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

[Huang Ying]

On Tue, 2008-11-11 at 09:10 +0800, Chris Snook wrote:
> Huang Ying wrote:
> > On Mon, 2008-11-10 at 15:35 +0800, 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.
> > 
> > Yes. atomic_inc() doesn't imply barrier on all architecture. But we
> > should not add barriers before all atomic_inc(), just ones needed. Can
> > you figure out which ones in the patch should has barrier added?
> 
> You need barriers *after* writes, and *before* reads.  Adding barriers to the 
> oops path should be extremely cheap for performance, unless oopsing is a common 
> occurrence, in which case we have bigger problems.

I just suspect why we need these barriers. Do we have some memory must
to be written after oops_in_progress? Or some memory must to be read
before oops_in_progress?

Best Regards,
Huang Ying

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