linux-kernel - Re: [patch 1/4] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v8

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Date:	Tue, 24 Jan 2012 11:07:30 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Andrew Vagin <avagin@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [patch 1/4] fs, proc: Introduce /proc/<pid>/task/<tid>/children
 entry v8

On Mon, 23 Jan 2012 18:20:37 +0400
Cyrill Gorcunov <gorcunov@...nvz.org> wrote:

> When we do checkpoint of a task we need to know the list of children
> the task, has but there is no easy and fast way to generate reverse
> parent->children chain from arbitrary <pid> (while a parent pid is
> provided in "PPid" field of /proc/<pid>/status).
> 
> So instead of walking over all pids in the system (creating one big process
> tree in memory, just to figure out which children a task has) -- we add
> explicit /proc/<pid>/task/<tid>/children entry, because the kernel already has
> this kind of information but it is not yet exported.
> 
> This is a first level children, not the whole process tree.
> 
> v2:
>  - Kame suggested to use a separated /proc/<pid>/children entry
>    instead of poking /proc/<pid>/status
>  - Andew suggested to use rcu facility instead of locking
>    tasklist_lock
>  - Tejun pointed that non-seekable seq file might not be
>    enough for tasks with large number of children
> 
> v3:
>  - To be on a safe side use %lu format for pid_t printing
> 
> v4:
>  - New line get printed when sequence ends not at seq->stop,
>    a nit pointed by Tejun
>  - Documentation update
>  - tasklist_lock is back, Oleg pointed that ->children list
>    is actually not rcu-safe
> 
> v5:
>  - Oleg suggested to make /proc/<pid>/task/<tid>/children
>    instead of global /proc/<pid>/children, which eliminates
>    hardness related to threads and children migration, and
>    allows patch to be a way simplier.
> 
> v6:
>  - Drop ptrace_may_access tests, pids are can be found anyway
>    so nothing to protect here.
>  - Update comments and docs, pointed by Oleg.
> 
> v7:
>  - Use get_pid over proc-pid directly, to simplify
>    code, pointed by Oleg.
> 
> v8:
>  - Obtain a starting pid from the proc's inode directly.
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
> Reviewed-by: Oleg Nesterov <oleg@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Pavel Emelyanov <xemul@...allels.com>
> Cc: Serge Hallyn <serge.hallyn@...onical.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

>From viewpoint I played with seq_file, yesterday.

> +static void *children_seq_start(struct seq_file *seq, loff_t *pos)
> +{
> +	return get_children_pid(seq->private, NULL, *pos);
> +}
> +
> +static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> +{
> +	struct pid *pid = NULL;
> +
> +	pid = get_children_pid(seq->private, v, *pos + 1);
> +	if (!pid)
> +		seq_printf(seq, "\n");
> +	put_pid(v);

Because seq_printf() may fail. This seems dangeorus.

If seq_printf() fails and returns NULL, "\n" will not be
printed out and user land parser will go wrong.

I think all seq_printf() should be handled in ->show().
(And you can use seq_putc() for "\n".)

Thanks,
-Kame


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