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From: Huang Ying <ying.huang@intel.com>
To: Chris Snook <csnook@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATH -mm -v2] Fix a race condtion of oops_in_progress
Date: Tue, 11 Nov 2008 09:19:18 +0800	[thread overview]
Message-ID: <1226366358.6081.96.camel@yhuang-dev.sh.intel.com> (raw)
In-Reply-To: <4918DB8E.2020004@redhat.com>

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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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      reply	other threads:[~2008-11-11  1:19 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-10-29  8:26 Huang Ying
2008-10-29  8:34 ` Ingo Molnar
2008-10-29  8:42   ` Huang Ying
2008-10-29 14:51 ` Chris Snook
2008-10-30  2:02   ` Huang Ying
2008-10-31 16:42     ` Chris Snook
2008-11-03  1:52       ` Huang Ying
2008-11-03 18:44         ` Chris Snook
2008-11-04  1:41           ` Huang Ying
2008-11-10  7:35             ` KOSAKI Motohiro
2008-11-10 18:45               ` Chris Snook
2008-11-11  1:05               ` Huang Ying
2008-11-11  1:10                 ` Chris Snook
2008-11-11  1:19                   ` Huang Ying [this message]

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