linux-kernel - Re: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2

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Date:	Thu, 8 Dec 2011 17:35:35 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	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>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Andrew Vagin <avagin@...nvz.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2

On 12/07, Cyrill Gorcunov wrote:
>
> On Wed, Dec 07, 2011 at 07:53:43PM +0100, Oleg Nesterov wrote:
> > Hi Cyrill,
> >
> > Sorry, I didn't read this patch yet, but
> >
> > On 12/06, Cyrill Gorcunov wrote:
> > >
> > > +static void *children_seq_start(struct seq_file *seq, loff_t *pos)
> > > +{
> > > +	struct task_struct *task;
> > > +
> > > +	rcu_read_lock();
> > > +	task = seq->private;
> > > +	if (task)
> > > +		return seq_list_start(&task->children, *pos);
> >
> > This looks "obviously wrong".
> >
> > We can not trust ->children->next after rcu_read_unlock(). Another
> > rcu_read_lock() can't help.
> >
> > Once again, I can be easily wrong, need to read the patch first.
> >
>
> Wait, Oleg, I might be wrong as well, but it's now a
>
> children_seq_open
>  get_proc_task (so ref to task increased)

Yes. task_struct itself can't go away.

> the children_seq_start/children_seq_stop work
> in iteration and every new iteration seq_list_next
> walks over the whole children list from the list
> head under rcu lock,

Yep, I misread this code, I though it does _next.

However, ->children list is not rcu-safe, this means that even
list_for_each() itself is not safe. Either you need tasklist or
we can probably make it rcu-safe...

As for /proc/pid/children, personally I think it is very useful.
But note that it obviously reports the children per-thread, while
in general this is the per-process thing. Not sure this really
makes sense, but perhaps /proc/pid/children and
/proc/pid/task/tid/children should act differently. Like, say,
proc_tid_stat/proc_tgid_stat. I won't insist.

Oleg.

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