Re: [PATCH 2/4] pid_ns: (BUG 11391) change ->child_reaper when init->group_leader exits

[sukadev@us.ibm.com]

Oleg Nesterov [oleg@tv-sign.ru] wrote:
| We don't change pid_ns->child_reaper when the main thread of the
| subnamespace init exits. As Robert Rex <robert.rex@exasol.com>
| pointed out this is wrong.
| 
| Yes, the re-parenting itself works correctly, but if the reparented
| task exits it needs ->parent->nsproxy->pid_ns in do_notify_parent(),
| and if the main thread is zombie its ->nsproxy was already cleared
| by exit_task_namespaces().
| 
| Introduce the new function, find_new_reaper(), which finds the new
| ->parent for the re-parenting and changes ->child_reaper if needed.
| Kill the now unneeded exit_child_reaper().
| 
| Also move the changing of ->child_reaper from zap_pid_ns_processes()
| to find_new_reaper(), this consolidates the games with ->child_reaper
| and makes it stable under tasklist_lock.
| 
| Reported-by: Robert Rex <robert.rex@exasol.com>
| Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

Acked-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>