[PATCH] arm64: barrier: implement wfe-base smp_cond_load_acquire

[Peter Zijlstra]

On Fri, Jun 17, 2016 at 04:38:03PM +0100, Will Deacon wrote:
> > Is there any case where we wouldn't want the memory clobber?
> 
> I don't think you'd need it for cmpwait_relaxed, because the CPU could
> reorder stuff anyway, so anything the compiler does is potentially futile.
> So actually, I can respin this without the clobber. I'll simplify
> the __CMPWAIT_CASE macro to drop the last two parameters as well.

In my previous patches I promoted cmpwait() as a cpu_relax()
'replacement'. Should we not retain the compiler barrier semantics
because of that?

That is, anywhere you use this, you basically _have_ to reread memory,
because the very fact that you return from this basically indicates its
been changed.