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Date:	Mon, 03 Nov 2008 09:52:41 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Chris Snook <csnook@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATH -mm -v2] Fix a race condtion of oops_in_progress

On Sat, 2008-11-01 at 00:42 +0800, Chris Snook wrote:
> Huang Ying wrote:
> > Hi, Chris,
> > 
> > On Wed, 2008-10-29 at 08:51 -0600, Chris Snook wrote:
> >> Huang Ying wrote:
> >>> Fix a race condition accessing oops_in_progress.  Which may be changed on
> >>> multiple CPU simultaneously, but it is changed via non-atomic operation
> >>> ++/--.  This patch changes the definition of oops_in_process from int to
> >>> atomic_t, and accessing method to atomic operations.
> >> You also need barriers.  I believe rmb() before atomic_read() and wmb() after 
> >> atomic_set() should suffice.
> > 
> > I don't think that is necessary. I haven't found there is particular
> > consistent requirement about oops_in_progress.
> 
> atomic_read() and atomic_set() don't inherently cause changes to be visible on 
> other CPUs any faster than ++ and -- operators.  Sometimes it happens to work 
> out that way as a result of how the compiler and the CPU order operations, but 
> there's no semantic guarantee, and it could even take arbitrarily long under 
> some circumstances.  If you want to use atomic ops to close the race, you need 
> to use barriers.

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.

Best Regards,
Huang Ying


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