gpio: of: assign and read the hog pointer atomically

The device nodes representing GPIO hogs cannot be deleted without
unregistering the GPIO chip so there's no need to serialize their access.
However we must ensure that users can get the right address so write and
read it atomically.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/gpio/gpiolib-of.c

@@ -801,7 +801,7 @@ static int of_gpiochip_add_hog(struct gpio_chip *chip, struct device_node *hog)
 			return ret;
 
 #ifdef CONFIG_OF_DYNAMIC
-		desc->hog = hog;
+		WRITE_ONCE(desc->hog, hog);
 #endif
 	}
 
@@ -849,7 +849,7 @@ static void of_gpiochip_remove_hog(struct gpio_chip *chip,
 	struct gpio_desc *desc;
 
 	for_each_gpio_desc_with_flag(chip, desc, FLAG_IS_HOGGED)
-		if (desc->hog == hog)
+		if (READ_ONCE(desc->hog) == hog)
 			gpiochip_free_own_desc(desc);
 }
 

drivers/gpio/gpiolib.c

@@ -2330,7 +2330,7 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
 		clear_bit(FLAG_EDGE_FALLING, &desc->flags);
 		clear_bit(FLAG_IS_HOGGED, &desc->flags);
 #ifdef CONFIG_OF_DYNAMIC
-		desc->hog = NULL;
+		WRITE_ONCE(desc->hog, NULL);
 #endif
 		ret = true;
 	}