net: no longer acquire RTNL in threaded_show()

dev->threaded can be read locklessly, if we add
corresponding READ_ONCE()/WRITE_ONCE() annotations.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240502173926.2010646-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netdevice.h

@@ -2370,8 +2370,8 @@ struct net_device {
 	struct sfp_bus		*sfp_bus;
 	struct lock_class_key	*qdisc_tx_busylock;
 	bool			proto_down;
+	bool			threaded;
 	unsigned		wol_enabled:1;
-	unsigned		threaded:1;
 
 	struct list_head	net_notifier_list;
 

net/core/dev.c

@@ -6531,7 +6531,7 @@ int dev_set_threaded(struct net_device *dev, bool threaded)
 		}
 	}
 
-	dev->threaded = threaded;
+	WRITE_ONCE(dev->threaded, threaded);
 
 	/* Make sure kthread is created before THREADED bit
 	 * is set.
@@ -6622,7 +6622,7 @@ void netif_napi_add_weight(struct net_device *dev, struct napi_struct *napi,
 	 * threaded mode will not be enabled in napi_enable().
 	 */
 	if (dev->threaded && napi_kthread_create(napi))
-		dev->threaded = 0;
+		dev->threaded = false;
 	netif_napi_set_irq(napi, -1);
 }
 EXPORT_SYMBOL(netif_napi_add_weight);

net/core/net-sysfs.c

@@ -605,13 +605,13 @@ static ssize_t threaded_show(struct device *dev,
 	struct net_device *netdev = to_net_dev(dev);
 	ssize_t ret = -EINVAL;
 
-	if (!rtnl_trylock())
-		return restart_syscall();
+	rcu_read_lock();
 
 	if (dev_isalive(netdev))
-		ret = sysfs_emit(buf, fmt_dec, netdev->threaded);
+		ret = sysfs_emit(buf, fmt_dec, READ_ONCE(netdev->threaded));
+
+	rcu_read_unlock();
 
-	rtnl_unlock();
 	return ret;
 }