net: add netdev->up protected by netdev_lock()

Some uAPI (netdev netlink) hide net_device's sub-objects while
the interface is down to ensure uniform behavior across drivers.
To remove the rtnl_lock dependency from those uAPIs we need a way
to safely tell if the device is down or up.

Add an indication of whether device is open or closed, protected
by netdev->lock. The semantics are the same as IFF_UP, but taking
netdev_lock around every write to ->flags would be a lot of code
churn.

We don't want to blanket the entire open / close path by netdev_lock,
because it will prevent us from applying it to specific structures -
core helpers won't be able to take that lock from any function
called by the drivers on open/close paths.

So the state of the flag is "pessimistic", as in it may report false
negatives, but never false positives.

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/20250115035319.559603-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 007bcfa383c9..cac81b0a166f 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2444,11 +2444,23 @@ struct net_device {
 	u32			napi_defer_hard_irqs;
 
 	/**
+	 * @up: copy of @state's IFF_UP, but safe to read with just @lock.
+	 *	May report false negatives while the device is being opened
+	 *	or closed (@lock does not protect .ndo_open, or .ndo_close).
+	 */
+	bool			up;
+
+	/**
 	 * @lock: netdev-scope lock, protects a small selection of fields.
 	 * Should always be taken using netdev_lock() / netdev_unlock() helpers.
 	 * Drivers are free to use it for other protection.
 	 *
-	 * Protects: @reg_state, @net_shaper_hierarchy.
+	 * Protects:
+	 *	@net_shaper_hierarchy, @reg_state
+	 *
+	 * Partially protects (writers must hold both @lock and rtnl_lock):
+	 *	@up
+	 *
 	 * Ordering: take after rtnl_lock.
 	 */
 	struct mutex		lock;