Re: [PATCH v9 1/2] arch/*: Add CONFIG_ARCH_HAVE_CMPXCHG64

[Christoph Hellwig]

> +config ARCH_HAVE_CMPXCHG64
> +	bool

64-bit architectures must support this as long is 64-bits wide.

So this should have a

	default y if 64BIT

which also means you only need to explicitly select in on 32-bit
architectures that support 64-bit cmpxchg.