Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last_pid's on ns hierarhy

[Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>]

On 02.05.2017 19:33, Oleg Nesterov wrote:
> sorry for delay, vacation...
> 
> On 04/28, Kirill Tkhai wrote:
>>
>> On 27.04.2017 19:22, Oleg Nesterov wrote:
>>>
>>> Ah, OK, I didn't notice the ns->child_reaper check in pidns_for_children_get().
>>>
>>> But note that it doesn't need tasklist_lock too.
>>
>> Hm, are there possible strange situations with memory ordering, when we see
>> ns->child_reaper of already died ns, which was placed in the same memory?
>> Do we have to use some memory barriers here?
> 
> Could you spell please? I don't understand your concerns...
> 
> I don't see how, say,
> 
> 	static struct ns_common *pidns_for_children_get(struct task_struct *task)
> 	{
> 		struct ns_common *ns = NULL;
> 		struct pid_namespace *pid_ns;
> 
> 		task_lock(task);
> 		if (task->nsproxy) {
> 			pid_ns = task->nsproxy->pid_ns_for_children;
> 			if (pid_ns->child_reaper) {
> 				ns = &pid_ns->ns;
> 				get_pid_ns(ns);
> 			}
> 		}
> 		task_unlock(task);
> 
> 		return ns;
> 	}
> 
> can be wrong. It also looks more clean to me.
> 
> ->child_reaper is not stable without tasklist, it can be dead/etc, but
> we do not care?

I mean the following. We had a pid_ns1 with a child_reaper set. Then
it became dead, and a new pid_ns2 were allocated in the same memory.

A task on another cpu opens the pid_for_children file, and because
of there is no memory ordering, it sees pid_ns1->child_reaper,
when it opens pid_ns2.

I forgot, what guarantees this situation is impossible? What guarantees,
the renewed content of pid_ns2 on another cpu is seen not later, than
we can't open it?