Unknown symbol error when using sysfs from kernel module

Hey, I'm trying to play around a with sysfs a little bit, to get some data from user space into my kernel module. Here is the code:

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/elf.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/kobject.h>
#include <linux/sysfs.h>

#define PRINT(x) printk(KERN_INFO x "\n")
#define ERROR(fmt,arg...) printk(KERN_ALERT "Error - " fmt,##arg)



ssize_t mystore (struct kobject *obj,
                    struct attribute *attr, const char *buff, size_t count)
{
 /* doing a little bit */
        return count;
}


ssize_t myshow (struct kobject *obj, struct attribute *attr, char *buff)
{
    return 0;
}

char file_name[] = "myfile";
static struct attribute myattribute = {
    .name = file_name,
    .mode = S_IRUSR | S_IWUSR
};
static struct sysfs_ops mysysfs_ops = {
    .show = myshow,
    .store = mystore
};
static struct kobj_type mykobj_type = {
    .sysfs_ops = &mysysfs_ops,
};
static struct kobject mykobject = {
    .name = "mykobject",
    .ktype = &mykobj_type
};

static int __init start(void)
{
    int tmp;

    PRINT("Initializing module\n");

    if (kobject_init_and_add(&mykobject, &mykobj_type, NULL, "mykobj") != 0) {
        ERROR ("Digsig key failed to register properly\n");
        return -1;
    }
    if ((tmp = sysfs_create_file(&mykobject, &myattribute)) != 0) {
        ERROR ("Create file failed\n");
        return -1;
    }
    PRINT("INIT CORRECT");
    return 0;
}

static void __exit close(void)
{
    PRINT("Deinitializing module\n");
    sysfs_remove_file(&mykobject, &myattribute);
    kobject_del(&mykobject);
}

module_init(start);
module_exit(close);

When I compile my module, all works fine, but when I try to run it I get a insmod: error inserting 'mytester.ko': -1 Unknown symbol in module

Using dmesg I get more details:

[18335.892462] mytester: Unknown symbol sysfs_remove_file (err 0)
[18335.892462] mytester: Unknown symbol sysfs_create_file (err 0)
[18335.892646] mytester: Unknown symbol kobject_init_and_add (err 0)

And that's the point. I don't understand this message, because both kobject.h and sysfs.h are included. So I can't really understand what is happening here. Even if I comment out my whole function mystore to a simple return (as shown) the error is the same. So the error is not elsewhere....