linux-kernel - Re: [PATCH] processes: reparent_thread: don't call kill_orphaned_pgrp() if task

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Message-Id: <20081204005203.95F6FFC3C0@magilla.sf.frob.com>
Date:	Wed, 26 Nov 2008 12:21:48 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] processes: reparent_thread: don't call
	kill_orphaned_pgrp() if task_detached()

> Needs an ack from someone who understands orphaned groups.

I understand what the semantics are supposed to be.
But I'm getting a headache trying to see if I really understand the code.

> If task_detached(p) == T, then either
> 
> 	a) p is not the main thread, we will find the group leader
> 	   on the ->children list.

Correct.

> or
> 	b) p is the group leader but its ->exit_state = EXIT_DEAD.
> 	   This can only happen when the last sub-thread has died,
> 	   but in that case that thread has already called
> 	   kill_orphaned_pgrp() from exit_notify().

I think that's right too.

> @@ -816,6 +816,8 @@ static void reparent_thread(struct task_
>  
>  	list_move_tail(&p->sibling, &p->real_parent->children);
>  
> +	if (task_detached(p))
> +		return;

Seems like it would be cleaner to reorganize the code a little.
reparent_thread has only one caller.  How about we move:

	if (p->pdeath_signal)
		/* We already hold the tasklist_lock here.  */
		group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);

	list_move_tail(&p->sibling, &p->real_parent->children);

into forget_original_parent and rename reparent_thread to something else,
called only:

	if (!task_detached(p) && !same_thread_group(p->real_parent, father))
		orphaned_process(p);


Thanks,
Roland
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