netdev - Re: [can/j1939] unregister_netdevice: waiting for vcan0 to become free. Usage count = 2

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Message-ID: <c95f5436-3e7e-43b8-820b-e380f059b9f8@I-love.SAKURA.ne.jp>
Date: Fri, 21 Nov 2025 19:19:24 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Oleksij Rempel <o.rempel@...gutronix.de>
Cc: linux-can@...r.kernel.org, Network Development <netdev@...r.kernel.org>,
        Marc Kleine-Budde <mkl@...gutronix.de>
Subject: Re: [can/j1939] unregister_netdevice: waiting for vcan0 to become
 free. Usage count = 2

On 2025/11/21 19:00, Oleksij Rempel wrote:
>> Do we want to update
>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/net/can/j1939?id=5ac798f79b48065b0284216c7a0057271185a882
>> in order to also try tracing refcount for j1939_session ?
> 
> Ack.
> 

I see.

By the way, I am thinking

diff --git a/net/can/j1939/socket.c b/net/can/j1939/socket.c
index 88e7160d4248..f12679446990 100644
--- a/net/can/j1939/socket.c
+++ b/net/can/j1939/socket.c
@@ -477,16 +477,22 @@ static int j1939_sk_bind(struct socket *sock, struct sockaddr *uaddr, int len)
 		struct net_device *ndev;
 
 		ndev = dev_get_by_index(net, addr->can_ifindex);
 		if (!ndev) {
 			ret = -ENODEV;
 			goto out_release_sock;
 		}
 
+		if (ndev->reg_state != NETREG_REGISTERED) {
+			dev_put(ndev);
+			ret = -ENODEV;
+			goto out_release_sock;
+		}
+
 		can_ml = can_get_ml_priv(ndev);
 		if (!can_ml) {
 			dev_put(ndev);
 			ret = -ENODEV;
 			goto out_release_sock;
 		}
 
 		if (!(ndev->flags & IFF_UP)) {

as an alternative approach for
https://lkml.kernel.org/r/9a3f9a95-1f58-4d67-9ab4-1ca360f86f79@I-love.SAKURA.ne.jp
because I consider that getting a new refcount on net_device should be avoided
when NETDEV_UNREGISTER event has already started.

Maybe we can do similar thing for j1939_session in order to avoid getting a new
refcount on j1939_priv when NETDEV_UNREGISTER event has already started.

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index fbf5c8001c9d..b22568fecba5 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -1492,6 +1492,8 @@ static struct j1939_session *j1939_session_new(struct j1939_priv *priv,
 	struct j1939_session *session;
 	struct j1939_sk_buff_cb *skcb;
 
+	if (priv->ndev->reg_state != NETREG_REGISTERED)
+		return NULL;
 	session = kzalloc(sizeof(*session), gfp_any());
 	if (!session)
 		return NULL;