xdp: double protect netdev->xdp_flags with netdev->lock

Protect xdp_features with netdev->lock. This way pure readers
no longer have to take rtnl_lock to access the field.

This includes calling NETDEV_XDP_FEAT_CHANGE under the lock.
Looks like that's fine for bonding, the only "real" listener,
it's the same as ethtool feature change.

In terms of normal drivers - only GVE need special consideration
(other drivers don't use instance lock or don't support XDP).
It calls xdp_set_features_flag() helper from gve_init_priv() which
in turn is called from gve_reset_recovery() (locked), or prior
to netdev registration. So switch to _locked.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Acked-by: Harshitha Ramamurthy <hramamurthy@google.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250408195956.412733-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

Documentation/networking/netdevices.rst

@@ -354,6 +354,7 @@ For devices with locked ops, currently only the following notifiers are
 running under the lock:
 * ``NETDEV_REGISTER``
 * ``NETDEV_UP``
+* ``NETDEV_XDP_FEAT_CHANGE``
 
 The following notifiers are running without the lock:
 * ``NETDEV_UNREGISTER``

drivers/net/ethernet/google/gve/gve_main.c

@@ -2185,7 +2185,7 @@ static void gve_set_netdev_xdp_features(struct gve_priv *priv)
 		xdp_features = 0;
 	}
 
-	xdp_set_features_flag(priv->dev, xdp_features);
+	xdp_set_features_flag_locked(priv->dev, xdp_features);
 }
 
 static int gve_init_priv(struct gve_priv *priv, bool skip_describe_device)

include/linux/netdevice.h

@@ -2526,7 +2526,7 @@ struct net_device {
 	 *	@net_shaper_hierarchy, @reg_state, @threaded
 	 *
 	 * Double protects:
-	 *	@up, @moving_ns, @nd_net
+	 *	@up, @moving_ns, @nd_net, @xdp_flags
 	 *
 	 * Double ops protects:
 	 *	@real_num_rx_queues, @real_num_tx_queues

include/net/xdp.h

@@ -616,6 +616,7 @@ struct xdp_metadata_ops {
 u32 bpf_xdp_metadata_kfunc_id(int id);
 bool bpf_dev_bound_kfunc_id(u32 btf_id);
 void xdp_set_features_flag(struct net_device *dev, xdp_features_t val);
+void xdp_set_features_flag_locked(struct net_device *dev, xdp_features_t val);
 void xdp_features_set_redirect_target(struct net_device *dev, bool support_sg);
 void xdp_features_clear_redirect_target(struct net_device *dev);
 #else

net/core/lock_debug.c

@@ -20,6 +20,7 @@ int netdev_debug_event(struct notifier_block *nb, unsigned long event,
 	switch (cmd) {
 	case NETDEV_REGISTER:
 	case NETDEV_UP:
+	case NETDEV_XDP_FEAT_CHANGE:
 		netdev_ops_assert_locked(dev);
 		fallthrough;
 	case NETDEV_DOWN:
@@ -58,7 +59,6 @@ int netdev_debug_event(struct notifier_block *nb, unsigned long event,
 	case NETDEV_OFFLOAD_XSTATS_DISABLE:
 	case NETDEV_OFFLOAD_XSTATS_REPORT_USED:
 	case NETDEV_OFFLOAD_XSTATS_REPORT_DELTA:
-	case NETDEV_XDP_FEAT_CHANGE:
 		ASSERT_RTNL();
 		break;
 

net/core/xdp.c

@@ -17,6 +17,7 @@
 #include <net/page_pool/helpers.h>
 
 #include <net/hotdata.h>
+#include <net/netdev_lock.h>
 #include <net/xdp.h>
 #include <net/xdp_priv.h> /* struct xdp_mem_allocator */
 #include <trace/events/xdp.h>
@@ -991,17 +992,26 @@ static int __init xdp_metadata_init(void)
 }
 late_initcall(xdp_metadata_init);
 
-void xdp_set_features_flag(struct net_device *dev, xdp_features_t val)
+void xdp_set_features_flag_locked(struct net_device *dev, xdp_features_t val)
 {
 	val &= NETDEV_XDP_ACT_MASK;
 	if (dev->xdp_features == val)
 		return;
 
+	netdev_assert_locked_or_invisible(dev);
 	dev->xdp_features = val;
 
 	if (dev->reg_state == NETREG_REGISTERED)
 		call_netdevice_notifiers(NETDEV_XDP_FEAT_CHANGE, dev);
 }
+EXPORT_SYMBOL_GPL(xdp_set_features_flag_locked);
+
+void xdp_set_features_flag(struct net_device *dev, xdp_features_t val)
+{
+	netdev_lock(dev);
+	xdp_set_features_flag_locked(dev, val);
+	netdev_unlock(dev);
+}
 EXPORT_SYMBOL_GPL(xdp_set_features_flag);
 
 void xdp_features_set_redirect_target(struct net_device *dev, bool support_sg)