[PATCH 2/2] pidns: find_new_reaper() can no longer switch to init_pid_ns.child_reaper

[Oleg Nesterov <oleg@redhat.com>]

find_new_reaper() changes pid_ns->child_reaper, see add0d4df
"pid_ns: zap_pid_ns_processes: fix the ->child_reaper changing".

The original reason has gone away after the previous patch,
->children list must be empty after zap_pid_ns_processes().

However now we can not switch to init_pid_ns.child_reaper.
__unhash_process() relies on the "->child_reaper == parent"
check, but this check does not work if the last exiting task
is also the child reaper.

As Eric sugested, we can change __unhash_process() to use the
parent's pid_ns and remove this code.

Also, with this change we can move detach_pid(PIDTYPE_PID) back,
where it was before the previous fix.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/exit.c |   10 ++--------
 1 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 4fd4c55..ab972a7 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -64,6 +64,7 @@ static void exit_mm(struct task_struct * tsk);
 static void __unhash_process(struct task_struct *p, bool group_dead)
 {
 	nr_threads--;
+	detach_pid(p, PIDTYPE_PID);
 	if (group_dead) {
 		detach_pid(p, PIDTYPE_PGID);
 		detach_pid(p, PIDTYPE_SID);
@@ -78,13 +79,12 @@ static void __unhash_process(struct task_struct *p, bool group_dead)
 		if (IS_ENABLED(CONFIG_PID_NS)) {
 			struct task_struct *parent = p->real_parent;
 
-			if ((task_active_pid_ns(p)->child_reaper == parent) &&
+			if ((task_active_pid_ns(parent)->child_reaper == parent) &&
 			    list_empty(&parent->children) &&
 			    (parent->flags & PF_EXITING))
 				wake_up_process(parent);
 		}
 	}
-	detach_pid(p, PIDTYPE_PID);
 	list_del_rcu(&p->thread_group);
 }
 
@@ -731,12 +731,6 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
 
 		zap_pid_ns_processes(pid_ns);
 		write_lock_irq(&tasklist_lock);
-		/*
-		 * We can not clear ->child_reaper or leave it alone.
-		 * There may by stealth EXIT_DEAD tasks on ->children,
-		 * forget_original_parent() must move them somewhere.
-		 */
-		pid_ns->child_reaper = init_pid_ns.child_reaper;
 	} else if (father->signal->has_child_subreaper) {
 		struct task_struct *reaper;
 
-- 
1.5.5.1