I don't think you need cgroup_signal.h. It's only included in
cgroup_signal.c, and doesn't really contain any useful definitions
anyway. You should just use a cgroup_subsys_state object as your state
object, since you'll never need to do anything with it anyway.

>+static struct cgroup_subsys_state *signal_create(
>+	struct cgroup_subsys *ss, struct cgroup *cgroup)
>+{
>+	struct stateless *dummy;
>+
>+	if (!capable(CAP_SYS_ADMIN))
>+		return ERR_PTR(-EPERM);

This is unnecessary.

>+
+	dummy = kzalloc(sizeof(struct stateless), GFP_KERNEL);
+	if (!dummy)
+		return ERR_PTR(-ENOMEM);
+	return  &dummy->css;
+}

This function could be simplified to:

struct cgroup_subsys_state *css;
css = kzalloc(sizeof(*css), GFP_KERNEL);
return css ?: ERR_PTR(-ENOMEM);

>+static int signal_can_attach(struct cgroup_subsys *ss,
>+			     struct cgroup *new_cgroup,
>+			     struct task_struct *task)
>+{
>+	return 0;
>+}

No need for a can_attach() method if it just returns 0 - that's the default.

>+static int signal_kill(struct cgroup *cgroup, int signum)
>+{
>+	struct cgroup_iter it;
>+	struct task_struct *task;
>+	int retval = 0;
>+
>+	cgroup_iter_start(cgroup, &it);
>+	while ((task = cgroup_iter_next(cgroup, &it))) {
>+		retval = send_sig(signum, task, 1);
>+		if (retval)
>+			break;
>+	}
>+	cgroup_iter_end(cgroup, &it);
>+
>+	return retval;
>+}

cgroup_iter_start() takes a read lock - is send_sig() guaranteed not to sleep?

>+static ssize_t signal_write(struct cgroup *cgroup,
>+			     struct cftype *cft,
>+			     struct file *file,
>+			     const char __user *userbuf,
>+			     size_t nbytes, loff_t *unused_ppos)

This should just be a write_u64() method - cgroups will handle the
copying/parsing for you. See e.g.
kernel/sched.c:cpu_shares_write_u64()

>+static struct cftype kill_file = {
>+	.name = "kill",
>+	.write = signal_write,
>+	.private = 0,
>+};

I agree with PaulJ that "signal.send" would be a nicer name for this
than "signal.kill"
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