Why is wmb() a no-op on x86_64?

Hi, Andi -

I notice that wmb() is a no-op on x86_64 kernels unless
CONFIG_UNORDERED_IO is set.  Is there any particular reason for this?
It's not similarly conditional on other platforms, and as a consequence,
in our driver (which requires a write barrier in some situations for
correctness), I have to add the following piece of ugliness:

#if defined(CONFIG_X86_64) && !defined(CONFIG_UNORDERED_IO)
#define ipath_wmb() asm volatile("sfence" ::: "memory")
#else
#define ipath_wmb() wmb()
#endif

	<b

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