linux-kernel - Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG

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Date:   Tue, 30 Jun 2020 22:20:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Will Deacon <will@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Matt Turner <mattst88@...il.com>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Richard Henderson <rth@...ddle.net>,
        Alan Stern <stern@...land.harvard.edu>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-alpha@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when
 CLANG_LTO=y

On Tue, Jun 30, 2020 at 09:47:30PM +0200, Marco Elver wrote:
> I do wonder, though, if there is some way to make the compiler do
> something better for us. Clearly, implementing real
> memory_order_consume hasn't worked out until today. But maybe the
> compiler could promote dependent loads to acquires if it recognizes it
> lost dependencies during optimizations. Just thinking out loud, it
> probably still has some weird corner case that will break. ;-)

I'd be very hesitant to let the compiler upgrade the ordering for us,
specifically because we're not using C11 crud and are using a lot of
inline asm.