linux-kernel - Re: [PATCH 1/3] will_become_orphaned

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Date:	Sun, 09 Dec 2007 08:45:58 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] will_become_orphaned_pgrp: we have threads

Oleg Nesterov <oleg@...sign.ru> writes:

> On 12/08, Eric W. Biederman wrote:
>>
>> Oleg Nesterov <oleg@...sign.ru> writes:
>> 
>> > p->exit_state != 0 doesn't mean this process is dead, it may have
> sub-threads.
>> >
>> > However, the new "p->exit_state && thread_group_empty(p)" check is not
> correct
>> > either, this is just the temporary hack. Perhaps we can just remove this
> check,
>> > but I don't understand orphaned process groups magic. At all. However, I
> think
>> > exit_notify() is obviously and completely wrong wrt this helper.
>> 
>> The problem that orphaned processes groups address is what happens if
>> an entire process group is stopped, and there is not a process that
>> can wake them up.
>> 
>> The rule for an unprivileged process sending a signal to a process
>> group is that it must be in the same session as the process group.
>> 
>> The rule for sending a signal to a process group is that the signal sender
>> must be in the same session.
>> 
>> So we are testing for a process group that does not have a living
>> member with a parent outside of the process that can send the process
>> group a signal.
>
> Ah, thanks a lot Eric, I am _starting_ to understand this.
>
>> Oleg what do you see wrong with checking p->exit_state &&
>> thread_group_empty(p)?   Since non-leader threads all self reap
>> that seems to be a valid test for an dead thread group.
>
> There is a window when exit_notify() drops tasklist and before release_task().
> Suppose the last (non-leader) thread exits. This means that entire group exits,
> but thread_group_empty() is not true.


And if you are an observer this is important.

Equally messed up is a our status in /proc at that point.  Which
says our sleeping process is a zombie.

I'm thinking we need to do at least some of the thread group leadership
transfer in do_exit, instead of de_thread.  Then p->group_leader->exit_state
would be sufficient to see if the entire thread group was alive,
as the group_leader would be whoever was left alive.  The original
group_leader might still need to be kept around for it's pid...

I think that would solve most of the problems you have with a dead
thread group leader and sending SIG_STOP as well.

Eric
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