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

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Date:	Tue, 16 Jun 2015 21:27:59 +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()

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?
	
> 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.

> +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.

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);


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;

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