Hello,

The reproducer for this problem is attached to this message.

Below you can find its effect without this fix:

We have one alive process which stucks in
zap_pid_ns_processes:

$ ps axf 
... 
11831 pts/0    S      0:00 [ptrace_pidns] 
11833 pts/0    Zl     0:00  \_ [ptrace_pidns] <defunct> 
 
$ cat /proc/11831/stack  
[<0>] do_wait+0x1fa/0x2c0 
[<0>] kernel_wait4+0x9e/0x150 
[<0>] zap_pid_ns_processes+0x17d/0x270 
[<0>] do_exit+0xa15/0xbd0 
[<0>] do_group_exit+0x47/0xc0 
[<0>] get_signal+0x28c/0x850 
[<0>] do_signal+0x36/0x630 
[<0>] exit_to_usermode_loop+0x62/0xc0 
[<0>] prepare_exit_to_usermode+0xb4/0xe0 
[<0>] retint_user+0x8/0x18 
[<0>] 0xffffffffffffffff 


The child process has two threads which were ptraced by parent:
$ ls  /proc/11833/task/ 
11833  11834 

The parent can't wait the child, becase a thread group isn't empty, but
a thread is in the dead list:

$ cat /proc/1183{1,3,4}/status | grep '\(NSpid\|TracerPid\|State\)' 
State:  S (sleeping) 
TracerPid:      0 
NSpid:  11831   1 
State:  Z (zombie) 
TracerPid:      0 
NSpid:  11833   2 
State:  X (dead) 
TracerPid:      0 
NSpid:  11834   3

On Wed, Jan 02, 2019 at 12:59:39PM -0800, Andrei Vagin wrote:
> 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
>