linux-kernel - Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last

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Message-ID: <20170502163324.GA25036@redhat.com>
Date:   Tue, 2 May 2017 18:33:25 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     serge@...lyn.com, ebiederm@...ssion.com, agruenba@...hat.com,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        paul@...l-moore.com, viro@...iv.linux.org.uk, avagin@...nvz.org,
        linux-fsdevel@...r.kernel.org, mtk.manpages@...il.com,
        akpm@...ux-foundation.org, luto@...capital.net,
        gorcunov@...nvz.org, mingo@...nel.org, keescook@...omium.org
Subject: Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of
        ns_last_pid's on ns hierarhy

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.