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

[Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>]

Paul E. McKenney wrote:
> 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()

#define synchronize_sched() synchronize_rcu()
they are the same? what's the point?

> primitive will wait until all preempt/irq-disable code sequences complete.
> Therefore, it would wait for all write_lock_irq() code sequences to
> complete.

But we don't need this. Iff we get the nsproxy under rcu_read_lock() all
we need is to wait for RCU sections to complete.

> Does this work?
> 
> 						Thanx, Paul
>