1 files changed, 24 insertions, 4 deletions

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index db95d8eb761b..bc6d6a89b6e6 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -190,7 +190,11 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
         /* Don't allow any more processes into the pid namespace */
         disable_pid_allocation(pid_ns);
 
-        /* Ignore SIGCHLD causing any terminated children to autoreap */
+        /*
+         * Ignore SIGCHLD causing any terminated children to autoreap.
+         * This speeds up the namespace shutdown, plus see the comment
+         * below.
+         */
         spin_lock_irq(&me->sighand->siglock);
         me->sighand->action[SIGCHLD - 1].sa.sa_handler = SIG_IGN;
         spin_unlock_irq(&me->sighand->siglock);
@@ -223,15 +227,31 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
         }
         read_unlock(&tasklist_lock);
 
-        /* Firstly reap the EXIT_ZOMBIE children we may have. */
+        /*
+         * Reap the EXIT_ZOMBIE children we had before we ignored SIGCHLD.
+         * sys_wait4() will also block until our children traced from the
+         * parent namespace are detached and become EXIT_DEAD.
+         */
         do {
                 clear_thread_flag(TIF_SIGPENDING);
                 rc = sys_wait4(-1, NULL, __WALL, NULL);
         } while (rc != -ECHILD);
 
         /*
-         * sys_wait4() above can't reap the TASK_DEAD children.
-         * Make sure they all go away, see free_pid().
+         * sys_wait4() above can't reap the EXIT_DEAD children but we do not
+         * really care, we could reparent them to the global init. We could
+         * exit and reap ->child_reaper even if it is not the last thread in
+         * this pid_ns, free_pid(nr_hashed == 0) calls proc_cleanup_work(),
+         * pid_ns can not go away until proc_kill_sb() drops the reference.
+         *
+         * But this ns can also have other tasks injected by setns()+fork().
+         * Again, ignoring the user visible semantics we do not really need
+         * to wait until they are all reaped, but they can be reparented to
+         * us and thus we need to ensure that pid->child_reaper stays valid
+         * until they all go away. See free_pid()->wake_up_process().
+         *
+         * We rely on ignored SIGCHLD, an injected zombie must be autoreaped
+         * if reparented.
          */
         for (;;) {
                 set_current_state(TASK_UNINTERRUPTIBLE);