[RFC, PATCH] handle the multi-threaded init's exit() properly

[Andrew Morton]

On Fri, 3 Aug 2007 01:20:09 +0400 Oleg Nesterov <oleg at tv-sign.ru> wrote:

> With or without this patch, multi-threaded init's are not fully supported, but
> do_exit() is completely wrong. This becomes a real problem when we support pid
> namespaces.
> 
> 1. do_exit() panics when the main thread of /sbin/init exits. It should not
>    until the whole thread group exits. Move the code below, under the
>    "if (group_dead)" check.
> 
>    Note: this means that forget_original_parent() can use an already dead
>    child_reaper()'s task_struct. This is OK for /sbin/init because
> 
>    	- do_wait() from alive sub-thread still can reap a zombie, we iterate
>    	  over all sub-thread's ->children lists
> 
>    	- do_notify_parent() will wakeup some alive sub-thread because it sends
>    	  the group-wide signal
> 
>    However, we should remove choose_new_parent()->BUG_ON(reaper->exit_state)
>    for this.
> 
> 2. We are playing games with ->nsproxy->pid_ns. This code is bogus today, and
>    it has to be changed anyway when we really support pid namespaces, just
>    remove it.
> 
> Signed-off-by: Oleg Nesterov <oleg at tv-sign.ru>
> 
> --- t/kernel/exit.c~	2007-08-03 00:10:28.000000000 +0400
> +++ t/kernel/exit.c	2007-08-03 01:12:18.000000000 +0400
> @@ -604,11 +604,6 @@ static void exit_mm(struct task_struct *
>  static inline void
>  choose_new_parent(struct task_struct *p, struct task_struct *reaper)
>  {
> -	/*
> -	 * Make sure we're not reparenting to ourselves and that
> -	 * the parent is not a zombie.
> -	 */
> -	BUG_ON(p == reaper || reaper->exit_state);
>  	p->real_parent = reaper;
>  }
>  
> @@ -895,6 +890,14 @@ static void check_stack_usage(void)
>  static inline void check_stack_usage(void) {}
>  #endif
>  
> +static inline void exit_child_reaper(struct task_struct *tsk)
> +{
> +	if (likely(tsk->group_leader != child_reaper(tsk)))
> +		return;
> +
> +	panic("Attempted to kill init!");
> +}
> +
>  fastcall NORET_TYPE void do_exit(long code)
>  {
>  	struct task_struct *tsk = current;
> @@ -908,13 +911,6 @@ fastcall NORET_TYPE void do_exit(long co
>  		panic("Aiee, killing interrupt handler!");
>  	if (unlikely(!tsk->pid))
>  		panic("Attempted to kill the idle task!");
> -	if (unlikely(tsk == child_reaper(tsk))) {
> -		if (tsk->nsproxy->pid_ns != &init_pid_ns)
> -			tsk->nsproxy->pid_ns->child_reaper = init_pid_ns.child_reaper;
> -		else
> -			panic("Attempted to kill init!");
> -	}
> -
>  
>  	if (unlikely(current->ptrace & PT_TRACE_EXIT)) {
>  		current->ptrace_message = code;
> @@ -964,6 +960,7 @@ fastcall NORET_TYPE void do_exit(long co
>  	}
>  	group_dead = atomic_dec_and_test(&tsk->signal->live);
>  	if (group_dead) {
> +		exit_child_reaper(tsk);
>  		hrtimer_cancel(&tsk->signal->real_timer);
>  		exit_itimers(tsk->signal);
>  	}

This patch broke

pid-namespaces-define-and-use-task_active_pid_ns-wrapper.patch.  This hunk:

***************
*** 908,915 ****
  	if (unlikely(!tsk->pid))
  		panic("Attempted to kill the idle task!");
  	if (unlikely(tsk == child_reaper(tsk))) {
- 		if (tsk->nsproxy->pid_ns != &init_pid_ns)
- 			tsk->nsproxy->pid_ns->child_reaper = init_pid_ns.child_reaper;
  		else
  			panic("Attempted to kill init!");
  	}
--- 908,916 ----
  	if (unlikely(!tsk->pid))
  		panic("Attempted to kill the idle task!");
  	if (unlikely(tsk == child_reaper(tsk))) {
+ 		if (task_active_pid_ns(tsk) != &init_pid_ns)
+ 			task_active_pid_ns(tsk)->child_reaper =
+ 					init_pid_ns.child_reaper;
  		else
  			panic("Attempted to kill init!");
  	}

has no place to live any more, so I just removed it.