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

[Pavel Emelyanov]

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).

We may, but the intention of this patch is just (!) to make the access 
to other's task namespaces lockless. Solving the accessing parent's
nsproxy in do_notify_parent() is a (good) side effect :)

>> +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.
> 
>