driver model: add per-class rwsem and protect list accesses with it.

This is similar to struct bus_type's rwsem, though classes aren't doing 
anything nearly as fancy with it. We just make sure to take it when ever
we add or remove any devices or drivers. 

drivers/base/class.c

@@ -10,27 +10,29 @@ static LIST_HEAD(class_list);
 
 int devclass_add_driver(struct device_driver * drv)
 {
-	if (drv->devclass) {
-		pr_debug("Registering driver %s:%s with class %s\n",
-			 drv->bus->name,drv->name,drv->devclass->name);
-
-		spin_lock(&device_lock);
-		list_add_tail(&drv->class_list,&drv->devclass->drivers);
-		spin_unlock(&device_lock);
+	struct device_class * cls = get_devclass(drv->devclass);
+	if (cls) {
+		down_write(&cls->rwsem);
+		pr_debug("device class %s: adding driver %s:%s\n",
+			 cls->name,drv->bus->name,drv->name);
+		list_add_tail(&drv->class_list,&cls->drivers);
 		devclass_drv_link(drv);
+		up_write(&cls->rwsem);
 	}
 	return 0;
 }
 
 void devclass_remove_driver(struct device_driver * drv)
 {
-	if (drv->devclass) {
-		pr_debug("Removing driver %s:%s:%s\n",
-			 drv->devclass->name,drv->bus->name,drv->name);
-		spin_lock(&device_lock);
+	struct device_class * cls = drv->devclass;
+	if (cls) {
+		down_write(&cls->rwsem);
+		pr_debug("device class %s: removing driver %s:%s\n",
+			 cls->name,drv->bus->name,drv->name);
 		list_del_init(&drv->class_list);
-		spin_unlock(&device_lock);
 		devclass_drv_unlink(drv);
+		up_write(&cls->rwsem);
+		put_devclass(cls);
 	}
 }
 
@@ -38,9 +40,7 @@ void devclass_remove_driver(struct device_driver * drv)
 static void enum_device(struct device_class * cls, struct device * dev)
 {
 	u32 val;
-	spin_lock(&device_lock);
 	val = cls->devnum++;
-	spin_unlock(&device_lock);
 	dev->class_num = val;
 	devclass_dev_link(cls,dev);
 }
@@ -51,22 +51,44 @@ static void unenum_device(struct device_class * cls, struct device * dev)
 	dev->class_num = 0;
 }
 
+/**
+ *	devclass_add_device - register device with device class
+ *	@dev:   device to be registered 
+ *
+ *	This is called when a device is either registered with the 
+ *	core, or after the a driver module is loaded and bound to
+ *	the device. 
+ *	The class is determined by looking at @dev's driver, so one
+ *	way or another, it must be bound to something. Once the 
+ *	class is determined, it's set to prevent against concurrent
+ *	calls for the same device stomping on each other. 
+ *
+ *	/sbin/hotplug should be called once the device is added to 
+ *	class and all the interfaces. 
+ */
 int devclass_add_device(struct device * dev)
 {
 	struct device_class * cls;
 	int error = 0;
 
 	if (dev->driver) {
-		cls = dev->driver->devclass;
+		cls = get_devclass(dev->driver->devclass);
 		if (cls) {
-			pr_debug("adding device '%s' to class '%s'\n",
-				 dev->name,cls->name);
+			down_write(&cls->rwsem);
+			pr_debug("device class %s: adding device %s\n",
+				 cls->name,dev->name);
 			if (cls->add_device) 
 				error = cls->add_device(dev);
 			if (!error) {
 				enum_device(cls,dev);
 				interface_add(cls,dev);
 			}
+
+			/* notify userspace (call /sbin/hotplug) here */
+
+			up_write(&cls->rwsem);
+			if (error)
+				put_devclass(cls);
 		}
 	}
 	return error;
@@ -74,14 +96,21 @@ int devclass_add_device(struct device * dev)
 
 void devclass_remove_device(struct device * dev)
 {
-	struct device_class * cls = dev->driver->devclass;
+	struct device_class * cls;
+
+	if (dev->driver) {
+		cls = dev->driver->devclass;
 		if (cls) {
-		pr_debug("removing device '%s' from class '%s'\n",
-			 dev->name,cls->name);
+			down_write(&cls->rwsem);
+			pr_debug("device class %s: removing device %s\n",
+				 cls->name,dev->name);
 			interface_remove(cls,dev);
 			unenum_device(cls,dev);
 			if (cls->remove_device)
 				cls->remove_device(dev);
+			up_write(&cls->rwsem);
+			put_devclass(cls);
+		}
 	}
 }
 
@@ -111,6 +140,7 @@ int devclass_register(struct device_class * cls)
 {
 	INIT_LIST_HEAD(&cls->drivers);
 	INIT_LIST_HEAD(&cls->intf_list);
+	init_rwsem(&cls->rwsem);
 	atomic_set(&cls->refcount,2);
 	cls->present = 1;
 	pr_debug("device class '%s': registering\n",cls->name);

include/linux/device.h

@@ -163,6 +163,8 @@ extern void driver_remove_file(struct device_driver *, struct driver_attribute *
  */
 struct device_class {
 	char			* name;
+	struct rw_semaphore	rwsem;
+
 	atomic_t		refcount;
 	u32			present;