net: Add per-netns netdev unregistration infra.

When we need to unregister a netdev in a different netns, we will
delegate its unregistration to per-netns work.

There are three types of such cross-netns devices:

  1. Paired devices (e.g., netkit, veth, vxcan)
     -> Unregistering one device also deletes its peer, which
        may reside in another netns.

  2. Tunnel devices (e.g., bareudp, geneve, etc)
     -> Destroying a netns removes devices in another netns if
        their backend sockets reside in the dying netns

  3. Stacked devices (e.g., ipvlan, macvlan, etc)
     -> Removing the lower device also removes multiple upper
        devices, each of which may reside in different namespaces.

In these cases, we will use unregister_netdevice_queue_net() to
queue such potential cross-netns devices for destruction.

Each driver must not call both unregister_netdevice_queue_net()
and unregister_netdevice_queue() for the same device.  See the
subsequent veth/bareudp/ipvlan patches for how they avoid double
queueing.

unregister_netdevice_queue_net() takes net and dev.  If dev resides
in the net, it simply calls unregister_netdevice_queue().

If dev_net(dev) is different from the net, it enqueues the device
to dev_net(dev)->dev_unreg_head and schedules the per-netns work.

When __rtnl_net_unlock() is called from the per-netns work (or another
thread already holding the lock), unregister_netdevice_many_net()
collects the queued devices and calls unregister_netdevice_many()
to perform the actual unregistration.

During netns dismantle, rtnl_net_flush_workqueue() is called at the
end of default_device_exit_batch() to ensure that cross-netns
devices in the other alive netns are unregistered.

Once RTNL is removed, a device could be moved to another netns while
being queued to net->dev_unreg_head.

__dev_change_net_namespace() handles this race by acquiring
net->dev_unreg_lock of both the old and new netns after dev_set_net()
and moving the device between their dev_unreg_head lists.

Since dev_set_net() and unregister_netdevice_queue_net() are
synchronised by netdev_lock(), the device is either queued to the
old netns's dev_unreg_head and then moved, or queued directly to
the new netns.

Note that unregister_netdevice_move_net() does not need to call
rtnl_net_queue_work() because __dev_change_net_namespace() is
(supposed to be) called with rtnl_net_lock().  (Not all callers
hold it yet, but the race does not happen until all callers
are converted and RTNL is removed.)

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260703001009.1572444-7-kuniyu@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/netdevice.h

@@ -1845,6 +1845,8 @@ enum netdev_reg_state {
  *	@napi_list:	List entry used for polling NAPI devices
  *	@unreg_list:	List entry  when we are unregistering the
  *			device; see the function unregister_netdev
+ *	@unreg_list_net:List entry when we are unregistering the cross-netns
+ *			device; see the function unregister_netdevice_queue_net()
  *	@close_list:	List entry used when we are closing the device
  *	@ptype_all:     Device-specific packet handlers for all protocols
  *	@ptype_specific: Device-specific, protocol-specific packet handlers
@@ -2241,6 +2243,9 @@ struct net_device {
 	struct list_head	dev_list;
 	struct list_head	napi_list;
 	struct list_head	unreg_list;
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+	struct list_head	unreg_list_net;
+#endif
 	struct list_head	close_list;
 	struct list_head	ptype_all;
 
@@ -3472,6 +3477,19 @@ static inline void unregister_netdevice(struct net_device *dev)
 	unregister_netdevice_queue(dev, NULL);
 }
 
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+void unregister_netdevice_queue_net(struct net *net, struct net_device *dev,
+				    struct list_head *head);
+void unregister_netdevice_many_net(struct net *net);
+#else
+static inline void unregister_netdevice_queue_net(struct net *net,
+						  struct net_device *dev,
+						  struct list_head *head)
+{
+	unregister_netdevice_queue(dev, head);
+}
+#endif
+
 int netdev_refcnt_read(const struct net_device *dev);
 void free_netdev(struct net_device *dev);
 

include/net/net_namespace.h

@@ -198,6 +198,8 @@ struct net {
 	/* Move to a better place when the config guard is removed. */
 	struct mutex		rtnl_mutex;
 	struct work_struct	rtnl_work;
+	struct list_head	dev_unreg_head;
+	spinlock_t		dev_unreg_lock;
 #endif
 #if IS_ENABLED(CONFIG_VSOCKETS)
 	struct netns_vsock	vsock;

net/core/dev.c

@@ -12097,6 +12097,9 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 
 	INIT_LIST_HEAD(&dev->napi_list);
 	INIT_LIST_HEAD(&dev->unreg_list);
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+	INIT_LIST_HEAD(&dev->unreg_list_net);
+#endif
 	INIT_LIST_HEAD(&dev->close_list);
 	INIT_LIST_HEAD(&dev->link_watch_list);
 	INIT_LIST_HEAD(&dev->adj_list.upper);
@@ -12314,6 +12317,10 @@ void unregister_netdevice_queue(struct net_device *dev, struct list_head *head)
 {
 	ASSERT_RTNL();
 
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+	DEBUG_NET_WARN_ON_ONCE(!list_empty(&dev->unreg_list_net));
+#endif
+
 	if (head) {
 		list_move_tail(&dev->unreg_list, head);
 	} else {
@@ -12490,6 +12497,16 @@ void unregister_netdevice_many_notify(struct list_head *head,
 	synchronize_net();
 
 	list_for_each_entry(dev, head, unreg_list) {
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+		struct net *net = dev_net(dev);
+
+		/* spin_lock() can be moved outside of the loop
+		 * once the per-netns RTNL conversion completes.
+		 */
+		spin_lock(&net->dev_unreg_lock);
+		list_del(&dev->unreg_list_net);
+		spin_unlock(&net->dev_unreg_lock);
+#endif
 		netdev_put(dev, &dev->dev_registered_tracker);
 		net_set_todo(dev);
 		cnt++;
@@ -12512,6 +12529,74 @@ void unregister_netdevice_many(struct list_head *head)
 }
 EXPORT_SYMBOL(unregister_netdevice_many);
 
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+void unregister_netdevice_queue_net(struct net *net, struct net_device *dev,
+				    struct list_head *head)
+{
+	netdev_lock(dev);
+
+	if (net_eq(dev_net(dev), net)) {
+		netdev_unlock(dev);
+		unregister_netdevice_queue(dev, head);
+		return;
+	}
+
+	net = dev_net(dev);
+
+	spin_lock(&net->dev_unreg_lock);
+
+	DEBUG_NET_WARN_ON_ONCE(!list_empty(&dev->unreg_list));
+	DEBUG_NET_WARN_ON_ONCE(!list_empty(&dev->unreg_list_net));
+
+	list_add_tail(&dev->unreg_list_net, &net->dev_unreg_head);
+	rtnl_net_queue_work(net);
+
+	spin_unlock(&net->dev_unreg_lock);
+
+	netdev_unlock(dev);
+}
+EXPORT_SYMBOL(unregister_netdevice_queue_net);
+
+static void unregister_netdevice_move_net(struct net *net_old,
+					  struct net *net,
+					  struct net_device *dev)
+{
+	if (net_old > net) {
+		spin_lock(&net->dev_unreg_lock);
+		spin_lock_nested(&net_old->dev_unreg_lock, SINGLE_DEPTH_NESTING);
+	} else {
+		spin_lock(&net_old->dev_unreg_lock);
+		spin_lock_nested(&net->dev_unreg_lock, SINGLE_DEPTH_NESTING);
+	}
+
+	if (!list_empty(&dev->unreg_list_net)) {
+		list_del(&dev->unreg_list_net);
+		list_add_tail(&dev->unreg_list_net, &net->dev_unreg_head);
+	}
+
+	spin_unlock(&net_old->dev_unreg_lock);
+	spin_unlock(&net->dev_unreg_lock);
+}
+
+void unregister_netdevice_many_net(struct net *net)
+{
+	struct net_device *dev, *tmp;
+	LIST_HEAD(unreg_head_net);
+	LIST_HEAD(unreg_head);
+
+	spin_lock(&net->dev_unreg_lock);
+	list_splice_init(&net->dev_unreg_head, &unreg_head_net);
+	spin_unlock(&net->dev_unreg_lock);
+
+	list_for_each_entry_safe(dev, tmp, &unreg_head_net, unreg_list_net) {
+		list_del_init(&dev->unreg_list_net);
+		list_add_tail(&dev->unreg_list, &unreg_head);
+	}
+
+	unregister_netdevice_many(&unreg_head);
+}
+#endif
+
 /**
  *	unregister_netdev - remove device from the kernel
  *	@dev: device
@@ -12668,6 +12753,10 @@ int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 	netdev_unlock(dev);
 	dev->ifindex = new_ifindex;
 
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+	unregister_netdevice_move_net(net_old, net, dev);
+#endif
+
 	if (new_name[0]) {
 		/* Rename the netdev to prepared name */
 		write_seqlock_bh(&netdev_rename_lock);
@@ -13110,6 +13199,8 @@ static void __net_exit default_device_exit_batch(struct list_head *net_list)
 	}
 	unregister_netdevice_many(&dev_kill_list);
 	rtnl_unlock();
+
+	rtnl_net_flush_workqueue();
 }
 
 static struct pernet_operations __net_initdata default_device_ops = {

net/core/net_namespace.c

@@ -423,6 +423,8 @@ static __net_init int preinit_net(struct net *net, struct user_namespace *user_n
 	mutex_init(&net->rtnl_mutex);
 	lock_set_cmp_fn(&net->rtnl_mutex, rtnl_net_lock_cmp_fn, NULL);
 	INIT_WORK(&net->rtnl_work, rtnl_net_work_func);
+	INIT_LIST_HEAD(&net->dev_unreg_head);
+	spin_lock_init(&net->dev_unreg_lock);
 #endif
 
 	INIT_LIST_HEAD(&net->ptype_all);

net/core/rtnetlink.c

@@ -197,6 +197,7 @@ void __rtnl_net_unlock(struct net *net)
 {
 	ASSERT_RTNL();
 
+	unregister_netdevice_many_net(net);
 	mutex_unlock(&net->rtnl_mutex);
 }
 EXPORT_SYMBOL(__rtnl_net_unlock);
@@ -290,6 +291,9 @@ void rtnl_net_work_func(struct work_struct *work)
 {
 	struct net *net = container_of(work, struct net, rtnl_work);
 
+	if (list_empty(&net->dev_unreg_head))
+		return;
+
 	rtnl_net_lock(net);
 	rtnl_net_unlock(net);
 }