Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761341Ab3DDNx7 (ORCPT ); Thu, 4 Apr 2013 09:53:59 -0400 Received: from mail-da0-f48.google.com ([209.85.210.48]:45858 "EHLO mail-da0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761221Ab3DDNx6 (ORCPT ); Thu, 4 Apr 2013 09:53:58 -0400 Date: Thu, 4 Apr 2013 06:53:53 -0700 From: Tejun Heo To: Michal Hocko 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: <20130404133706.GA9425@htj.dyndns.org> References: <515BF233.6070308@huawei.com> <515BF2A4.1070703@huawei.com> <20130404113750.GH29911@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130404113750.GH29911@dhcp22.suse.cz> 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: 1245 Lines: 30 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? 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. -- tejun -- 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/