driver core: bus: move dev_root out of struct bus_type

Now that all accesses of dev_root is through the bus_get_dev_root()
call, move the pointer out of struct bus_type and into the private
dynamic structure, subsys_private.

With this change, there is no modifiable portions of struct bus_type so
it can be marked as a constant structure and moved to read-only memory.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230313182918.1312597-22-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/base.h

@@ -27,6 +27,7 @@
  *                 on this bus.
  * @bus - pointer back to the struct bus_type that this structure is associated
  *        with.
+ * @dev_root: Default device to use as the parent.
  *
  * @glue_dirs - "glue" directory to put in-between the parent device to
  *              avoid namespace conflicts
@@ -49,6 +50,7 @@ struct subsys_private {
 	struct blocking_notifier_head bus_notifier;
 	unsigned int drivers_autoprobe:1;
 	struct bus_type *bus;
+	struct device *dev_root;
 
 	struct kset glue_dirs;
 	struct class *class;

drivers/base/bus.c

@@ -935,8 +935,8 @@ void bus_unregister(const struct bus_type *bus)
 		return;
 
 	pr_debug("bus: '%s': unregistering\n", bus->name);
-	if (bus->dev_root)
-		device_unregister(bus->dev_root);
+	if (sp->dev_root)
+		device_unregister(sp->dev_root);
 
 	bus_kobj = &sp->subsys.kobj;
 	sysfs_remove_groups(bus_kobj, bus->bus_groups);
@@ -1198,6 +1198,7 @@ static int subsys_register(struct bus_type *subsys,
 			   const struct attribute_group **groups,
 			   struct kobject *parent_of_root)
 {
+	struct subsys_private *sp;
 	struct device *dev;
 	int err;
 
@@ -1205,6 +1206,12 @@ static int subsys_register(struct bus_type *subsys,
 	if (err < 0)
 		return err;
 
+	sp = bus_to_subsys(subsys);
+	if (!sp) {
+		err = -EINVAL;
+		goto err_sp;
+	}
+
 	dev = kzalloc(sizeof(struct device), GFP_KERNEL);
 	if (!dev) {
 		err = -ENOMEM;
@@ -1223,7 +1230,8 @@ static int subsys_register(struct bus_type *subsys,
 	if (err < 0)
 		goto err_dev_reg;
 
-	subsys->dev_root = dev;
+	sp->dev_root = dev;
+	subsys_put(sp);
 	return 0;
 
 err_dev_reg:
@@ -1232,6 +1240,8 @@ static int subsys_register(struct bus_type *subsys,
 err_name:
 	kfree(dev);
 err_dev:
+	subsys_put(sp);
+err_sp:
 	bus_unregister(subsys);
 	return err;
 }
@@ -1349,9 +1359,15 @@ bool bus_is_registered(const struct bus_type *bus)
  */
 struct device *bus_get_dev_root(const struct bus_type *bus)
 {
-	if (bus)
-		return get_device(bus->dev_root);
-	return NULL;
+	struct subsys_private *sp = bus_to_subsys(bus);
+	struct device *dev_root;
+
+	if (!sp)
+		return NULL;
+
+	dev_root = get_device(sp->dev_root);
+	subsys_put(sp);
+	return dev_root;
 }
 EXPORT_SYMBOL_GPL(bus_get_dev_root);
 

include/linux/device/bus.h

@@ -26,7 +26,6 @@ struct fwnode_handle;
  *
  * @name:	The name of the bus.
  * @dev_name:	Used for subsystems to enumerate devices like ("foo%u", dev->id).
- * @dev_root:	Default device to use as the parent.
  * @bus_groups:	Default attributes of the bus.
  * @dev_groups:	Default attributes of the devices on the bus.
  * @drv_groups: Default attributes of the device drivers on the bus.
@@ -82,7 +81,6 @@ struct fwnode_handle;
 struct bus_type {
 	const char		*name;
 	const char		*dev_name;
-	struct device		*dev_root;
 	const struct attribute_group **bus_groups;
 	const struct attribute_group **dev_groups;
 	const struct attribute_group **drv_groups;