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

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Date:   Tue, 21 Nov 2017 17:23:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'paulmck@...ux.vnet.ibm.com'" <paulmck@...ux.vnet.ibm.com>,
        "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-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

From: Paul E. McKenney
> Sent: 20 November 2017 20:54
> 
> On Mon, Nov 20, 2017 at 08:32:56PM +0100, Peter Zijlstra wrote:
> > On Mon, Nov 20, 2017 at 06:05:55PM +0000, Will Deacon wrote:
> > > Although the current direction of the C++ committee is to prefer
> > > that dependencies are explicitly "marked", this is not deemed to be
> > > acceptable for the kernel (in other words, everything is always considered
> > > "marked").
> >
> > Yeah, that is an attitude not compatible with existing code. Much like
> > the proposal to allow temporary/wide stores on everything not explicitly
> > declared atomic. Such stuff instantly breaks all extant code that does
> > multi-threading with no recourse.
> 
> If someone suggests temporary/wide stores, even on non-atomics, tell
> them that the standard does not permit them to introduce data races.

The C standard doesn't say anything about multi-threading.

The x86 bis (bit set) family are well known for being problematic
because they always do a 32bit wide rmw cycle.

	David