[PATCH net-next v2 02/11] net: make netdev_lock() protect netdev->reg_state

[Jakub Kicinski <kuba@kernel.org>]

Protect writes to netdev->reg_state with netdev_lock().
From now on holding netdev_lock() is sufficient to prevent
the net_device from getting unregistered, so code which
wants to hold just a single netdev around no longer needs
to hold rtnl_lock.

We do not protect the NETREG_UNREGISTERED -> NETREG_RELEASED
transition. We'd need to move mutex_destroy(netdev->lock)
to .release, but the real reason is that trying to stop
the unregistration process mid-way would be unsafe / crazy.
Taking references on such devices is not safe, either.
So the intended semantics are to lock REGISTERED devices.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
v2:
 - reorder with next patch
v1: https://lore.kernel.org/20250114035118.110297-4-kuba@kernel.org
---
 include/linux/netdevice.h | 2 +-
 net/core/dev.c            | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 891c5bdb894c..30963c5d409b 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2448,7 +2448,7 @@ struct net_device {
 	 * Should always be taken using netdev_lock() / netdev_unlock() helpers.
 	 * Drivers are free to use it for other protection.
 	 *
-	 * Protects: @net_shaper_hierarchy.
+	 * Protects: @reg_state, @net_shaper_hierarchy.
 	 * Ordering: take after rtnl_lock.
 	 */
 	struct mutex		lock;
diff --git a/net/core/dev.c b/net/core/dev.c
index fda4e1039bf0..6603c08768f6 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10668,7 +10668,9 @@ int register_netdevice(struct net_device *dev)
 
 	ret = netdev_register_kobject(dev);
 
+	netdev_lock(dev);
 	WRITE_ONCE(dev->reg_state, ret ? NETREG_UNREGISTERED : NETREG_REGISTERED);
+	netdev_unlock(dev);
 
 	if (ret)
 		goto err_uninit_notify;
@@ -10942,7 +10944,9 @@ void netdev_run_todo(void)
 			continue;
 		}
 
+		netdev_lock(dev);
 		WRITE_ONCE(dev->reg_state, NETREG_UNREGISTERED);
+		netdev_unlock(dev);
 		linkwatch_sync_dev(dev);
 	}
 
@@ -11548,7 +11552,9 @@ void unregister_netdevice_many_notify(struct list_head *head,
 	list_for_each_entry(dev, head, unreg_list) {
 		/* And unlink it from device chain. */
 		unlist_netdevice(dev);
+		netdev_lock(dev);
 		WRITE_ONCE(dev->reg_state, NETREG_UNREGISTERING);
+		netdev_unlock(dev);
 	}
 	flush_all_backlogs();
 
-- 
2.48.0