Adding own sysfs device attribute group to existing standard device class attributes - linux-kernel

Question

One of Linux kernel training courses task is to add sysfs support to one of previously written device drivers. I choose my ds1307 rtc driver and want to add device attributes, not to replace rtc device ones.
But I am not sure that my solution is correct.
At the time when the probe function is called i2c client device is already created, so that any manipulation on device attributes will cause race condition.
Thus I decide to add my attributes to rtc device before device registration. I just count default rtc attribute groups, allocate new array with one extra position for my attribute group and replace rtc device groups pointer with allocated array.
static int ds1307x_probe(struct i2c_client *client,
const struct i2c_device_id *id)
{
struct device *dev;
const struct attribute_group **grp, **ngrp;
struct rtc_device *rtc;
int ret, cnt;
/* device presence check code skipped */
dev = &client->dev;
rtc = devm_rtc_allocate_device(dev);
if (!rtc)
return -ENOMEM;
dev = &rtc->dev;
grp = dev->groups;
if (!grp) {
/* no default groups, just use own groups array */
dev->groups = ds1307x_groups;
} else {
/* copy rtc groups array, add own group at end */
cnt = 0;
while (grp[cnt])
++cnt;
ngrp = devm_kmalloc(&rtc->dev, (cnt+2) * sizeof(*ngrp),
GFP_KERNEL);
if (!ngrp)
return -ENOMEM;
memcpy(ngrp, grp, cnt * sizeof(*ngrp));
ngrp[cnt] = &ds1307x_group;
ngrp[cnt+1] = NULL;
dev->groups = ngrp;
}
rtc->uie_unsupported = 1;
rtc->ops = &ds1307x_ops;
ret = rtc_register_device(rtc);
/* remaining of code skipped */
}
Attribute group ds1307x_control with attributes clock_halt and out_ctrl appears in rtc0 device directory among standard rtc class attributes:
~ # ls /sys/class/rtc/rtc0/
date ds1307x_control name subsystem
dev hctosys power time
device max_user_freq since_epoch uevent
~ # ls /sys/class/rtc/rtc0/ds1307x_control/
clock_halt out_ctrl
This solution works for me and I hope it is good enough for training courses.
But what about real life? Are there any hidden problems?