linux-kernel - Re: [PATCH v2 -mm 1/1] pidns: find_new_reaper() can no longer switch to init_pid_ns.child

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Date:	Tue, 29 May 2012 00:34:27 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Louis Rilling <louis.rilling@...labs.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH v2 -mm 1/1] pidns: find_new_reaper() can no longer switch to init_pid_ns.child_reaper

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

> find_new_reaper() changes pid_ns->child_reaper, see add0d4df
> "pid_ns: zap_pid_ns_processes: fix the ->child_reaper changing".
>
> The original reason has gone away after the previous patch,
> ->children list must be empty after zap_pid_ns_processes().
>
> However now we can not switch to init_pid_ns.child_reaper.
> __unhash_process() relies on the "->child_reaper == parent"
> check, but this check does not work if the last exiting task
> is also the child reaper.
>
> As Eric sugested, we can change __unhash_process() to use the
> parent's pid_ns and remove this code.
>
> Also, with this change we can move detach_pid(PIDTYPE_PID) back,
> where it was before the previous fix.

This looks good to me. 

Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

I will be on the road for next two days so I don't expect I will
be particularly active in this converation for a while.

Eric


> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  kernel/exit.c |   10 ++--------
>  1 files changed, 2 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 6d66cd2..6424e6b 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -64,6 +64,7 @@ static void exit_mm(struct task_struct * tsk);
>  static void __unhash_process(struct task_struct *p, bool group_dead)
>  {
>  	nr_threads--;
> +	detach_pid(p, PIDTYPE_PID);
>  	if (group_dead) {
>  		detach_pid(p, PIDTYPE_PGID);
>  		detach_pid(p, PIDTYPE_SID);
> @@ -79,13 +80,12 @@ static void __unhash_process(struct task_struct *p, bool group_dead)
>  		if (IS_ENABLED(CONFIG_PID_NS)) {
>  			struct task_struct *parent = p->real_parent;
>  
> -			if ((task_active_pid_ns(p)->child_reaper == parent) &&
> +			if ((task_active_pid_ns(parent)->child_reaper == parent) &&
>  			    list_empty(&parent->children) &&
>  			    (parent->flags & PF_EXITING))
>  				wake_up_process(parent);
>  		}
>  	}
> -	detach_pid(p, PIDTYPE_PID);
>  	list_del_rcu(&p->thread_group);
>  }
>  
> @@ -732,12 +732,6 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
>  
>  		zap_pid_ns_processes(pid_ns);
>  		write_lock_irq(&tasklist_lock);
> -		/*
> -		 * We can not clear ->child_reaper or leave it alone.
> -		 * There may by stealth EXIT_DEAD tasks on ->children,
> -		 * forget_original_parent() must move them somewhere.
> -		 */
> -		pid_ns->child_reaper = init_pid_ns.child_reaper;
>  	} else if (father->signal->has_child_subreaper) {
>  		struct task_struct *reaper;
--
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