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Date: Wed, 26 Jun 2013 17:57:28 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] proc: Expose /proc/<pid>/task/<tid>/children
	unconditionally
Message-ID: <20130626155728.GA2141@redhat.com>
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On 06/25, Andy Lutomirski wrote:
>
> On Tue, Jun 25, 2013 at 2:52 PM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> > On Tue, Jun 25, 2013 at 02:36:31PM -0700, Andy Lutomirski wrote:
> >> On Tue, Jun 25, 2013 at 1:17 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> >> > On 06/26, Cyrill Gorcunov wrote:
> >> >>
> >> >> On Tue, Jun 25, 2013 at 12:51:45PM -0700, Andy Lutomirski wrote:
> >> >> > This is currently only available if CONFIG_CHECKPOINT_RESTORE, which
> >> >> > is hidden under CONFIG_EXPERT.  It's generally useful functionality,
> >> >> > though, so expose it unconditionally.
> >> >> >
> >> >> > Cc: Cyrill Gorcunov <gorcunov@openvz.org>
> >> >> > Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> >> >> Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
> >> >
> >> > I didn't see the patch but I guess it is trivial and I agree with intent ;)
> >>
> >> The patch works, but "children" is only listed under task/<tid>, not
> >> under /proc/<pid>.  Is that intentional?  Fixing it would be a
> >> one-liner.
> >
> > Yeah, it's intentional. Here some explanations from Oleg (check out
> > the whole thread, it's not that big https://lkml.org/lkml/2011/12/9/220)
> > in short this might require some more code, but i'll re-check tomorrow.
>
> This is a little strange.  It looks like ppid (in status) shows the
> tgid,

Yes,

> but the actual real_parent can refer to a thread (as opposed to
> a thread group leader),

Yes.

This is mostly the internal implementation detail. And probably it
would be nice to move ->children from task_struct to signal_struct.

However. See __WNOTHREAD in man waitpid. I think this is the only
(historical) reason. Otherwise the real_parent's tid doesn't matter,
the parent is always the whole process, not sub-thread.

> and task/tid/children respects that.

Well, it should respect if you want to restart and keep __WNOTHREAD
working.

But there is another reason. It is not trivial to list all children
under /proc/<pid>, and the fix would not be a one-liner ;) You need
to fight with the exiting sub-threads and reparenting. 

> So the
> tree that you get by following task/tid/ children won't be quite the
> same as the tree you get by following ppid.

Not sure I understand... but in any case, yes you need to read
/proc/pid/task/*/children to construct the tree.

> I wonder if the ptid should be added to status.  Is there anything
> (other than task/tid/children)

Perhaps, I dunno.

Better yet, we should kill __WNOTHREAD ;) I am wondering if it is still
used.

Oleg.

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