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

[Paul E. McKenney]

On Wed, Aug 08, 2007 at 08:41:07PM +0400, Oleg Nesterov wrote:
> This time Paul E. McKenney actually cc'ed, sorry for the extra
> noise...
> 
> On 08/08, Pavel Emelyanov wrote:
> >
> > When someone wants to deal with some other taks's namespaces
> > it has to lock the task and then to get the desired namespace
> > if the one exists. This is slow on read-only paths and may be
> > impossible in some cases.
> > 
> > E.g. Oleg recently noticed a race between unshare() and the
> > (just sent for review) pid namespaces - when the task notifies
> > the parent it has to know the parent's namespace, but taking
> > the task_lock() is impossible there - the code is under write
> > locked tasklist lock.
> > 
> > On the other hand switching the namespace on task (daemonize)
> > and releasing the namespace (after the last task exit) is rather
> > rare operation and we can sacrifice its speed to solve the
> > issues above.
> 
> Still it is a bit sad we slow down process's exit. Perhaps I missed
> some other ->nsproxy access, but can't we make a simpler patch?
> 
> --- kernel/fork.c	2007-07-28 16:58:17.000000000 +0400
> +++ /proc/self/fd/0	2007-08-08 20:30:33.325216944 +0400
> @@ -1633,7 +1633,9 @@ asmlinkage long sys_unshare(unsigned lon
> 
>  		if (new_nsproxy) {
>  			old_nsproxy = current->nsproxy;
> +			read_lock(&tasklist_lock);
>  			current->nsproxy = new_nsproxy;
> +			read_unlock(&tasklist_lock);
>  			new_nsproxy = old_nsproxy;
>  		}
> 
> 
> This way ->nsproxy is stable under task_lock() or write_lock(tasklist).
> 
> > +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.

Does this work?

						Thanx, Paul
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