Re: [RFC][PATCH] Make access to taks's nsproxy liter

[Oleg Nesterov <oleg-6lXkIZvqkOAvJsYlp49lxw@public.gmane.org>]

On 08/08, Paul E. McKenney wrote:
>
> On Wed, Aug 08, 2007 at 08:41:07PM +0400, Oleg Nesterov wrote:
> > > +void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
> > > +{
> > > +	struct nsproxy *ns;
> > > +
> > > +	might_sleep();
> > > +
> > > +	ns = p->nsproxy;
> > > +	if (ns == new)
> > > +		return;
> > > +
> > > +	if (new)
> > > +		get_nsproxy(new);
> > > +	rcu_assign_pointer(p->nsproxy, new);
> > > +
> > > +	if (ns && atomic_dec_and_test(&ns->count)) {
> > > +		/*
> > > +		 * wait for others to get what they want from this
> > > +		 * nsproxy. cannot release this nsproxy via the
> > > +		 * call_rcu() since put_mnt_ns will want to sleep
> > > +		 */
> > > +		synchronize_rcu();
> > > +		free_nsproxy(ns);
> > > +	}
> > > +}
> > 
> > (I may be wrong, Paul cc'ed)
> > 
> > This is correct with the current implementation of RCU, but strictly speaking,
> > we can't use synchronize_rcu() here, because write_lock_irq() doesn't imply
> > rcu_read_lock() in theory.
> 
> Can you use synchronize_sched() instead?  The synchronize_sched()
> primitive will wait until all preempt/irq-disable code sequences complete.
> Therefore, it would wait for all write_lock_irq() code sequences to
> complete.

Thanks Paul!

But we also need to cover the case when ->nsproxy is used under rcu_read_lock(),
so if we go this way, we'd better add rcu_read_lock() to do_notify_parent.*() as
Eric suggested.

Oleg.