Re: INFO: suspicious rcu_dereference_check() usage -include/linux/cgroup.h:492 invoked rcu_dereference_check() withoutprotection!

From: Paul E. McKenney
Date: Mon Apr 19 2010 - 19:01:43 EST

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
> > > > <paulmck@xxxxxxxxxxxxxxxxxx> 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] [<ffffffff8108584f>] lockdep_rcu_dereference+0xaf/0xc0
> > > > [ 0.046010] [<ffffffff81044812>] 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 <a.p.zijlstra@xxxxxxxxx>
> > ---
> > 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] [<ffffffff8108652f>] lockdep_rcu_dereference+0xaf/0xc0
> [ 0.048013] [<ffffffff810a3453>] freezer_fork+0xb3/0xd0
> [ 0.049007] [<ffffffff8109d61c>] cgroup_fork_callbacks+0x2c/0x40
> [ 0.050007] [<ffffffff81055e4a>] copy_process+0xb6a/0x11e0
> [ 0.051006] [<ffffffff8105657e>] do_fork+0xbe/0x3e0
> [ 0.052007] [<ffffffff81012519>] ? sched_clock+0x9/0x10
> [ 0.053008] [<ffffffff81077d45>] ? sched_clock_local+0x15/0x80
> [ 0.054006] [<ffffffff81077e69>] ? sched_clock_cpu+0xb9/0xf0
> [ 0.055006] [<ffffffff81076cd5>] ? up+0x35/0x50
> [ 0.056006] [<ffffffff81084073>] ? get_lock_stats+0x23/0x70
> [ 0.057006] [<ffffffff810840ce>] ? put_lock_stats+0xe/0x30
> [ 0.058010] [<ffffffff81cade20>] ? kernel_init+0x0/0x2e0
> [ 0.059006] [<ffffffff810136dd>] kernel_thread+0x8d/0xa0
> [ 0.060006] [<ffffffff81cade20>] ? kernel_init+0x0/0x2e0
> [ 0.061007] [<ffffffff8100bc20>] ? kernel_thread_helper+0x0/0x10
> [ 0.062006] [<ffffffff81cad140>] ? early_idt_handler+0x0/0x71
> [ 0.063011] [<ffffffff814e40c1>] rest_init+0x21/0x110
> [ 0.064005] [<ffffffff81cadd3f>] start_kernel+0x3af/0x490
> [ 0.065006] [<ffffffff81cad29c>] x86_64_start_reservations+0x7c/0xd0
> [ 0.066005] [<ffffffff81cad000>] ? early_idt_handlers+0x0/0x140
> [ 0.067006] [<ffffffff81cad3e8>] 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 <paulmck@xxxxxxxxxxxxxxxxxx>
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 <eparis@xxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

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
--
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