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

Jeremy Fitzhardinge
2010-04-19T16:11:55Z

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