linux-kernel - Re: [PATCH] exit: Clarify choice of new parent in forget_original

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Message-ID: <20150616200352.GA30871@redhat.com>
Date:	Tue, 16 Jun 2015 22:03:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kirill Tkhai <ktkhai@...n.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] exit: Clarify choice of new parent in
	forget_original_parent()

forgot to mention,

On 06/16, Oleg Nesterov wrote:
>
> On 06/16, Kirill Tkhai wrote:
> >
> > +	 * child_reaper doesn't have children after zap_pid_ns_processes(),
> > +	 * therefore it can't enter this function.
> > +	 */
> > +	BUG_ON(child_reaper == father);
>
> Yes, we can add this BUG_ON(). But please see the comments in
> zap_pid_ns_processes(). We can change zap_pid_ns_processes() so that
> it returns with non-empty ->children list due to EXIT_DEAD children.
>
> Unlikely we will actually do this, at least soon, so I won't argue
> with this BUG_ON().
>
> But. In this case it would be better to add it into forget_original_parent(),
>
> 	reaper = find_new_reaper(...);
> 	BUG_ON(reaper == father);

because this way:

	1. This BUG_ON() will still be valid even if we actually change
	   zap_pid_ns_processes() to return with EXIT_DEAD children

	2. If we really want this sanity check, we should not tie it to
	   ->child_reaper case.

OTOH. If for some reason you want to check ->child_reaper only, then
you should probably do this right after list_empty(&father->children)
check, or at least before find_alive_thread(). Because otherwise it
looks confusing, it looks as if "child_reaper == father" is only wrong
if find_alive_thread(father) fails.

Oleg.

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