From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755518Ab0C3R2R (ORCPT ); Tue, 30 Mar 2010 13:28:17 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:37905 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754807Ab0C3R2Q (ORCPT ); Tue, 30 Mar 2010 13:28:16 -0400 Date: Tue, 30 Mar 2010 10:28:08 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Paul Menage , 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: <20100330172808.GK2513@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100329211525.GA17703@linux.vnet.ibm.com> <6599ad831003291543r71300bcfv2957004bf2e927bb@mail.gmail.com> <1269939034.5109.511.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1269939034.5109.511.camel@twins> 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 Tue, Mar 30, 2010 at 10:50:34AM +0200, Peter Zijlstra wrote: > 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. Certainly the lack of clarity about why this access is safe indicates that recording some of the locking design in rcu_dereferenceI() would be quite helpful. ;-) Thanx, Paul