linux-kernel - Re: [PATCH 1/5] Add a global synchronization point for pvclock

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Date:	Mon, 19 Apr 2010 16:46:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Glauber Costa <glommer@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

On Mon, 2010-04-19 at 17:33 +0300, Avi Kivity wrote:
> On 04/19/2010 05:21 PM, Glauber Costa wrote:
> >
> >> Oh yes, just trying to avoid a patch with both atomic64_read() and
> >> ACCESS_ONCE().
> >>      
> > you're mixing the private version of the patch you saw with this one.
> > there isn't any atomic reads in here. I'll use a barrier then
> >    
> 
> This patch writes last_value atomically, but reads it non-atomically.  A 
> barrier is insufficient.

What avi says! :-)

On a 32bit machine a 64bit read are two 32bit reads, so

  last = last_value;

becomes:

  last.high = last_value.high;
  last.low  = last_vlue.low;

(or the reverse of course)

Now imagine a write getting interleaved with that ;-)



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