net: protect threaded status of NAPI with netdev_lock()

Now that NAPI instances can't come and go without holding
netdev->lock we can trivially switch from rtnl_lock() to
netdev_lock() for setting netdev->threaded via sysfs.

Note that since we do not lock netdev_lock around sysfs
calls in the core we don't have to "trylock" like we do
with rtnl_lock.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250115035319.559603-9-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netdevice.h

@@ -384,7 +384,7 @@ struct napi_struct {
 	int			rx_count; /* length of rx_list */
 	unsigned int		napi_id; /* protected by netdev_lock */
 	struct hrtimer		timer;
-	struct task_struct	*thread;
+	struct task_struct	*thread; /* protected by netdev_lock */
 	unsigned long		gro_flush_timeout;
 	unsigned long		irq_suspend_timeout;
 	u32			defer_hard_irqs;
@@ -2451,11 +2451,13 @@ struct net_device {
 	 * Drivers are free to use it for other protection.
 	 *
 	 * Protects:
-	 *	@napi_list, @net_shaper_hierarchy, @reg_state
+	 *	@napi_list, @net_shaper_hierarchy, @reg_state, @threaded
 	 *
 	 * Partially protects (writers must hold both @lock and rtnl_lock):
 	 *	@up
 	 *
+	 * Also protects some fields in struct napi_struct.
+	 *
 	 * Ordering: take after rtnl_lock.
 	 */
 	struct mutex		lock;
@@ -2697,6 +2699,13 @@ static inline void netdev_assert_locked(struct net_device *dev)
 	lockdep_assert_held(&dev->lock);
 }
 
+static inline void netdev_assert_locked_or_invisible(struct net_device *dev)
+{
+	if (dev->reg_state == NETREG_REGISTERED ||
+	    dev->reg_state == NETREG_UNREGISTERING)
+		netdev_assert_locked(dev);
+}
+
 static inline void netif_napi_set_irq(struct napi_struct *napi, int irq)
 {
 	napi->irq = irq;

net/core/dev.c

@@ -6785,6 +6785,8 @@ int dev_set_threaded(struct net_device *dev, bool threaded)
 	struct napi_struct *napi;
 	int err = 0;
 
+	netdev_assert_locked_or_invisible(dev);
+
 	if (dev->threaded == threaded)
 		return 0;
 

net/core/net-sysfs.c

@@ -36,7 +36,7 @@ static const char fmt_uint[] = "%u\n";
 static const char fmt_ulong[] = "%lu\n";
 static const char fmt_u64[] = "%llu\n";
 
-/* Caller holds RTNL or RCU */
+/* Caller holds RTNL, netdev->lock or RCU */
 static inline int dev_isalive(const struct net_device *dev)
 {
 	return READ_ONCE(dev->reg_state) <= NETREG_REGISTERED;
@@ -108,6 +108,36 @@ static ssize_t netdev_store(struct device *dev, struct device_attribute *attr,
 	return ret;
 }
 
+/* Same as netdev_store() but takes netdev_lock() instead of rtnl_lock() */
+static ssize_t
+netdev_lock_store(struct device *dev, struct device_attribute *attr,
+		  const char *buf, size_t len,
+		  int (*set)(struct net_device *, unsigned long))
+{
+	struct net_device *netdev = to_net_dev(dev);
+	struct net *net = dev_net(netdev);
+	unsigned long new;
+	int ret;
+
+	if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+		return -EPERM;
+
+	ret = kstrtoul(buf, 0, &new);
+	if (ret)
+		return ret;
+
+	netdev_lock(netdev);
+
+	if (dev_isalive(netdev)) {
+		ret = (*set)(netdev, new);
+		if (ret == 0)
+			ret = len;
+	}
+	netdev_unlock(netdev);
+
+	return ret;
+}
+
 NETDEVICE_SHOW_RO(dev_id, fmt_hex);
 NETDEVICE_SHOW_RO(dev_port, fmt_dec);
 NETDEVICE_SHOW_RO(addr_assign_type, fmt_dec);
@@ -638,7 +668,7 @@ static ssize_t threaded_store(struct device *dev,
 			      struct device_attribute *attr,
 			      const char *buf, size_t len)
 {
-	return netdev_store(dev, attr, buf, len, modify_napi_threaded);
+	return netdev_lock_store(dev, attr, buf, len, modify_napi_threaded);
 }
 static DEVICE_ATTR_RW(threaded);