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

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Date:	Wed, 18 Jan 2012 15:36:31 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	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 01/18, Cyrill Gorcunov wrote:
>
> So Oleg, I think you meant something like below? Comment is moved down an
> list_empty over siblings remans, right?

Yes, except the comment still looks misleading to me.

Otherwise looks correct, but I'll try to re-check once again with
the fresh head. Although I think you should remove me from CC: after
I found the nonexistent bug ;)

> +get_children_pid(struct proc_pid_children_iter *iter, struct pid *pid_prev, loff_t pos)
> +{
> ...
> +	/*
> +	 * Slow search case
> +	 *
> +	 * We might miss some freshly created children
> +	 * here, but it was never promised to be
> +	 * accurate.
> +	 */
> +	list_for_each_entry(task, &start->children, sibling) {
> +		if (pos-- == 0) {
> +			pid = get_pid(task_pid(task));
> +			break;
> +		}
> +	}

This is minor, but "freshly created" looks very confusing to me.
What does it mean? We hold tasklist, we can't race with fork().

Yes we can miss a child, but this has nothing to do with "freshly".
Just suppose that the parent sleeps, but N children exit after we
printed their tids. Now the slow paths skips N extra children, we
miss N tasks.

Oleg.

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