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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Huang Ying <ying.huang@intel.com>
Subject: Re: [PATH -mm -v2] Fix a race condtion of oops_in_progress
Cc: kosaki.motohiro@jp.fujitsu.com, Chris Snook <csnook@redhat.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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> > > As far as I know, barriers don't cause changes to be visible on other
> > > CPUs faster too. It just guarantees corresponding operations after will
> > > not get executed until that before have finished. And, I don't think we
> > > need make changes to be visible on other CPUs faster.
> > 
> > You're correct that barrier() has no impact on other CPUs.  wmb() and rmb() do. 
> >   If we don't need to make changes visible any faster, what's the point in using 
> > atomic_set()?  It's not any less racy.  atomic_inc() and atomic_dec() would be 
> > less racy, but you're not using those.
> 
> In default bust_spinlocks() implementation in lib/bust_spinlocks.c,
> atomic_inc() and atomic_dec_and_test() is used. Which is used by x86
> too. In some other architecture, atomic_set() is used to replace
> "oops_in_progress = <xxx>". So this patch fixes architectures which use
> default bust_spinlocks(), other architectures can be fixed by
> corresponding architecture developers.

I think Chris is right.
So, I reccomend to read Documentation/memory-barriers.txt

Almost architecture gurantee atomic_inc cause barrier implicitly.
but not _all_ architecture.



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