From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752850AbaBLCcd (ORCPT ); Tue, 11 Feb 2014 21:32:33 -0500 Received: from szxga02-in.huawei.com ([119.145.14.65]:1803 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751919AbaBLCcc (ORCPT ); Tue, 11 Feb 2014 21:32:32 -0500 Message-ID: <52FADD2B.3080401@huawei.com> Date: Wed, 12 Feb 2014 10:32:11 +0800 From: Li Zefan User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20130801 Thunderbird/17.0.8 MIME-Version: 1.0 To: Michal Hocko CC: Tejun Heo , LKML , Cgroups Subject: Re: [PATCH cgroup/for-3.14-fixes] cgroup: protect modifications to cgroup_idr with cgroup_mutex References: <52F9D9DA.7040108@huawei.com> <20140211154105.GC24490@htj.dyndns.org> <20140211162625.GP11946@dhcp22.suse.cz> <52FAD958.6020505@huawei.com> In-Reply-To: <52FAD958.6020505@huawei.com> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.18.230] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2014/2/12 10:15, Li Zefan wrote: > On 2014/2/12 0:26, Michal Hocko wrote: >> On Tue 11-02-14 10:41:05, Tejun Heo wrote: >> [...] >>> @@ -4254,12 +4256,12 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry, >>> >>> return 0; >>> >>> -err_unlock: >>> - mutex_unlock(&cgroup_mutex); >>> - /* Release the reference count that we took on the superblock */ >>> - deactivate_super(sb); >>> err_free_id: >>> idr_remove(&root->cgroup_idr, cgrp->id); >>> + /* Release the reference count that we took on the superblock */ >>> + deactivate_super(sb); >>> +err_unlock: >>> + mutex_unlock(&cgroup_mutex); >>> err_free_name: >>> kfree(rcu_dereference_raw(cgrp->name)); >>> err_free_cgrp: >> >> Do I have to change deactivate_super vs. mutex_unlock ordering in my >> backport for 3.12 as well? >> > > Your change is wrong that you shouldn't drop sb refcnt in err_unlock path. > > But you made me think if it's OK to hold cgroup_mutex while calling deactivate_super(), > and the answer is NO! deactive_super() may call cgroup_kill_sb() which will > acquire cgroup_mutex. > > I'll update the patch. > > Thank Tejun we won't be entangled with vfs internal anymore after coverting > to kernfs. > On second thought, it should be safe to call deactivate_super() before releasing cgroup_mutex, as cgroup_create() is called through vfs, so vfs should guanrantee the superblock won't disapear, so this deactivate_super() won't drop sb refcnt to 0. Still this is just my guess without diving into vfs code, and we'd better not depend on it even my guess is correct.