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

[Oleg Nesterov]

On 08/02, Roland McGrath wrote:
>
> This looks fine to me, though I don't know anything about the nsproxy bit.
> Now that choose_new_parent is one trivial line, you might go on to get rid
> of it and roll its one line into reparent_thread.

OK, thanks. Please find the same patch + "kill one-liner reparent_thread()"
below.


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

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~MTINIT	2007-07-28 16:58:17.000000000 +0400
+++ t/kernel/exit.c	2007-08-03 18:05:52.000000000 +0400
@@ -601,17 +601,6 @@ static void exit_mm(struct task_struct *
 	mmput(mm);
 }
 
-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;
-}
-
 static void
 reparent_thread(struct task_struct *p, struct task_struct *father, int traced)
 {
@@ -719,7 +708,7 @@ forget_original_parent(struct task_struc
 
 		if (father == p->real_parent) {
 			/* reparent with a reaper, real father it's us */
-			choose_new_parent(p, reaper);
+			p->real_parent = reaper;
 			reparent_thread(p, father, 0);
 		} else {
 			/* reparent ptraced task to its real parent */
@@ -740,7 +729,7 @@ forget_original_parent(struct task_struc
 	}
 	list_for_each_safe(_p, _n, &father->ptrace_children) {
 		p = list_entry(_p, struct task_struct, ptrace_list);
-		choose_new_parent(p, reaper);
+		p->real_parent = reaper;
 		reparent_thread(p, father, 1);
 	}
 }
@@ -895,6 +884,14 @@ 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 +905,6 @@ 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 +954,7 @@ 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);
 	}