Re: [PATH -mm -v2] Fix a race condtion of oops_in_progress

[Chris Snook]

KOSAKI Motohiro wrote:
> > > > 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.

The rmb() before atomic_read() is even more critical, since that's a 
non-barrier operation on nearly all platforms.

-- Chris
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