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Date: Mon, 20 Nov 2017 12:52:04 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO_CLANG
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>, Sami Tolvanen <samitolvanen@google.com>, Nick Desaulniers <ndesaulniers@google.com>, Alex Matveev <alxmtvv@gmail.com>, Andi Kleen <ak@linux.intel.com>, Ard Biesheuvel <ard.biesheuvel@linaro.org>, Greg Hackmann <ghackmann@google.com>, Kees Cook <keescook@chromium.org>, linux-arm-kernel@lists.infradead.org, Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, Mark Rutland <mark.rutland@arm.com>, Masahiro Yamada <yamada.masahiro@socionext.com>, Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>, Michal Marek <michal.lkml@markovi.net>, Yury Norov <ynorov@caviumnetworks.com>, Matthias Kaehlcke <mka@chromium.org>, Alexander Potapenko <glider@google.com>, Stephen Hines <srhines@google.com>, Pirama Arumuga Nainar <pirama@google.com>, Manoj Gupta <manojgupta@google.com>

On Mon, Nov 20, 2017 at 08:28:06PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 20, 2017 at 06:05:55PM +0000, Will Deacon wrote:
> > This is a thorny issue, but RCU (specifically rcu_dereference but probably
> > also some READ_ONCEs) relies on being able to utilise syntactic dependency
> > chains to order local accesses to shared variables.
> 
> Well, we used to have READ_ONCE() and smp_read_barrier_depends(), but
> we recently munged them together, in the process getting rid of
> lockless_dereference().
> 
> So for sure, READ_ONCE() must be able to do the address dependency
> thing, otherwise tons of code comes apart.

I do hope that they track the volatile accesses produced by READ_ONCE().
Otherwise, it would not be good to apply this to anything touching MMIO.

							Thanx, Paul