linux-kernel - Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO

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

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