Re: [PATCH] proc: Expose /proc/<pid>/task/<tid>/children unconditionally

[Andy Lutomirski]

On Wed, Jun 26, 2013 at 8:57 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 06/25, Andy Lutomirski wrote:
>>
>> On Tue, Jun 25, 2013 at 2:52 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
>> >> >> > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> >> >> Acked-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
>> >> >
>> >> > 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.

Ugh.  I'll submit a follow-up patch to
Documentation/filesystems/proc.txt to explain the situation.  All I
really care about is being able to iterate the children of a
single-threaded process, and the current semantics are fine for that.

--Andy
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