netdev: add "ops compat locking" helpers

Add helpers to "lock a netdev in a backward-compatible way",
which for ops-locked netdevs will mean take the instance lock.
For drivers which haven't opted into the ops locking we'll take
rtnl_lock.

The scoped foreach is dropping and re-taking the lock for each
device, even if prev and next are both under rtnl_lock.
I hope that's fine since we expect that netdev nl to be mostly
supported by modern drivers, and modern drivers should also
opt into the instance locking.

Note that these helpers are mostly needed for queue related state,
because drivers modify queue config in their ops in a non-atomic
way. Or differently put, queue changes don't have a clear-cut API
like NAPI configuration. Any state that can should just use the
instance lock directly, not the "compat" hacks.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250408195956.412733-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 51 insertions, 0 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 8c13e5339cc9..b52efa4cec56 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1052,6 +1052,20 @@ struct net_device *__netdev_put_lock(struct net_device *dev, struct net *net)
 	return dev;
 }
 
+static struct net_device *
+__netdev_put_lock_ops_compat(struct net_device *dev, struct net *net)
+{
+	netdev_lock_ops_compat(dev);
+	if (dev->reg_state > NETREG_REGISTERED ||
+	    dev->moving_ns || !net_eq(dev_net(dev), net)) {
+		netdev_unlock_ops_compat(dev);
+		dev_put(dev);
+		return NULL;
+	}
+	dev_put(dev);
+	return dev;
+}
+
 /**
  *	netdev_get_by_index_lock() - find a device by its ifindex
  *	@net: the applicable net namespace
@@ -1075,6 +1089,18 @@ struct net_device *netdev_get_by_index_lock(struct net *net, int ifindex)
 }
 
 struct net_device *
+netdev_get_by_index_lock_ops_compat(struct net *net, int ifindex)
+{
+	struct net_device *dev;
+
+	dev = dev_get_by_index(net, ifindex);
+	if (!dev)
+		return NULL;
+
+	return __netdev_put_lock_ops_compat(dev, net);
+}
+
+struct net_device *
 netdev_xa_find_lock(struct net *net, struct net_device *dev,
 		    unsigned long *index)
 {
@@ -1099,6 +1125,31 @@ netdev_xa_find_lock(struct net *net, struct net_device *dev,
 	} while (true);
 }
 
+struct net_device *
+netdev_xa_find_lock_ops_compat(struct net *net, struct net_device *dev,
+			       unsigned long *index)
+{
+	if (dev)
+		netdev_unlock_ops_compat(dev);
+
+	do {
+		rcu_read_lock();
+		dev = xa_find(&net->dev_by_index, index, ULONG_MAX, XA_PRESENT);
+		if (!dev) {
+			rcu_read_unlock();
+			return NULL;
+		}
+		dev_hold(dev);
+		rcu_read_unlock();
+
+		dev = __netdev_put_lock_ops_compat(dev, net);
+		if (dev)
+			return dev;
+
+		(*index)++;
+	} while (true);
+}
+
 static DEFINE_SEQLOCK(netdev_rename_lock);
 
 void netdev_copy_name(struct net_device *dev, char *name)