From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754056Ab0C2V3l (ORCPT ); Mon, 29 Mar 2010 17:29:41 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:52333 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752847Ab0C2V3j (ORCPT ); Mon, 29 Mar 2010 17:29:39 -0400 Date: Mon, 29 Mar 2010 14:29:32 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, abogani@texware.it Subject: Re: [PATCH tip/core/urgent] rcu: protect fork-time cgroup access Message-ID: <20100329212932.GO2569@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100329211525.GA17703@linux.vnet.ibm.com> <1269897564.12097.372.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1269897564.12097.372.camel@laptop> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 29, 2010 at 11:19:23PM +0200, Peter Zijlstra wrote: > On Mon, 2010-03-29 at 14:15 -0700, Paul E. McKenney wrote: > > Add an rcu_read_lock() / rcu_read_unlock() pair to protect a fork-time > > cgroup access. This seems likely to be a false positive. > > > > Located by: Alessio Igor Bogani > > Signed-off-by: Paul E. McKenney > > --- > > > > sched.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/kernel/sched.c b/kernel/sched.c > > index 9ab3cd7..d4bb5e0 100644 > > --- a/kernel/sched.c > > +++ b/kernel/sched.c > > @@ -2621,7 +2621,9 @@ void sched_fork(struct task_struct *p, int clone_flags) > > if (p->sched_class->task_fork) > > p->sched_class->task_fork(p); > > > > + rcu_read_lock(); > > set_task_cpu(p, cpu); > > + rcu_read_unlock(); > > > > #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT) > > if (likely(sched_info_on())) > > What got accessed? This patch just looks wrong. It might well be wrong. The lockdep RCU splat triggered in task_subsys_state(): static inline struct cgroup_subsys_state *task_subsys_state( struct task_struct *task, int subsys_id) { return rcu_dereference_check(task->cgroups->subsys[subsys_id], rcu_read_lock_held() || cgroup_lock_is_held()); } My thought was that this access was safe due to the fact that we were in the middle of creating a task, so that no other CPU could access it. But I am not certain of this, given the fact that this is digging through cgroup state. The lockdep splat is here: http://pastebin.ubuntu.com/406131/ Suggestions? Thanx, Paul