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

Oleg Nesterov [off-list ref] writes:

sorry for delay, vacation...

On 04/28, Kirill Tkhai wrote:

quoted

On 27.04.2017 19:22, Oleg Nesterov wrote:

quoted

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?

It breaks a number of assumptions if you can join a pid namespace before
an init process is created in that pid namespace.  Checking for
child_reaper is a bit heavy handed but appears to ensure all of the
assumptions of initial pid namespace creation have been met.

Which means your simplified pidns_for_children_get is a bit insufficient.

Eric