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

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Message-ID: <4BCC80CA.90902@goop.org>
Date:	Mon, 19 Apr 2010 09:11:54 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Glauber Costa <glommer@...hat.com>,
	Peter Zijlstra <peterz@...radead.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 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

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