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

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Message-ID: <20111207191916.GR21678@moon>
Date:	Wed, 7 Dec 2011 23:19:16 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Oleg Nesterov <oleg@...hat.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 Wed, Dec 07, 2011 at 11:03:40PM +0400, 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 as
> 
> children_seq_open
>  get_proc_task (so ref to task increased)
> 
> the children_seq_start/children_seq_stop works
> in iteration and every new iteration seq_list_next
> walks over the whole children list from the list
> head under rcu lock, so even if task is removed
> or added the link should exsist until rcu is unlocked
> and sync'ed no?
> 

On the other hands some if (task) tests are redundant
and might be dropped since we have a reference to a
task until seq-file is not released. I'll update it
and shrink a patch some more.

	Cyrill
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