[PATCH 6/9] signal: Always call do_notify_parent_cldstop with siglock held

[Oleg Nesterov]

On 04/26, Eric W. Biederman wrote:
>
> @@ -2164,7 +2166,9 @@ static void do_notify_parent_cldstop(struct task_struct *tsk,
>   	}
>
>  	sighand = parent->sighand;
> -	spin_lock_irqsave(&sighand->siglock, flags);
> +	lock = tsk->sighand != sighand;
> +	if (lock)
> +		spin_lock_nested(&sighand->siglock, SINGLE_DEPTH_NESTING);

But why is it safe?

Suppose we have two tasks, they both trace each other, both call
ptrace_stop() at the same time. Of course this is ugly, they both
will block.

But with this patch in this case we have the trivial ABBA deadlock,
no?

Oleg.