linux-kernel - Re: [RFC] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v6

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Date:	Mon, 16 Jan 2012 17:11:14 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>, Andrew Vagin <avagin@...nvz.org>,
	Vasiliy Kulikov <segoon@...nwall.com>
Subject: Re: [RFC] fs, proc: Introduce /proc/<pid>/task/<tid>/children
	entry v6

On 01/16, Cyrill Gorcunov wrote:
>
> +get_children_pid(struct proc_pid_children_iter *iter, struct pid *pid_prev, loff_t pos)
> +{
> +	struct task_struct *start, *task;
> +	struct pid *pid = NULL;
> +
> +	read_lock(&tasklist_lock);
> +
> +	start = pid_task(iter->pid_start, PIDTYPE_PID);
> +	if (!start)
> +		goto out;
> +
> +	/*
> +	 * Lets try to continue searching first, this gives
> +	 * us significant speedup on children-rich processes.
> +	 */
> +	if (pid_prev) {
> +		task = pid_task(pid_prev, PIDTYPE_PID);
> +		if (task && task->real_parent == start &&
> +		    !(list_empty(&task->sibling))) {

Damn. No, this is wrong.

Damn! Yes, it was we who told you to check list_empty(sibling) ;)

But this is not enough. exit_ptrace() can do list_move() without
changing ->real_parent.

I'll try to think. At first glance we can rely on EXIT_DEAD, but
I'd like to avoid this, I think EXIT_DEAD should die.

Oleg.

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