net: protect NAPI config fields with netdev_lock()

Protect the following members of netdev and napi by netdev_lock:
 - defer_hard_irqs,
 - gro_flush_timeout,
 - irq_suspend_timeout.

The first two are written via sysfs (which this patch switches
to new lock), and netdev genl which holds both netdev and rtnl locks.

irq_suspend_timeout is only written by netdev genl.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250115035319.559603-11-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

include/linux/netdevice.h

@@ -384,11 +384,11 @@ 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; /* protected by netdev_lock */
+	/* all fields past this point are write-protected by netdev_lock */
+	struct task_struct	*thread;
 	unsigned long		gro_flush_timeout;
 	unsigned long		irq_suspend_timeout;
 	u32			defer_hard_irqs;
-	/* all fields past this point are write-protected by netdev_lock */
 	/* control-path-only fields follow */
 	struct list_head	dev_list;
 	struct hlist_node	napi_hash_node;
@@ -2452,7 +2452,8 @@ struct net_device {
 	 * Drivers are free to use it for other protection.
 	 *
 	 * Protects:
-	 *	@napi_list, @net_shaper_hierarchy, @reg_state, @threaded
+	 *	@gro_flush_timeout, @napi_defer_hard_irqs, @napi_list,
+	 *	@net_shaper_hierarchy, @reg_state, @threaded
 	 *
 	 * Partially protects (writers must hold both @lock and rtnl_lock):
 	 *	@up

net/core/net-sysfs.c

@@ -450,7 +450,7 @@ static ssize_t gro_flush_timeout_store(struct device *dev,
 	if (!capable(CAP_NET_ADMIN))
 		return -EPERM;
 
-	return netdev_store(dev, attr, buf, len, change_gro_flush_timeout);
+	return netdev_lock_store(dev, attr, buf, len, change_gro_flush_timeout);
 }
 NETDEVICE_SHOW_RW(gro_flush_timeout, fmt_ulong);
 
@@ -470,7 +470,8 @@ static ssize_t napi_defer_hard_irqs_store(struct device *dev,
 	if (!capable(CAP_NET_ADMIN))
 		return -EPERM;
 
-	return netdev_store(dev, attr, buf, len, change_napi_defer_hard_irqs);
+	return netdev_lock_store(dev, attr, buf, len,
+				 change_napi_defer_hard_irqs);
 }
 NETDEVICE_SHOW_RW(napi_defer_hard_irqs, fmt_uint);