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

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Date:	Wed, 17 Jun 2015 20:23:14 +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 21:27 +0200, Oleg Nesterov пишет:
> On 06/16, Kirill Tkhai wrote:
> >
> > Second parameter of find_new_reaper() and the similarity of its name
> > and find_child_reaper()'s name confuse a reader.
> 
> OK, I agree that
> 
> 	reaper = find_child_reaper(father);
> 	...
> 	reaper = find_new_reaper(father, reaper);
> 
> can look confusing and probably deserves a cleanup. How about the patch
> below then?

Good, IMO it improves the readability.

> 	
> > Rename find_child_reaper() for better conformity of its name and its
> > function.
> 
> I never argueus with renames ;) Probably the new name looks better.
> 
> > Also delete the second parameter of find_new_reaper().
> 
> Yes, we can do this. But this 2nd argument avoids another another
> task_active_pid_ns(father)->child_reaper, so this is optimization.
> 
> I agree, this optimization is minor, but still I think this change
> needs some justification.

It looks like gcc inlines both of these function, so it seems there won't
be a problem...

> 
> > +static struct task_struct *find_new_reaper(struct task_struct *father)
> >  {
> > -	struct task_struct *thread, *reaper;
> > +	struct task_struct *thread, *reaper, *child_reaper;
> >  
> >  	thread = find_alive_thread(father);
> >  	if (thread)
> >  		return thread;
> >  
> > +	child_reaper = task_active_pid_ns(father)->child_reaper;
> > +	/*
> > +	 * 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.

Yes, I saw. Since zap_pid_ns_processes() waits for nr_hashed,
and __unhash_process() deletes from pid chain and sibling list
at the same time, pid_ns child_reaper can't have a child after
nr_hashed == init_pids.

> 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);

Yeah, I'm agree.

> Oh. Off-topic, but this reminds me that I forgot about another bug with
> ->has_child_subreaper... this needs another discussion.
> 
> Oleg.
> 
> --- x/kernel/exit.c
> +++ x/kernel/exit.c
> @@ -551,17 +551,17 @@ static void reparent_leader(struct task_
>  static void forget_original_parent(struct task_struct *father,
>  					struct list_head *dead)
>  {
> -	struct task_struct *p, *t, *reaper;
> +	struct task_struct *p, *t, *child_reaper, *reaper;
>  
>  	if (unlikely(!list_empty(&father->ptraced)))
>  		exit_ptrace(father, dead);
>  
>  	/* Can drop and reacquire tasklist_lock */
> -	reaper = find_child_reaper(father);
> +	child_reaper = find_child_reaper(father);
>  	if (list_empty(&father->children))
>  		return;
>  
> -	reaper = find_new_reaper(father, reaper);
> +	reaper = find_new_reaper(father, child_reaper);
>  	list_for_each_entry(p, &father->children, sibling) {
>  		for_each_thread(p, t) {
>  			t->real_parent = reaper;
> 

Thanks,
Kirill

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