Subject: Re: [PATCH net-next v1] dev: remove netdev_lock() and netdev_lock_ops() in register_netdevice().

From: Stanislav Fomichev <[email-masked]>
Date: Sat, 8 Mar 2025 14:41:41 -0800

On 03/08, Jakub Kicinski wrote:
> On Sun, 9 Mar 2025 05:37:18 +0900 Kohei Enju wrote:
> > Both netdev_lock() and netdev_lock_ops() are called before
> > list_netdevice() in register_netdevice().
> > No other context can access the struct net_device, so we don't need these
> > locks in this context.

That's technically true, but it will set off a bunch of lockdep
warnings :-(

> Doesn't sysfs get registered earlier?
> I'm afraid not being able to take the lock from the registration
> path ties our hands too much. Maybe we need to make a more serious
> attempt at letting the caller take the lock?

This looks like another case of upper/lower :-( So maybe we need to solve
it for real? With an extra upper_lock pointer in the netdev?
Untested patch to convey the general idea:

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index d3c549f73909..9c85179431e6 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2520,6 +2520,7 @@ struct net_device {
 	 * Ordering: take after rtnl_lock.
 	 */
 	struct mutex		lock;
+	struct mutex		*upper_lock;
 
 #if IS_ENABLED(CONFIG_NET_SHAPER)
 	/**
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 90597bf84e3d..3d0fda6e9bca 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -3022,6 +3022,9 @@ static int do_setlink(const struct sk_buff *skb, struct net_device *dev,
 	char ifname[IFNAMSIZ];
 	int err;
 
+	/* TODO: add another wrapper for this */
+	if (dev->upper_lock)
+		mutex_lock(dev->upper_lock);
 	netdev_lock_ops(dev);
 
 	err = validate_linkmsg(dev, tb, extack);
@@ -3394,6 +3397,8 @@ static int do_setlink(const struct sk_buff *skb, struct net_device *dev,
 	}
 
 	netdev_unlock_ops(dev);
+	if (dev->upper_lock)
+		mutex_unlock(dev->upper_lock);
 
 	return err;
 }
diff --git a/net/mac80211/iface.c b/net/mac80211/iface.c
index b0423046028c..818ff487b363 100644
--- a/net/mac80211/iface.c
+++ b/net/mac80211/iface.c
@@ -304,7 +304,7 @@ static int ieee80211_change_mac(struct net_device *dev, void *addr)
 	if (!dev->ieee80211_ptr->registered)
 		return 0;
 
-	guard(wiphy)(local->hw.wiphy);
+	/* TODO: remove guard from other places */
 
 	return _ieee80211_change_mac(sdata, addr);
 }
@@ -2227,6 +2227,8 @@ int ieee80211_if_add(struct ieee80211_local *local, const char *name,
 			free_netdev(ndev);
 			return ret;
 		}
+
+		ndev->upper_lock = &local->hw.wiphy.mtx;
 	}
 
 	mutex_lock(&local->iflist_mtx);