[PATCH -mm 1/1] 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, "can't clear ->child_reaper or leave it alone" is
still true, and now we can not use 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.

Change find_new_reaper() to use pid_ns->parent->child_reaper.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/exit.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index b3e6e0e..9f9af91 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -733,11 +733,11 @@ 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.
+		 * Our parent can be ->child_reaper as well, make sure
+		 * we don't break the "child_reaper == parent" logic in
+		 * __unhash_process().
 		 */
-		pid_ns->child_reaper = init_pid_ns.child_reaper;
+		pid_ns->child_reaper = pid_ns->parent->child_reaper;
 	} else if (father->signal->has_child_subreaper) {
 		struct task_struct *reaper;
 
-- 
1.5.5.1