# Re: \[PATCH 2/2\] pid\_ns: Introduce ioctl to set vector of ns\_last\_pid's on ns hierarhy

- From: Oleg Nesterov \<oleg@redhat.com\>
- Date: 2017-05-02 16:34:17 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2017/05/121774
- Thread: https://ratatoskr.run/linux-api/2017/04/119712/t.md

```text
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?

Oleg.
```