On Wednesday 18 January 2006 17:23, Bryan O'Sullivan wrote:
> Hi, Andi -
>
> I notice that wmb() is a no-op
Actually it is a compiler optimizer barrier, not a no-op.
> on x86_64 kernels unless
> CONFIG_UNORDERED_IO is set.
Because x86 is architecturally defined as having ordered writes (unless you use
write combining or non temporal stores which normal kernel code doesn't). So it's
not needed.
> 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
Hmm, I suppose one could add a wc_wmb() or somesuch, but WC
is currently deeply architecture specific so I'm not sure
how you can even use it portably.
Why do you need the barrier?
-Andi
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