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

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Date:	Sun, 9 Dec 2007 17:21:16 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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

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.

Oleg.

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