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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