Here is the modified patch that applies on top of Oleg's patch
to fix the error for threaded-init. Hope the "big-fat-comment" in
exit_child_reaper() (about child-reaper inheriting children even
after exiting) makes sense.

---

From: Pavel Emelyanov <xemul@openvz.org>
Subject: [PATCH 14/15] Destroy pid namespace on init's death

From: Sukadev Bhattiprolu <sukadev@us.ibm.com>

Terminate all processes in a namespace when the reaper of the namespace
is exiting. We do this by walking the pidmap of the namespace and sending
SIGKILL to all processes.

Changelog:
	[Oleg Nesterov]: In zap_pid_ns_processes() wait for any child
		rather than iterating over all pid_ts in the pidmap. Clear
		TIF_SIGPENDING flag for successive wait() calls.

	[Oleg Nesterov]: Ensure the logic works even with multi-threaded
		container-init process.

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Acked-by: Pavel Emelyanov <xemul@openvz.org>

---

 include/linux/pid.h |    2 ++
 kernel/exit.c       |   27 ++++++++++++++++++++++++++-
 kernel/pid.c        |   41 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+), 1 deletion(-)

Index: lx26-23-rc1-mm1/include/linux/pid.h
===================================================================
--- lx26-23-rc1-mm1.orig/include/linux/pid.h	2007-08-02 11:03:39.000000000 -0700
+++ lx26-23-rc1-mm1/include/linux/pid.h	2007-08-02 11:06:47.000000000 -0700
@@ -118,6 +118,8 @@ extern struct pid *find_ge_pid(int nr, s
 
 extern struct pid *alloc_pid(struct pid_namespace *ns);
 extern void FASTCALL(free_pid(struct pid *pid));
+extern void zap_pid_ns_processes(struct pid_namespace *pid_ns,
+		struct task_struct *tsk);
 
 /*
  * the helpers to get the pid's id seen from different namespaces
Index: lx26-23-rc1-mm1/kernel/exit.c
===================================================================
--- lx26-23-rc1-mm1.orig/kernel/exit.c	2007-08-02 11:06:36.000000000 -0700
+++ lx26-23-rc1-mm1/kernel/exit.c	2007-08-02 23:06:47.000000000 -0700
@@ -916,7 +916,32 @@ static inline void exit_child_reaper(str
 	if (likely(tsk->group_leader != task_child_reaper(tsk)))
 		return;
 
-	panic("Attempted to kill init!");
+	if (tsk->nsproxy->pid_ns == &init_pid_ns)
+		panic("Attempted to kill init!");
+
+	/*
+	 * @tsk is the last thread in the 'container-init' and is exiting.
+	 * Terminate all remaining processes in the namespace and reap them
+	 * before exiting @tsk.
+	 *
+	 * Note that @tsk (last thread of container-init) may not necessarily
+	 * be the child-reaper (i.e main thread of container-init) of the
+	 * namespace i.e the child_reaper may have already exited.
+ 	 *
+	 * Even after a child_reaper exits, we let it inherit orphaned children,
+	 * because, pid_ns->child_reaper remains valid as long as there is
+	 * at least one living sub-thread in the container init.
+
+	 * This living sub-thread of the container-init will be notified when
+	 * a child inherited by the 'child-reaper' exits (do_notify_parent()
+	 * uses __group_send_sig_info()). Further, when reaping child processes,
+	 * do_wait() iterates over children of all living sub threads.
+
+	 * i.e even though 'child_reaper' thread is listed as the parent of the
+	 * orphaned children, any living sub-thread in the  container-init can
+	 * receive notification of the child exiting and reap the child.
+	 */
+	zap_pid_ns_processes(tsk->nsproxy->pid_ns, tsk);
 }
 
 fastcall NORET_TYPE void do_exit(long code)
Index: lx26-23-rc1-mm1/kernel/pid.c
===================================================================
--- lx26-23-rc1-mm1.orig/kernel/pid.c	2007-08-02 11:03:39.000000000 -0700
+++ lx26-23-rc1-mm1/kernel/pid.c	2007-08-02 23:06:40.000000000 -0700
@@ -29,6 +29,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/init_task.h>
 #include <linux/proc_fs.h>
+#include <linux/syscalls.h>
 
 #define pid_hashfn(nr, ns)	\
 	hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
@@ -593,6 +594,46 @@ out:
 	return new_ns;
 }
 
+void zap_pid_ns_processes(struct pid_namespace *pid_ns,
+			struct task_struct *reaper)
+{
+	int nr;
+	int rc;
+	pid_t reaper_pid = pid_nr_ns(task_pid(reaper), pid_ns);
+
+	/*
+	 * The last thread in the container-init thread group is terminating.
+	 * Find remaining pid_ts in the namespace, signal and wait for them
+	 * to exit.
+	 *
+	 * Note:  This signals each threads in the namespace - even those that
+	 * 	  belong to the same thread group, To avoid this, we would have
+	 * 	  to walk the entire tasklist looking a processes in this
+	 * 	  namespace, but that could be unnecessarily expensive if the
+	 * 	  pid namespace has just a few processes. Or we need to
+	 * 	  maintain a tasklist for each pid namespace.
+	 *
+	 */
+	read_lock(&tasklist_lock);
+	nr = next_pidmap(pid_ns, 0);
+	while (nr > 0) {
+		if (reaper_pid != nr)
+			kill_proc_info(SIGKILL, SEND_SIG_PRIV, nr);
+		nr = next_pidmap(pid_ns, nr);
+	}
+	read_unlock(&tasklist_lock);
+
+	do {
+		clear_thread_flag(TIF_SIGPENDING);
+		rc = sys_wait4(-1, NULL, __WALL, NULL);
+	} while (rc != -ECHILD);
+
+
+	/* Don't need a child reaper for this pid namespace anymore */
+	pid_ns->child_reaper = NULL;
+	return;
+}
+
 static void do_free_pid_ns(struct work_struct *w)
 {
 	struct pid_namespace *ns, *parent;
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