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Date:   Wed, 30 Jun 2021 09:55:55 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Arnd Bergmann <arnd@arndb.de>
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Subject: Re: [PATCH v2 00/33] locking/atomic: convert all architectures to
 ARCH_ATOMIC
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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().