From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759477Ab0FPWlP (ORCPT ); Wed, 16 Jun 2010 18:41:15 -0400 Received: from e8.ny.us.ibm.com ([32.97.182.138]:40753 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754545Ab0FPWlN (ORCPT ); Wed, 16 Jun 2010 18:41:13 -0400 Date: Wed, 16 Jun 2010 15:41:10 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org, akpm@linux-foundation.org, tglx@linutronix.de, daniel.blueman@gmail.com, lizf@cn.fujitsu.com, miles.lane@gmail.com, manfred@colorfullife.com Subject: Re: [GIT PULL rcu/urgent] yet more lockdep-RCU splat fixes Message-ID: <20100616224110.GI2457@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100616042224.GA3892@linux.vnet.ibm.com> <20100616055358.GB17936@elte.hu> <1276669431.1745.595.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1276669431.1745.595.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 Wed, Jun 16, 2010 at 08:23:51AM +0200, Peter Zijlstra wrote: > On Wed, 2010-06-16 at 07:53 +0200, Ingo Molnar wrote: > > * Paul E. McKenney wrote: > > > > > Hello, Ingo, > > > > > > Here are a few more fixes for RCU-lockdep splats. > > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git rcu/urgent > > > > > > This is based off of v2.6.35-rc3. > > > > > > Thanx, Paul > > > > > > ------------------> > > > Daniel J Blueman (1): > > > rcu: fix lockdep splat in wake_affine() > > > > > > Paul E. McKenney (2): > > > rcu: fix scope of wake_affine()'s new RCU read-side critical section > > > idr: fix RCU lockdep splat in idr_get_next() > > > > > > kernel/sched_fair.c | 2 ++ > > > lib/idr.c | 4 ++-- > > > 2 files changed, 4 insertions(+), 2 deletions(-) > > > > Pulled, thanks a lot Paul! > > I'm not at all sure of those two wake_affine() ones.. Hello, Peter! Here is the story as I understand it: o wake_affine() calls task_group() and uses the resulting pointer, for example, passing it to effective_load(). This pointer is to a struct task_group, which contains a struct rcu_head, which is passed to call_rcu in sched_destroy_group(). So some protection really is needed -- or is it enough that wake_affine seems to be invoked on the current task? If the latter, we would need to add a "task == current" check to task_subsys_state(). o task_group() calls task_subsys_state(), returning a pointer to the enclosing task_group structure. o task_subsys_state() returns an rcu_dereference_check()ed pointer. The caller must either be in an RCU read-side critical section, hold the ->alloc_lock, or hold the cgroup lock. Now wake_affine() appears to be doing load calculations, so it does not seem reasonable to acquire the lock. Hence the use of RCU. So, what should we be doing instead? ;-) Thanx, Paul