Hello,

On Tue, Sep 11, 2018 at 12:40:36PM +0800, jianchao.wang wrote:

(long)(sum (every cpu's refcounter)) + atomic refcounter

and when switch to atomic mode, there could be value left in atomic refcounter.
then the unsigned long percpu refcounter cound be decreased from 0.

It can always do that.  Imagine a task getting a ref on one cpu,
migrating and putting it on another.  The only thing which matters is
that the sum of everything adds upto what it should be.

Thanks.

-- 
tejun