pidns: find_new_reaper() can no longer switch to init_pid_ns.child_reaper

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>
Cc: Louis Rilling <louis.rilling@kerlabs.com>
Cc: Mike Galbraith <efault@gmx.de>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Tested-by: Andrew Wagin <avagin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 8 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index a85efd2348bd..2f59cc334516 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);
 }
 
@@ -732,12 +732,6 @@ 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;