From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753971Ab1LUDOH (ORCPT ); Tue, 20 Dec 2011 22:14:07 -0500 Received: from cn.fujitsu.com ([222.73.24.84]:62458 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1750703Ab1LUDOF (ORCPT ); Tue, 20 Dec 2011 22:14:05 -0500 Message-ID: <4EF14F7B.2040507@cn.fujitsu.com> Date: Wed, 21 Dec 2011 11:16:11 +0800 From: Li Zefan User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc14 Thunderbird/3.1.4 MIME-Version: 1.0 To: Frederic Weisbecker CC: Tejun Heo , LKML , Containers , Cgroups , KAMEZAWA Hiroyuki , Oleg Nesterov , Andrew Morton , Paul Menage , Mandeep Singh Baines Subject: Re: [PATCH 2/2 v2] cgroup: Drop task_lock(parent) on cgroup_fork() References: <1324432958-20414-1-git-send-email-fweisbec@gmail.com> <1324432958-20414-3-git-send-email-fweisbec@gmail.com> <4EF14176.9040206@cn.fujitsu.com> <20111221024849.GC17668@somewhere> In-Reply-To: <20111221024849.GC17668@somewhere> X-MIMETrack: Itemize by SMTP Server on mailserver/fnst(Release 8.5.1FP4|July 25, 2010) at 2011-12-21 11:13:19, Serialize by Router on mailserver/fnst(Release 8.5.1FP4|July 25, 2010) at 2011-12-21 11:13:23, Serialize complete at 2011-12-21 11:13:23 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>> @@ -4556,7 +4556,7 @@ static const struct file_operations proc_cgroupstats_operations = { >>> * >>> * A pointer to the shared css_set was automatically copied in >>> * fork.c by dup_task_struct(). However, we ignore that copy, since >>> - * it was not made under the protection of RCU or cgroup_mutex, so >>> + * it was not made under the protection of threadgroup_change_begin(), so >> >> I think the original comment still stands, but now threadgroup_change_begin() >> can also protect the cgroup pointer from becoming invalid. > > Right but I'm not sure it's worth quoting RCU and cgroup_mutex. The reason > why we use threadgroup_change_begin() is not only to ensure the pointer > validity but also to synchronize the whole cgroup proc logic. This way > when we attach a whole proc with cgroup_attach_proc(), we are sure that > no thread forked too soon or too late such that it wouldn't be migrated with > the rest. > > RCU or cgroup_mutex on dup_task_struct() (+ a get_css_set()) would have > protected the pointer validity but not the whole above described machinery. > So I don't think it's even worth quoting those solutions. But if you prefer > I can keep the old comment. > No, I don't have strong opinion here. So I'll ack this patch. > OTOH what I think is missing in the comment is that explanation on the synchronization > against entire proc migration. I can edit that. > I would appreciate this. :) >> >>> * might no longer be a valid cgroup pointer. cgroup_attach_task() might >>> * have already changed current->cgroups, allowing the previously >>> * referenced cgroup group to be removed and freed. >>> @@ -4566,10 +4566,14 @@ static const struct file_operations proc_cgroupstats_operations = { >>> */ >>> void cgroup_fork(struct task_struct *child) >>> { >>> - task_lock(current); >>> + /* >>> + * We don't need to task_lock() current because current->cgroups >>> + * can't be changed concurrently here. The parent obviously hasn't >>> + * exited and called cgroup_exit(), and we are synchronized against >>> + * cgroup migration through threadgroup_change_begin(). >>> + */ >>> child->cgroups = current->cgroups; >>> get_css_set(child->cgroups); >>> - task_unlock(current); >>> INIT_LIST_HEAD(&child->cg_list); >>> } >>> >