[PATCH] kernel: release ptraced tasks before zap_pid_ns_processes

[Andrei Vagin <avagin@gmail.com>]

Currently, exit_ptrace() adds all ptraced tasks in a dead list, than
zap_pid_ns_processes() waits all tasks in a current pid ns, and only
then tasks from the dead list are released.

zap_pid_ns_processes() can stuck on waiting tasks from the dead list. In
this case, we will have one unkillable process with one or more dead
children.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrei Vagin <avagin@gmail.com>
---
 kernel/exit.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 0e21e6d21f35..ccaa6f6549ba 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -664,9 +664,6 @@ static void forget_original_parent(struct task_struct *father,
 {
 	struct task_struct *p, *t, *reaper;
 
-	if (unlikely(!list_empty(&father->ptraced)))
-		exit_ptrace(father, dead);
-
 	/* Can drop and reacquire tasklist_lock */
 	reaper = find_child_reaper(father);
 	if (list_empty(&father->children))
@@ -705,8 +702,18 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 	LIST_HEAD(dead);
 
 	write_lock_irq(&tasklist_lock);
-	forget_original_parent(tsk, &dead);
+	if (unlikely(!list_empty(&tsk->ptraced)))
+		exit_ptrace(tsk, &dead);
+	write_unlock_irq(&tasklist_lock);
+
+	/* Ptraced tasks have to be released before zap_pid_ns_processes(). */
+	list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
+		list_del_init(&p->ptrace_entry);
+		release_task(p);
+	}
 
+	write_lock_irq(&tasklist_lock);
+	forget_original_parent(tsk, &dead);
 	if (group_dead)
 		kill_orphaned_pgrp(tsk->group_leader, NULL);
 
-- 
2.17.2