Hello, Michal.

On Tue, Dec 04, 2012 at 04:14:20PM +0100, Michal Hocko wrote:

OK, I read this as "generic helper doesn't make much sense". Then I
would just ask. Does cgroup core really care whether we do
list_empty test? Is this something that we have to care about in memcg
and should fix? If yes then just try to do it as simple as possible.

The thing is, what does the test mean when it doesn't have proper
synchronization?  list_empty(&cgroup->children) doesn't really have a
precise meaning if you're not synchronized.  There could be cases
where such correct-most-of-the-time results are okay but depending on
stuff like that is a sure-fire way to subtle bugs.

So, my recommendation would be to bite the bullet and implement
properly synchronized on/offline state and teach the memcg iterator
about it so that memcg can definitively tell what's online and what's
not while holding memcg_mutex, and use such knowledge consistently.

Thanks.

-- 
tejun

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