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 11:26:04PM +0200, Peter Zijlstra wrote:
> On Mon, 2010-03-29 at 23:19 +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.
> 
> So the only cgroup thing I can find is set_task_rq()'s task_group()
> usage, which does indeed look like it wants RCU, but then, why would
> only the sched_fork() usage of set_task_cpu() need this.
> 
> If it's needed (possible) then set_task_rq() needs it unconditionally.

Easy patch, if appropriate!

> That said, the whole task_group stuff is tied to the cgroup muck, so it
> shouldn't be possible for the task_group to disappear on us, but then, I
> always sorta glaze over when I get near the cgroup core.

Hey, if it was easy, I might have gotten it right on the first try!  ;-)

							Thanx, Paul