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From: Roland Dreier <rdreier@cisco.com>
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Subject: Re: MMIO and gcc re-ordering issue
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 > > Cool... I assume you do this for mutex_unlock() etc?

 > That's a good point... I don't think we do. Maybe we should.

I think it's needed -- take a look at 76d7cc03, which came from a real
bug seen on Altix boxes.