Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762382Ab3DDPUd (ORCPT ); Thu, 4 Apr 2013 11:20:33 -0400 Received: from cantor2.suse.de ([195.135.220.15]:45669 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762351Ab3DDPUc (ORCPT ); Thu, 4 Apr 2013 11:20:32 -0400 Date: Thu, 4 Apr 2013 17:20:28 +0200 From: Michal Hocko To: Tejun Heo Cc: Li Zefan , linux-mm@kvack.org, LKML , Cgroups , Glauber Costa , KAMEZAWA Hiroyuki , Johannes Weiner Subject: Re: [RFC][PATCH 5/7] cgroup: make sure parent won't be destroyed before its children Message-ID: <20130404152028.GK29911@dhcp22.suse.cz> References: <515BF233.6070308@huawei.com> <515BF2A4.1070703@huawei.com> <20130404113750.GH29911@dhcp22.suse.cz> <20130404133706.GA9425@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130404133706.GA9425@htj.dyndns.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1520 Lines: 38 On Thu 04-04-13 06:53:53, Tejun Heo wrote: > Hey, > > On Thu, Apr 04, 2013 at 01:37:50PM +0200, Michal Hocko wrote: > > On Wed 03-04-13 17:13:08, Li Zefan wrote: > > > Suppose we rmdir a cgroup and there're still css refs, this cgroup won't > > > be freed. Then we rmdir the parent cgroup, and the parent is freed due > > > to css ref draining to 0. Now it would be a disaster if the child cgroup > > > tries to access its parent. > > > > Hmm, I am not sure what is the correct layer for this to handle - cgroup > > core or memcg. But we have enforced that in mem_cgroup_css_online where > > we take an additional reference to the memcg. > > > > Handling it in the memcg code would have an advantage of limiting an > > additional reference only to use_hierarchy cases which is sufficient > > as we never touch the parent otherwise (parent_mem_cgroup). > > But what harm does an additional reference do? No harm at all. I just wanted to be sure that this is not yet another "for memcg" hack. So if this is useful for other controllers then I have no objections of course. > And given that there are cgroup core interfaces which access ->parent, > I think it'd be a good idea that parent always exists while there are > children. > > Thanks. -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/