 ` Paul E. McKenney
2020-07-06 17:05 ` Dave Martin
2020-07-06 17:36 ` Paul E. McKenney
2020-07-07 10:29 ` Dave Martin
2020-07-07 22:51 ` Paul E. McKenney
2020-07-07 23:01 ` Nick Desaulniers
2020-07-08 7:15 ` Marco Elver
2020-07-08 9:16 ` Peter Zijlstra
2020-07-08 18:20 ` Paul E. McKenney
2020-07-06 18:35 ` Will Deacon
2020-07-06 19:23 ` Marco Elver
2020-07-06 19:42 ` Paul E. McKenney
2020-07-06 16:08 ` Dave Martin
2020-07-06 18:35 ` Will Deacon
2020-07-07 10:10 ` Dave Martin
2020-07-01 7:38 ` [PATCH 00/18] Allow architectures to override __READ_ONCE() Josh Triplett
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