Re: [PATCH tip/core/urgent] rcu: protect fork-time cgroup access

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Mar 29, 2010 at 02:29:32PM -0700, Paul E. McKenney wrote:
> On Mon, Mar 29, 2010 at 11:19:23PM +0200, Peter Zijlstra wrote:
> > On Mon, 2010-03-29 at 14:15 -0700, 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 <abogani@texware.it>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > ---
> > > 
> > >  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();
> > >  
> > >  #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
> > >  	if (likely(sched_info_on()))
> > 
> > What got accessed? This patch just looks wrong.
> 
> It might well be wrong.  The lockdep RCU splat triggered in
> task_subsys_state():
> 
> static inline struct cgroup_subsys_state *task_subsys_state(
> 	struct task_struct *task, int subsys_id)
> {
> 	return rcu_dereference_check(task->cgroups->subsys[subsys_id],
> 				     rcu_read_lock_held() ||
> 				     cgroup_lock_is_held());
> }
> 
> My thought was that this access was safe due to the fact that we were
> in the middle of creating a task, so that no other CPU could access it.
> But I am not certain of this, given the fact that this is digging through
> cgroup state.
> 
> The lockdep splat is here: http://pastebin.ubuntu.com/406131/
> 
> Suggestions?

And it appears that my patch is at best insufficient:
http://paste.ubuntu.com/406189/

Left to myself, I would wrap copy_process() with rcu_read_lock(),
but I would rather hear your thoughts before doing too much more
semi-random hacking.  ;-)

						Thanx, Paul