wwan: core: no more hold netdev ops owning module

The WWAN netdev ops owner holding was used to protect from the
unexpected memory disappear. This approach causes a dependency cycle
(driver -> core -> driver) and effectively prevents a WWAN driver
unloading. E.g. WWAN hwsim could not be unloaded until all simulated
devices are removed:

~# modprobe wwan_hwsim devices=2
~# lsmod | grep wwan
wwan_hwsim             16384  2
wwan                   20480  1 wwan_hwsim
~# rmmod wwan_hwsim
rmmod: ERROR: Module wwan_hwsim is in use
~# echo > /sys/kernel/debug/wwan_hwsim/hwsim0/destroy
~# echo > /sys/kernel/debug/wwan_hwsim/hwsim1/destroy
~# lsmod | grep wwan
wwan_hwsim             16384  0
wwan                   20480  1 wwan_hwsim
~# rmmod wwan_hwsim

For a real device driver this will cause an inability to unload module
until a served device is physically detached.

Since the last commit we are removing all child netdev(s) when a driver
unregister the netdev ops. This allows us to permit the driver
unloading, since any sane driver will call ops unregistering on a device
deinitialization. So, remove the holding of an ops owner to make it
easier to unload a driver module. The owner field has also beed removed
from the ops structure as there are no more users of this field.
Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/mhi/net.c

@@ -383,7 +383,6 @@ static void mhi_net_dellink(void *ctxt, struct net_device *ndev,
 }
 
 static const struct wwan_ops mhi_wwan_ops = {
-	.owner = THIS_MODULE,
 	.priv_size = sizeof(struct mhi_net_dev),
 	.setup = mhi_net_setup,
 	.newlink = mhi_net_newlink,
@@ -436,7 +435,7 @@ static void mhi_net_remove(struct mhi_device *mhi_dev)
 	struct mhi_net_dev *mhi_netdev = dev_get_drvdata(&mhi_dev->dev);
 	struct mhi_controller *cntrl = mhi_dev->mhi_cntrl;
 
-	/* rtnetlink takes care of removing remaining links */
+	/* WWAN core takes care of removing remaining links */
 	wwan_unregister_ops(&cntrl->mhi_dev->dev);
 
 	if (create_default_iface)

drivers/net/wwan/wwan_core.c

@@ -929,11 +929,6 @@ int wwan_register_ops(struct device *parent, const struct wwan_ops *ops,
 		return -EBUSY;
 	}
 
-	if (!try_module_get(ops->owner)) {
-		wwan_remove_dev(wwandev);
-		return -ENODEV;
-	}
-
 	wwandev->ops = ops;
 	wwandev->ops_ctxt = ctxt;
 
@@ -960,7 +955,6 @@ static int wwan_child_dellink(struct device *dev, void *data)
 void wwan_unregister_ops(struct device *parent)
 {
 	struct wwan_device *wwandev = wwan_dev_get_by_parent(parent);
-	struct module *owner;
 	LIST_HEAD(kill_list);
 
 	if (WARN_ON(IS_ERR(wwandev)))
@@ -976,8 +970,6 @@ void wwan_unregister_ops(struct device *parent)
 	 */
 	put_device(&wwandev->dev);
 
-	owner = wwandev->ops->owner;	/* Preserve ops owner */
-
 	rtnl_lock();	/* Prevent concurent netdev(s) creation/destroying */
 
 	/* Remove all child netdev(s), using batch removing */
@@ -989,8 +981,6 @@ void wwan_unregister_ops(struct device *parent)
 
 	rtnl_unlock();
 
-	module_put(owner);
-
 	wwandev->ops_ctxt = NULL;
 	wwan_remove_dev(wwandev);
 }

drivers/net/wwan/wwan_hwsim.c

@@ -94,7 +94,6 @@ static void wwan_hwsim_netdev_setup(struct net_device *ndev)
 }
 
 static const struct wwan_ops wwan_hwsim_wwan_rtnl_ops = {
-	.owner = THIS_MODULE,
 	.priv_size = 0,			/* No private data */
 	.setup = wwan_hwsim_netdev_setup,
 };

include/linux/wwan.h

@@ -128,14 +128,12 @@ void *wwan_port_get_drvdata(struct wwan_port *port);
 
 /**
  * struct wwan_ops - WWAN device ops
- * @owner: module owner of the WWAN ops
  * @priv_size: size of private netdev data area
  * @setup: set up a new netdev
  * @newlink: register the new netdev
  * @dellink: remove the given netdev
  */
 struct wwan_ops {
-	struct module *owner;
 	unsigned int priv_size;
 	void (*setup)(struct net_device *dev);
 	int (*newlink)(void *ctxt, struct net_device *dev,