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

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

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.

Oleg.