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Subject[PATCH v2 -mm 1/1] 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>
---
kernel/exit.c | 10 ++--------
1 files changed, 2 insertions(+), 8 deletions(-)
diff --git a/kernel/exit.c b/kernel/exit.c
index 6d66cd2..6424e6b 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);
@@ -79,13 +80,12 @@ 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;

--
1.5.5.1



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