linux-kernel - Re: [PATCH 2/3] pidns: Guarantee that the pidns init will be the last pidns process reaped.

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Message-ID: <20120518123911.GA417@redhat.com>
Date:	Fri, 18 May 2012 14:39:11 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Louis Rilling <louis.rilling@...labs.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 2/3] pidns: Guarantee that the pidns init will be the
	last pidns process reaped.

On 05/17, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > What do you think?
>
> I think there is something very compelling about your solution,
> we do need my bit about making the init process ignore SIGCHLD
> so all of init's children self reap.

Not sure I understand. This can work with or without 3/3 which
changes zap_pid_ns_processes() to ignore SIGCHLD. And just in
case, I think 3/3 is fine.

And once again, this wait_event() + __wake_up_parent() is very
simple and straightforward, we can cleanup this code later if
needed.

> > Do you mean the "if (tsk->ptrace)" code in exit_notify() ? Nobody
> > understand it ;) Last time this code was modified by me (iirc), but
> > I simply tried to preserve the previous behaviour.
>
> Yes.  It is some pretty strange code.

Yes. In particular, I think it should always use SIGCHLD.

> Especially where we are reading
> a return result which is always false.  I think there is a bug somewhere
> between that code and ptrace detach

Yes. This is the known oddity. We always notify the tracer if the
leader exits, even if !thread_group_empty(). But after that the
tracer can't detach, and it can't do do_wait(WEXITED).

The problem is not that we can't "fix" this. Just any discussed
fix adds the subtle/incompatible user-visible change.

Oleg.

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