Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753042Ab0DSXBk (ORCPT ); Mon, 19 Apr 2010 19:01:40 -0400 Received: from e8.ny.us.ibm.com ([32.97.182.138]:56139 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752786Ab0DSXBi (ORCPT ); Mon, 19 Apr 2010 19:01:38 -0400 Date: Mon, 19 Apr 2010 16:01:36 -0700 From: "Paul E. McKenney" To: Eric Paris Cc: Peter Zijlstra , Eric Paris , Miles Lane , LKML Subject: Re: INFO: suspicious rcu_dereference_check() usage - include/linux/cgroup.h:492 invoked rcu_dereference_check() without protection! Message-ID: <20100419230136.GA16856@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100311032843.GE6767@linux.vnet.ibm.com> <1271098032.4807.137.camel@twins> <1271242058.32749.19.camel@laptop> <1271701612.2972.5.camel@dhcp231-113.rdu.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1271701612.2972.5.camel@dhcp231-113.rdu.redhat.com> 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 Content-Length: 6830 Lines: 154 On Mon, Apr 19, 2010 at 02:26:52PM -0400, Eric Paris wrote: > On Wed, 2010-04-14 at 12:47 +0200, Peter Zijlstra wrote: > > On Mon, 2010-04-12 at 20:47 +0200, Peter Zijlstra wrote: > > > On Mon, 2010-04-12 at 14:44 -0400, Eric Paris wrote: > > > > On Wed, Mar 10, 2010 at 11:28 PM, Paul E. McKenney > > > > wrote: > > > > > > > I know you indicated this was fixed in mainline and I see that set of > > > > commits objects, but I'm seeing the below spew from linux-next today. > > > > > > > > tree: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git > > > > tag: next-20100412 > > > > commit: bbeecf185fe464ccd7ee97ce6d3646ad686995b4 > > > > > > > > [ 0.035602] =================================================== > > > > [ 0.036003] [ INFO: suspicious rcu_dereference_check() usage. ] > > > > [ 0.037006] --------------------------------------------------- > > > > [ 0.038004] include/linux/cgroup.h:533 invoked > > > > rcu_dereference_check() without protection! > > > > [ 0.039003] > > > > [ 0.039004] other info that might help us debug this: > > > > [ 0.039004] > > > > [ 0.040003] > > > > [ 0.040004] rcu_scheduler_active = 1, debug_locks = 0 > > > > [ 0.041004] no locks held by swapper/0. > > > > [ 0.042003] > > > > [ 0.042004] stack backtrace: > > > > [ 0.043005] Pid: 0, comm: swapper Not tainted 2.6.34-rc3-next-20100412+ #65 > > > > [ 0.044003] Call Trace: > > > > [ 0.045015] [] lockdep_rcu_dereference+0xaf/0xc0 > > > > [ 0.046010] [] set_task_cpu+0x2d2/0x370 > > > > > > Oh, right, I still have to sort that out. > > > > > > I need to figure out how all that scheduler and cgroup muck interact to > > > fix this. > > > > I think the below should cure this.. > > > > > > Signed-off-by: Peter Zijlstra > > --- > > kernel/sched.c | 10 ++++++++++ > > 1 files changed, 10 insertions(+), 0 deletions(-) > > > > diff --git a/kernel/sched.c b/kernel/sched.c > > index 3acf694..2e06d87 100644 > > --- a/kernel/sched.c > > +++ b/kernel/sched.c > > @@ -323,6 +323,15 @@ static inline struct task_group *task_group(struct task_struct *p) > > /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */ > > static inline void set_task_rq(struct task_struct *p, unsigned int cpu) > > { > > + /* > > + * Strictly speaking this rcu_read_lock() is not needed since the > > + * task_group is tied to the cgroup, which in turn can never go away > > + * as long as there are tasks attached to it. > > + * > > + * However since task_group() uses task_subsys_state() which is an > > + * rcu_dereference() user, this quiets CONFIG_PROVE_RCU. > > + */ > > + rcu_read_lock(); > > #ifdef CONFIG_FAIR_GROUP_SCHED > > p->se.cfs_rq = task_group(p)->cfs_rq[cpu]; > > p->se.parent = task_group(p)->se[cpu]; > > @@ -332,6 +341,7 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu) > > p->rt.rt_rq = task_group(p)->rt_rq[cpu]; > > p->rt.parent = task_group(p)->rt_se[cpu]; > > #endif > > + rcu_read_unlock(); > > } > > > > #else > > So I'm back with another one even with this patch. Would people prefer > another thread? > > [ 0.037175] =================================================== > [ 0.038003] [ INFO: suspicious rcu_dereference_check() usage. ] > [ 0.039003] --------------------------------------------------- > [ 0.040004] include/linux/cgroup.h:533 invoked rcu_dereference_check() without protection! > [ 0.041003] > [ 0.041004] other info that might help us debug this: > [ 0.041005] > [ 0.042004] > [ 0.042004] rcu_scheduler_active = 1, debug_locks = 0 > [ 0.043004] no locks held by swapper/0. > [ 0.044003] > [ 0.044004] stack backtrace: > [ 0.045005] Pid: 0, comm: swapper Not tainted 2.6.34-rc4-next-20100415+ #94 > [ 0.046004] Call Trace: > [ 0.047014] [] lockdep_rcu_dereference+0xaf/0xc0 > [ 0.048013] [] freezer_fork+0xb3/0xd0 > [ 0.049007] [] cgroup_fork_callbacks+0x2c/0x40 > [ 0.050007] [] copy_process+0xb6a/0x11e0 > [ 0.051006] [] do_fork+0xbe/0x3e0 > [ 0.052007] [] ? sched_clock+0x9/0x10 > [ 0.053008] [] ? sched_clock_local+0x15/0x80 > [ 0.054006] [] ? sched_clock_cpu+0xb9/0xf0 > [ 0.055006] [] ? up+0x35/0x50 > [ 0.056006] [] ? get_lock_stats+0x23/0x70 > [ 0.057006] [] ? put_lock_stats+0xe/0x30 > [ 0.058010] [] ? kernel_init+0x0/0x2e0 > [ 0.059006] [] kernel_thread+0x8d/0xa0 > [ 0.060006] [] ? kernel_init+0x0/0x2e0 > [ 0.061007] [] ? kernel_thread_helper+0x0/0x10 > [ 0.062006] [] ? early_idt_handler+0x0/0x71 > [ 0.063011] [] rest_init+0x21/0x110 > [ 0.064005] [] start_kernel+0x3af/0x490 > [ 0.065006] [] x86_64_start_reservations+0x7c/0xd0 > [ 0.066005] [] ? early_idt_handlers+0x0/0x140 > [ 0.067006] [] x86_64_start_kernel+0xf8/0x130 Yep, different code path to the same location. Does the following patch help? Thanx, Paul ------------------------------------------------------------------------ commit 2836f18139267ea918ed2cf39023fb0eb38c4361 Author: Paul E. McKenney Date: Mon Apr 19 15:59:50 2010 -0700 rcu: fix RCU lockdep splat on freezer_fork path Add an RCU read-side critical section to suppress this false positive. Located-by: Eric Paris Signed-off-by: Paul E. McKenney diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c index da5e139..e5c0244 100644 --- a/kernel/cgroup_freezer.c +++ b/kernel/cgroup_freezer.c @@ -205,9 +205,12 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task) * No lock is needed, since the task isn't on tasklist yet, * so it can't be moved to another cgroup, which means the * freezer won't be removed and will be valid during this - * function call. + * function call. Nevertheless, apply RCU read-side critical + * section to suppress RCU lockdep false positives. */ + rcu_read_lock(); freezer = task_freezer(task); + rcu_read_unlock(); /* * The root cgroup is non-freezable, so we can skip the -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/