[PATCH][BUGFIX] cgroups: fix pid namespace bug

[Serge E. Hallyn]

Quoting Paul Menage (menage at google.com):
> On Wed, Jul 1, 2009 at 6:24 PM, Li Zefan<lizf at cn.fujitsu.com> wrote:
> > +       cp = kzalloc(sizeof(*cp), GFP_KERNEL);
> > +       if (!cp) {
> > +               up_write(&cgrp->pids_mutex);
> > +               kfree(pidarray);
> > +               return -ENOMEM;
> > +       }
> > +       cp->cgrp = cgrp;
> > +       cp->pid_ns = pid_ns;
> 
> You're storing an uncounted reference to the pid ns here - there's no
> guarantee that the pid_ns will outlive the open file.

Yeah I was thinking about that, but

	1. the only way it won't outlive the open file is if the 
		task opens the file, hands the open fd over a
		unix socket, then exits as the last task of its
		pidns
	2. We don't dereference the pid_ns, so there is no actual
		safety issue.  So it would become a problem only
		if a new pidns gets created at that same address
		*and* a task in the new pidns opens the same
		tasks file.

Still, it wouldn't hurt to do get_pid_ns/put_pid_ns at the open
and release :)

-serge