netdev - [PATCH net-next 07/10] wwan: core: no more hold netdev ops owning module

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Date:   Tue, 15 Jun 2021 03:30:13 +0300
From:   Sergey Ryazanov <ryazanov.s.a@...il.com>
To:     Loic Poulain <loic.poulain@...aro.org>,
        Johannes Berg <johannes@...solutions.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org
Subject: [PATCH net-next 07/10] 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@...il.com>
---
 drivers/net/mhi/net.c         |  1 -
 drivers/net/wwan/wwan_core.c  | 10 ----------
 drivers/net/wwan/wwan_hwsim.c |  1 -
 include/linux/wwan.h          |  2 --
 4 files changed, 14 deletions(-)

diff --git a/drivers/net/mhi/net.c b/drivers/net/mhi/net.c
index 64af1e518484..ebd1dbf0a536 100644
--- a/drivers/net/mhi/net.c
+++ b/drivers/net/mhi/net.c
@@ -383,7 +383,6 @@ static void mhi_net_dellink(void *ctxt, struct net_device *ndev,
 }
 
 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,
diff --git a/drivers/net/wwan/wwan_core.c b/drivers/net/wwan/wwan_core.c
index 3f04e674cdaa..28590405172c 100644
--- a/drivers/net/wwan/wwan_core.c
+++ b/drivers/net/wwan/wwan_core.c
@@ -921,11 +921,6 @@ 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;
 
@@ -952,7 +947,6 @@ 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)))
@@ -968,8 +962,6 @@ 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 */
@@ -981,8 +973,6 @@ void wwan_unregister_ops(struct device *parent)
 
 	rtnl_unlock();
 
-	module_put(owner);
-
 	wwandev->ops_ctxt = NULL;
 	wwan_remove_dev(wwandev);
 }
diff --git a/drivers/net/wwan/wwan_hwsim.c b/drivers/net/wwan/wwan_hwsim.c
index c1e850b9c087..a8582a58a385 100644
--- a/drivers/net/wwan/wwan_hwsim.c
+++ b/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,
 };
diff --git a/include/linux/wwan.h b/include/linux/wwan.h
index 430a3a0817de..656a571b52ed 100644
--- a/include/linux/wwan.h
+++ b/include/linux/wwan.h
@@ -119,14 +119,12 @@ 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,
-- 
2.26.3