Possible race between cgroup_attach_proc and de_thread, and questionable code in de_thread.

[Paul E. McKenney]

On Sun, Aug 14, 2011 at 07:51:19PM +0200, Oleg Nesterov wrote:
> On 07/28, Paul E. McKenney wrote:
> >
> > On Thu, Jul 28, 2011 at 11:08:13AM +1000, NeilBrown wrote:
> > >
> > > I disagree.  It also requires - by virtue of the use of while_each_thread() -
> > > that 'g' remains on the list that 't' is walking along.
> >
> > Doesn't the following code in the loop body deal with this possibilty?
> >
> > 	/* Exit if t or g was unhashed during refresh. */
> > 	if (t->state == TASK_DEAD || g->state == TASK_DEAD)
> > 		goto unlock;
> 
> This code is completely wrong even if while_each_thread() was fine.
> 
> I sent the patch but it was ignored.
> 
> 	[PATCH] fix the racy check_hung_uninterruptible_tasks()->rcu_lock_break()
> 	http://marc.info/?l=linux-kernel&m=127688790019041

If it helps...

Acked-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>

							Thanx, Paul