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

[Oleg Nesterov <oleg@tv-sign.ru>]

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@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);
 	}