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

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Message-ID: <1434561864.1711.51.camel@odin.com>
Date:	Wed, 17 Jun 2015 20:24:24 +0300
From:	Kirill Tkhai <ktkhai@...n.com>
To:	Oleg Nesterov <oleg@...hat.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()

В Вт, 16/06/2015 в 22:03 +0200, Oleg Nesterov пишет:
> 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.

Sure, it's more logical. Thanks, Oleg.

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