rtc: rk808: fix possible race condition

The probe function is not allowed to fail after registering the RTC because
the following may happen:

CPU0:                                CPU1:
sys_load_module()
 do_init_module()
  do_one_initcall()
   cmos_do_probe()
    rtc_device_register()
     __register_chrdev()
     cdev->owner = struct module*
                                     open("/dev/rtc0")
    rtc_device_unregister()
  module_put()
  free_module()
   module_free(mod->module_core)
   /* struct module *module is now
      freed */
                                      chrdev_open()
                                       spin_lock(cdev_lock)
                                       cdev_get()
                                        try_module_get()
                                         module_is_live()
                                         /* dereferences already
                                            freed struct module* */

Switch to devm_rtc_allocate_device/rtc_register_device to register the rtc
as late as possible.
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-rk808.c

@@ -405,12 +405,11 @@ static int rk808_rtc_probe(struct platform_device *pdev)
 
 	device_init_wakeup(&pdev->dev, 1);
 
-	rk808_rtc->rtc = devm_rtc_device_register(&pdev->dev, "rk808-rtc",
-						  &rk808_rtc_ops, THIS_MODULE);
-	if (IS_ERR(rk808_rtc->rtc)) {
-		ret = PTR_ERR(rk808_rtc->rtc);
-		return ret;
-	}
+	rk808_rtc->rtc = devm_rtc_allocate_device(&pdev->dev);
+	if (IS_ERR(rk808_rtc->rtc))
+		return PTR_ERR(rk808_rtc->rtc);
+
+	rk808_rtc->rtc->ops = &rk808_rtc_ops;
 
 	rk808_rtc->irq = platform_get_irq(pdev, 0);
 	if (rk808_rtc->irq < 0) {
@@ -427,9 +426,10 @@ static int rk808_rtc_probe(struct platform_device *pdev)
 	if (ret) {
 		dev_err(&pdev->dev, "Failed to request alarm IRQ %d: %d\n",
 			rk808_rtc->irq, ret);
+		return ret;
 	}
 
-	return ret;
+	return rtc_register_device(rk808_rtc->rtc);
 }
 
 static struct platform_driver rk808_rtc_driver = {