From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755942Ab0C3Ivl (ORCPT ); Tue, 30 Mar 2010 04:51:41 -0400 Received: from casper.infradead.org ([85.118.1.10]:50624 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755649Ab0C3Ivk convert rfc822-to-8bit (ORCPT ); Tue, 30 Mar 2010 04:51:40 -0400 Subject: Re: [PATCH tip/core/urgent] rcu: protect fork-time cgroup access From: Peter Zijlstra To: Paul Menage Cc: paulmck@linux.vnet.ibm.com, 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: <6599ad831003291543r71300bcfv2957004bf2e927bb@mail.gmail.com> References: <20100329211525.GA17703@linux.vnet.ibm.com> <6599ad831003291543r71300bcfv2957004bf2e927bb@mail.gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Tue, 30 Mar 2010 10:50:34 +0200 Message-ID: <1269939034.5109.511.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2010-03-29 at 15:43 -0700, Paul Menage wrote: > On Mon, Mar 29, 2010 at 2:15 PM, 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(); > > I think you're right that this is a false positive - it would only be > a problem if it were possible for the task to be moved to a different > cgroup, and I think that shouldn't be the case at this point in the > fork path since the new process isn't visible on the tasklist yet, > right? Well the thing is, this fork time invocation of set_task_cpu()->set_task_rq() is in no way special, there's multiple places like that.