From: Joel Fernandes <joelaf@google.com>
To: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Jason Wang <jasowang@redhat.com>,
virtualization@lists.linux-foundation.org,
"Joel Fernandes \(Google\)" <joel@joelfernandes.org>,
Arnd Bergmann <arnd@arndb.de>,
Alan Stern <stern@rowland.harvard.edu>,
Sami Tolvanen <samitolvanen@google.com>,
Matt Turner <mattst88@gmail.com>,
"Cc: Android Kernel" <kernel-team@android.com>,
Marco Elver <elver@google.com>, Kees Cook <keescook@chromium.org>,
"Paul E. McKenney" <paulmck@kernel.org>,
Boqun Feng <boqun.feng@gmail.com>,
Josh Triplett <josh@joshtriplett.org>,
Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
"moderated list:ARM64 PORT \(AARCH64 ARCHITECTURE\)"
<linux-arm-kernel@lists.infradead.org>,
Richard Henderson <rth@twiddle.net>,
Nick Desaulniers <ndesaulniers@google.com>,
LKML <linux-kernel@vger.kernel.org>,
linux-alpha@vger.kernel.org
Subject: Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation
Date: Thu, 2 Jul 2020 11:07:19 -0400 [thread overview]
Message-ID: <CAJWu+ooEsf70ri4J+M5+Fkz6VrH1kN5541j71LE8=y=rmSLDJQ@mail.gmail.com> (raw)
In-Reply-To: <20200702145532.GB16999@willie-the-truck>
On Thu, Jul 2, 2020 at 10:55 AM Will Deacon <will@kernel.org> wrote:
> On Thu, Jul 02, 2020 at 10:43:55AM -0400, Joel Fernandes wrote:
> > On Tue, Jun 30, 2020 at 1:38 PM Will Deacon <will@kernel.org> wrote:
> > > diff --git a/arch/alpha/include/asm/barrier.h b/arch/alpha/include/asm/barrier.h
> > > index 92ec486a4f9e..2ecd068d91d1 100644
> > > --- a/arch/alpha/include/asm/barrier.h
> > > +++ b/arch/alpha/include/asm/barrier.h
> > > - * For example, the following code would force ordering (the initial
> > > - * value of "a" is zero, "b" is one, and "p" is "&a"):
> > > - *
> > > - * <programlisting>
> > > - * CPU 0 CPU 1
> > > - *
> > > - * b = 2;
> > > - * memory_barrier();
> > > - * p = &b; q = p;
> > > - * read_barrier_depends();
> > > - * d = *q;
> > > - * </programlisting>
> > > - *
> > > - * because the read of "*q" depends on the read of "p" and these
> > > - * two reads are separated by a read_barrier_depends(). However,
> > > - * the following code, with the same initial values for "a" and "b":
> > > - *
> >
> > Would it be Ok to keep this example in the kernel sources? I think it
> > serves as good documentation and highlights the issue in the Alpha
> > architecture well.
>
> I'd _really_ like to remove it, as I think it only serves to confuse people
> on a topic that is confusing enough already. Paul's perfbook [1] already has
> plenty of information about this, so I don't think we need to repeat that
> here. I could add a citation, perhaps?
True, and also found that LKMM docs and the memory-barriers.txt talks
about it, so removing it here sounds good to me. Maybe a reference
here to either documentation should be Ok.
> > BTW, do you know any architecture where speculative execution of
> > address-dependent loads can cause similar misorderings? That would be
> > pretty insane though. In Alpha's case it is not speculation but rather
> > the split local cache design as the docs mention. The reason I ask
> > is it is pretty amusing that control-dependent loads do have such
> > misordering issues due to speculative branch execution and I wondered
> > what other games the CPUs are playing. FWIW I ran into [1] which talks
> > about analogy between memory dependence and control dependence.
>
> I think you're asking about value prediction, and the implications it would
> have on address-dependent loads where the address can itself be predicted.
Yes.
> I'm not aware of an CPUs where that is observable architecturally.
I see.
> arm64 has some load instructions that do not honour address dependencies,
> but I believe that's mainly to enable alternative cache designs for things
> like non-temporal and large vector loads.
Good to know this, thanks.
- Joel
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next prev parent reply other threads:[~2020-07-02 15:25 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
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 [this message]
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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