From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753917Ab0C2V0L (ORCPT ); Mon, 29 Mar 2010 17:26:11 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:58775 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752872Ab0C2V0K (ORCPT ); Mon, 29 Mar 2010 17:26:10 -0400 Subject: Re: [PATCH tip/core/urgent] rcu: protect fork-time cgroup access From: Peter Zijlstra To: paulmck@linux.vnet.ibm.com 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 In-Reply-To: <1269897564.12097.372.camel@laptop> References: <20100329211525.GA17703@linux.vnet.ibm.com> <1269897564.12097.372.camel@laptop> Content-Type: text/plain; charset="UTF-8" Date: Mon, 29 Mar 2010 23:26:04 +0200 Message-ID: <1269897964.12097.375.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2010-03-29 at 23:19 +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. So the only cgroup thing I can find is set_task_rq()'s task_group() usage, which does indeed look like it wants RCU, but then, why would only the sched_fork() usage of set_task_cpu() need this. If it's needed (possible) then set_task_rq() needs it unconditionally. That said, the whole task_group stuff is tied to the cgroup muck, so it shouldn't be possible for the task_group to disappear on us, but then, I always sorta glaze over when I get near the cgroup core.