Hello, Michal. On Wed, Dec 05, 2012 at 03:35:37PM +0100, Michal Hocko wrote: > On Tue 04-12-12 07:22:25, Tejun Heo wrote: > > 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. > > For the cases memcg use this test it is perfectly valid because the only > important information is whether there is a child group. We do not care > about its current state. The test is rather strict because we could set > use_hierarchy to 1 even if there is child which is not online yet (after > the value is copied in css_online of course). But do we care about this > race? If yes, patches with the use case are welcome. Please just implement properly synchronized onlineness. There is absoluately *NO* reason not to do it. It's gonna be error/race-prone like hell if memcg keeps trying to dance around it. > > 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. > > I would rather not complicate the iterators with even more logic but if > it turns out being useful then why not. It's gonna be as simple as the following. I doubt it's gonna add much to the complexity. if (!memcg_online(pos)) continue; // or goto next; whatever Thanks. -- tejun -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>