From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Blum Subject: Re: [PATCH v4 2/2] cgroups: make procs file writable Date: Wed, 4 Aug 2010 00:38:49 -0400 Message-ID: <20100804043849.GC11950@ghc17.ghc.andrew.cmu.edu> References: <20100730235649.GA22644@ghc17.ghc.andrew.cmu.edu> <20100730235902.GC22644@ghc17.ghc.andrew.cmu.edu> <20100804100811.199d73ba.kamezawa.hiroyu@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Paul Menage Cc: Ben Blum , containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org List-Id: containers.vger.kernel.org On Tue, Aug 03, 2010 at 09:30:00PM -0700, Paul Menage wrote: > >> --- a/kernel/cpuset.c > >> +++ b/kernel/cpuset.c > >> @@ -1404,6 +1404,10 @@ static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cont, > >> ? ? ? ? ? ? ? struct task_struct *c; > >> > >> ? ? ? ? ? ? ? rcu_read_lock(); > >> + ? ? ? ? ? ? if (!thread_group_leader(tsk)) { > >> + ? ? ? ? ? ? ? ? ? ? rcu_read_unlock(); > >> + ? ? ? ? ? ? ? ? ? ? return -EAGAIN; > >> + ? ? ? ? ? ? } > > Why are you adding this requirement, here and in sched.c? (ns_cgroup.c > doesn't matter since it's being deleted). > > Paul It was either this or: rcu_read_lock(); for_each_subsys(...) { can_attach(...); } rcu_read_unlock(); Which forces all can_attaches to not sleep. So by dropping rcu_read_lock(), we allow the possibility of the exec race I described in my last email, and therefore we have to check each time we re-acquire rcu_read to iterate thread_group. Yeah, it is not pretty. I call it "double-double-toil-and-trouble-check locking". But it is safe. -- Ben From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756063Ab0HDEjh (ORCPT ); Wed, 4 Aug 2010 00:39:37 -0400 Received: from SMTP.ANDREW.CMU.EDU ([128.2.11.96]:45613 "EHLO smtp.andrew.cmu.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755741Ab0HDEjf (ORCPT ); Wed, 4 Aug 2010 00:39:35 -0400 Date: Wed, 4 Aug 2010 00:38:49 -0400 From: Ben Blum To: Paul Menage Cc: KAMEZAWA Hiroyuki , Ben Blum , linux-kernel@vger.kernel.org, containers@lists.linux-foundation.org, akpm@linux-foundation.org, ebiederm@xmission.com, lizf@cn.fujitsu.com, matthltc@us.ibm.com, oleg@redhat.com Subject: Re: [PATCH v4 2/2] cgroups: make procs file writable Message-ID: <20100804043849.GC11950@ghc17.ghc.andrew.cmu.edu> References: <20100730235649.GA22644@ghc17.ghc.andrew.cmu.edu> <20100730235902.GC22644@ghc17.ghc.andrew.cmu.edu> <20100804100811.199d73ba.kamezawa.hiroyu@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) X-PMX-Version: 5.5.9.388399, Antispam-Engine: 2.7.2.376379, Antispam-Data: 2010.4.9.4220 X-SMTP-Spam-Clean: 8% ( BODY_SIZE_1000_1099 0, BODY_SIZE_2000_LESS 0, BODY_SIZE_5000_LESS 0, BODY_SIZE_7000_LESS 0, __BOUNCE_CHALLENGE_SUBJ 0, __BOUNCE_NDR_SUBJ_EXEMPT 0, __CD 0, __CT 0, __CT_TEXT_PLAIN 0, __HAS_MSGID 0, __MIME_TEXT_ONLY 0, __MIME_VERSION 0, __SANE_MSGID 0, __TO_MALFORMED_2 0, __USER_AGENT 0) X-SMTP-Spam-Score: 8% Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 03, 2010 at 09:30:00PM -0700, Paul Menage wrote: > >> --- a/kernel/cpuset.c > >> +++ b/kernel/cpuset.c > >> @@ -1404,6 +1404,10 @@ static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cont, > >> ? ? ? ? ? ? ? struct task_struct *c; > >> > >> ? ? ? ? ? ? ? rcu_read_lock(); > >> + ? ? ? ? ? ? if (!thread_group_leader(tsk)) { > >> + ? ? ? ? ? ? ? ? ? ? rcu_read_unlock(); > >> + ? ? ? ? ? ? ? ? ? ? return -EAGAIN; > >> + ? ? ? ? ? ? } > > Why are you adding this requirement, here and in sched.c? (ns_cgroup.c > doesn't matter since it's being deleted). > > Paul It was either this or: rcu_read_lock(); for_each_subsys(...) { can_attach(...); } rcu_read_unlock(); Which forces all can_attaches to not sleep. So by dropping rcu_read_lock(), we allow the possibility of the exec race I described in my last email, and therefore we have to check each time we re-acquire rcu_read to iterate thread_group. Yeah, it is not pretty. I call it "double-double-toil-and-trouble-check locking". But it is safe. -- Ben