linux-kernel - Re: [PATCH v2 00/33] locking/atomic: convert all architectures to ARCH

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Date:   Wed, 30 Jun 2021 09:55:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Brian Cain <bcain@...eaurora.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Chris Zankel <chris@...kel.net>, Rich Felker <dalias@...c.org>,
        David Miller <davem@...emloft.net>,
        Vincent Chen <deanbo422@...il.com>,
        Helge Deller <deller@....de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Greentime Hu <green.hu@...il.com>, Guo Ren <guoren@...nel.org>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Max Filippov <jcmvbkbc@...il.com>,
        Jonas Bonn <jonas@...thpole.se>,
        Ley Foon Tan <ley.foon.tan@...el.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Matt Turner <mattst88@...il.com>,
        Michal Simek <monstr@...str.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nick Hu <nickhu@...estech.com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Richard Henderson <rth@...ddle.net>,
        Stafford Horne <shorne@...il.com>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Vineet Gupta <vgupta@...opsys.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH v2 00/33] locking/atomic: convert all architectures to
 ARCH_ATOMIC

On Tue, Jun 29, 2021 at 09:36:23AM +0200, Arnd Bergmann wrote:
> For the specific case of cmpxchg64(), I do think it would make sense to either
> have a Kconfig symbol that controls the few users, or to provide a spinlock
> based fallback for those that don't have a native implementation.

My preference goes to a Kconfig based solution; I so detest spinlock
based atomics. I dream of the day we can kill the lot of 'em
(sparc32-smp, parisc-smp are always the ones that come to mind).

This is doubly true for the xchg/cmpxchg/cmpxchg64 family of functions
that are supposed to work together with READ_ONCE/WRITE_ONCE but don't
(when 'emulated', we'd need WRITE_ONCE to also be spinlock based in
that case).

That is, at least for atomic64_*() the whole API is self contained so we
can (and do) frob atomic64_set() to behave sanely vs atomic64_cmpxchg().