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

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Date:	Wed, 16 May 2012 20:39:20 +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.

Eric, sorry for the huge delay, I was on vacation when you sent this patch...

On 05/06, Eric W. Biederman wrote:
>
> @@ -798,6 +815,12 @@ static void forget_original_parent(struct task_struct *father)
>  	exit_ptrace(father);
>  	reaper = find_new_reaper(father);
>
> +	/* Return immediately if we aren't going to reparent anything */
> +	if (unlikely(reaper == father)) {
> +		write_unlock_irq(&tasklist_lock);
> +		return;
> +	}

I was confused by the comment. Afaics, it is not that "we aren't
going to reparent", we need this change because we can't "reparent"
to the same thread, list_for_each_entry_safe() below can never stop.
But this is off-topic...

Hmm. I don't think the patch is 100% correct. Afaics, this needs more
delay_pidns_leader() checks.

For example. Suppose we have a CLONE_NEWPID zombie I, it has an
EXIT_DEAD child D so delay_pidns_leader(I) == T.

Now suppose that I->real_parent exits, lets denote this task as P.

Suppose that P->real_parent ignores SIGCHLD.

In this case P will do release_task(I) prematurely. And worse, when
D finally does realease_task(D) it will do realease_task(I) again.

Oleg.

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