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SubjectRe: [RFC, PATCH] handle the multi-threaded init's exit() properly
On Fri, 3 Aug 2007 01:20:09 +0400 Oleg Nesterov <oleg@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@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.

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