From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock
Date: Mon, 19 Apr 2010 09:11:54 -0700
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Cc: Glauber Costa <glommer@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Zachary Amsden <zamsden@redhat.com>
To: Avi Kivity <avi@redhat.com>
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On 04/19/2010 07:33 AM, 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.

Well, on a 32b system, you can explicitly order the updates of low and
high, then do a high-low-checkhigh read.  That would be much more
efficient than atomic64.  If we really care about 32b.

    J