driver model: make driver refcounting similar to devices'.

In the spirit of devices and buses, change driver refcounting model to 
match the way that devices and buses are done.

struct device_driver gets a ->present field, which is set on registration 
and cleared in driver_unregister(). get_device() checks the state of this
flag and returns NULL if cleared. 

Note that the horribly wrong remove_driver() is deprecated and simply BUG()s
when called. Please convert callers to use driver_unregister(). Updates to 
callers will be coming soon.

Note also that this still doesn't fix the race in which a driver module can
be removed while the refcount on a driver > 1. Near future work should help
to remedy it, but no solutions are guaranteed..

drivers/base/driver.c

@@ -39,6 +39,43 @@ int driver_for_each_dev(struct device_driver * drv, void * data,
 	return error;
 }
 
+struct device_driver * get_driver(struct device_driver * drv)
+{
+	struct device_driver * ret = drv;
+	spin_lock(&device_lock);
+	if (drv && drv->present && atomic_read(&drv->refcount) > 0)
+		atomic_inc(&drv->refcount);
+	else
+		ret = NULL;
+	spin_unlock(&device_lock);
+	return ret;
+}
+
+
+void remove_driver(struct device_driver * drv)
+{
+	BUG();
+}
+
+/**
+ * put_driver - decrement driver's refcount and clean up if necessary
+ * @drv:	driver in question
+ */
+void put_driver(struct device_driver * drv)
+{
+	struct bus_type * bus = drv->bus;
+	if (!atomic_dec_and_lock(&drv->refcount,&device_lock))
+		return;
+	list_del_init(&drv->bus_list);
+	spin_unlock(&device_lock);
+	BUG_ON(drv->present);
+	driver_detach(drv);
+	driver_remove_dir(drv);
+	if (drv->release)
+		drv->release(drv);
+	put_bus(bus);
+}
+
 /**
  * driver_register - register driver with bus
  * @drv:	driver to register
@@ -50,12 +87,13 @@ int driver_register(struct device_driver * drv)
 	if (!drv->bus)
 		return -EINVAL;
 
-	pr_debug("Registering driver '%s' with bus '%s'\n",drv->name,drv->bus->name);
+	pr_debug("driver %s:%s: registering\n",drv->bus->name,drv->name);
 
 	get_bus(drv->bus);
 	atomic_set(&drv->refcount,2);
 	rwlock_init(&drv->lock);
 	INIT_LIST_HEAD(&drv->devices);
+	drv->present = 1;
 	spin_lock(&device_lock);
 	list_add(&drv->bus_list,&drv->bus->drivers);
 	spin_unlock(&device_lock);
@@ -65,39 +103,17 @@ int driver_register(struct device_driver * drv)
 	return 0;
 }
 
-static void __remove_driver(struct device_driver * drv)
-{
-	pr_debug("Unregistering driver '%s' from bus '%s'\n",drv->name,drv->bus->name);
-	driver_detach(drv);
-	driver_remove_dir(drv);
-	if (drv->release)
-		drv->release(drv);
-	put_bus(drv->bus);
-}
-
-void remove_driver(struct device_driver * drv)
+void driver_unregister(struct device_driver * drv)
 {
 	spin_lock(&device_lock);
-	atomic_set(&drv->refcount,0);
-	list_del_init(&drv->bus_list);
-	spin_unlock(&device_lock);
-	__remove_driver(drv);
-}
-
-/**
- * put_driver - decrement driver's refcount and clean up if necessary
- * @drv:	driver in question
- */
-void put_driver(struct device_driver * drv)
-{
-	if (!atomic_dec_and_lock(&drv->refcount,&device_lock))
-		return;
-	list_del_init(&drv->bus_list);
+	drv->present = 0;
 	spin_unlock(&device_lock);
-	__remove_driver(drv);
+	pr_debug("driver %s:%s: unregistering\n",drv->bus->name,drv->name);
+	put_driver(drv);
 }
 
 EXPORT_SYMBOL(driver_for_each_dev);
 EXPORT_SYMBOL(driver_register);
+EXPORT_SYMBOL(driver_unregister);
+EXPORT_SYMBOL(get_driver);
 EXPORT_SYMBOL(put_driver);
-EXPORT_SYMBOL(remove_driver);

include/linux/device.h

@@ -110,6 +110,7 @@ struct device_driver {
 
 	rwlock_t		lock;
 	atomic_t		refcount;
+	u32			present;
 
 	struct list_head	bus_list;
 	struct list_head	class_list;
@@ -127,16 +128,10 @@ struct device_driver {
 };
 
 
-
 extern int driver_register(struct device_driver * drv);
+extern void driver_unregister(struct device_driver * drv);
 
-static inline struct device_driver * get_driver(struct device_driver * drv)
-{
-	BUG_ON(!atomic_read(&drv->refcount));
-	atomic_inc(&drv->refcount);
-	return drv;
-}
-
+extern struct device_driver * get_driver(struct device_driver * drv);
 extern void put_driver(struct device_driver * drv);
 extern void remove_driver(struct device_driver * drv);