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

[Chris Snook <csnook@redhat.com>]

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