ptrace: do_wait(traced_leader_killed_by_mt_exec) can block forever

Test-case:

	void *tfunc(void *arg)
	{
		execvp("true", NULL);
		return NULL;
	}

	int main(void)
	{
		int pid;

		if (fork()) {
			pthread_t t;

			kill(getpid(), SIGSTOP);

			pthread_create(&t, NULL, tfunc, NULL);

			for (;;)
				pause();
		}

		pid = getppid();
		assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0);

		while (wait(NULL) > 0)
			ptrace(PTRACE_CONT, pid, 0,0);

		return 0;
	}

It is racy, exit_notify() does __wake_up_parent() too. But in the
likely case it triggers the problem: de_thread() does release_task()
and the old leader goes away without the notification, the tracer
sleeps in do_wait() without children/tracees.

Change de_thread() to do __wake_up_parent(traced_leader->parent).
Since it is already EXIT_DEAD we can do this without ptrace_unlink(),
EXIT_DEAD threads do not exist from do_wait's pov.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>

fs/exec.c

@@ -967,6 +967,14 @@ static int de_thread(struct task_struct *tsk)
 
 		BUG_ON(leader->exit_state != EXIT_ZOMBIE);
 		leader->exit_state = EXIT_DEAD;
+
+		/*
+		 * We are going to release_task()->ptrace_unlink() silently,
+		 * the tracer can sleep in do_wait(). EXIT_DEAD guarantees
+		 * the tracer wont't block again waiting for this thread.
+		 */
+		if (unlikely(leader->ptrace))
+			__wake_up_parent(leader, leader->parent);
 		write_unlock_irq(&tasklist_lock);
 
 		release_task(leader);