linux-kernel - Re: [PATCH] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON

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Date:   Wed, 28 Nov 2018 09:23:39 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jürg Billeter <j@...ron.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON_EXIT

Oleg Nesterov <oleg@...hat.com> writes:

> On 11/27, Jürg Billeter wrote:
>>
>> @@ -704,6 +713,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>>  	struct task_struct *p, *n;
>>  	LIST_HEAD(dead);
>>  
>> +	if (group_dead && tsk->signal->kill_descendants_on_exit)
>> +		walk_process_tree(tsk, kill_descendant_visitor, NULL);
>
> Well, this is not exactly right, at least this is suboptimal in that
> other sub-threads can too call walk_process_tree(kill_descendant_visitor)
> later for no reason.

Oleg I think I am missing something.

Reading kernel/exit.c I see "group_dead = atomic_dec_and_test(&tsk->signal->live)".
Which seems like enough to ensure exactly one task/thread calls walk_process_tree.

Can you explain what I am missing?

Eric