[PATCH v2] Bluetooth: 6lowpan: fix cyclic locking warning on netdev unregister

[PATCH v2] Bluetooth: 6lowpan: fix cyclic locking warning on netdev unregister

[Pauli Virtanen]

6lowpan.c has theoretically conflicting lock orderings, which lockdep
complains about:

    a) rtnl_lock > hdev->workqueue

    from 6lowpan.c:delete_netdev -> rtnl_lock -> device_del
    -> put_device(parent) -> hci_release_dev -> destroy_workqueue

    b) hdev->workqueue > l2cap_conn->lock > chan->lock > rtnl_lock

    from hci_rx_work -> 6lowpan.c:chan_ready_cb
    -> lowpan_register_netdev, ifup -> rtnl_lock

Actual deadlock appears not possible, as hci_rx_work is disabled and
l2cap_conn flushed already on hdev unregister. Hence, do minimal thing
to make lockdep happy by breaking chain a) by holding hdev refcount
until after netdev put in 6lowpan.c.

Fixes the lockdep complaint:
WARNING: possible circular locking dependency detected.
kworker/0:1/11 is trying to acquire lock:
ffff8880023b3940 ((wq_completion)hci0#2){+.+.}-{0:0}, at: touch_wq_lockdep_map+0x8b/0x130
but task is already holding lock:
ffffffff95e4f9c0 (rtnl_mutex){+.+.}-{4:4}, at: lowpan_unregister_netdev+0xd/0x30
Workqueue: events delete_netdev

Signed-off-by: Pauli Virtanen <pav@iki.fi>
---

Notes:
    v2: initialize new_dev->hdev in disconnect_devices()
    
    Not very pretty, however breaking the theoretical cyclic locking chain
    in a proper way is more complicated as for b) we need to handle
    l2cap_chan going away in the middle of netdev setup, and for a) removing
    hdev as parent device maybe has some side effects.

 net/bluetooth/6lowpan.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/net/bluetooth/6lowpan.c b/net/bluetooth/6lowpan.c
index 2f03b780b40d..23a229ab6a33 100644
--- a/net/bluetooth/6lowpan.c
+++ b/net/bluetooth/6lowpan.c
@@ -758,13 +758,33 @@ static inline struct l2cap_chan *chan_new_conn_cb(struct l2cap_chan *pchan)
 	return chan;
 }
 
+static void unregister_dev(struct lowpan_btle_dev *dev)
+{
+	struct hci_dev *hdev = READ_ONCE(dev->hdev);
+
+	/* If netdev holds last reference to hci_dev (its parent device), this
+	 * leads to theoretical cyclic locking on lowpan_unregister_netdev:
+	 *
+	 * rtnl_lock -> put_device(parent) -> hci_release_dev ->
+	 * destroy_workqueue -> hci_rx_work -> l2cap_recv_acldata ->
+	 * chan_ready_cb -> ifup -> rtnl_lock
+	 *
+	 * However, hci_rx_work is disabled in hci_unregister_dev, so this
+	 * should not occur. Make lockdep happy by postponing hdev release after
+	 * netdev put.
+	 */
+	hci_dev_hold(hdev);
+	lowpan_unregister_netdev(dev->netdev);
+	hci_dev_put(hdev);
+}
+
 static void delete_netdev(struct work_struct *work)
 {
 	struct lowpan_btle_dev *entry = container_of(work,
 						     struct lowpan_btle_dev,
 						     delete_netdev);
 
-	lowpan_unregister_netdev(entry->netdev);
+	unregister_dev(entry);
 
 	/* The entry pointer is deleted by the netdev destructor. */
 }
@@ -1250,6 +1270,7 @@ static void disconnect_devices(void)
 			break;
 
 		new_dev->netdev = entry->netdev;
+		new_dev->hdev = entry->hdev;
 		INIT_LIST_HEAD(&new_dev->list);
 
 		list_add_rcu(&new_dev->list, &devices);
@@ -1261,7 +1282,7 @@ static void disconnect_devices(void)
 		ifdown(entry->netdev);
 		BT_DBG("Unregistering netdev %s %p",
 		       entry->netdev->name, entry->netdev);
-		lowpan_unregister_netdev(entry->netdev);
+		unregister_dev(entry);
 		kfree(entry);
 	}
 }
-- 
2.53.0

Re: [PATCH v2] Bluetooth: 6lowpan: fix cyclic locking warning on netdev unregister

[patchwork-bot+bluetooth]

Hello:

This patch was applied to bluetooth/bluetooth-next.git (master)
by Luiz Augusto von Dentz <luiz.von.dentz@intel.com>:

On Sat, 11 Apr 2026 21:15:09 +0300 you wrote:
> 6lowpan.c has theoretically conflicting lock orderings, which lockdep
> complains about:
> 
>     a) rtnl_lock > hdev->workqueue
> 
>     from 6lowpan.c:delete_netdev -> rtnl_lock -> device_del
>     -> put_device(parent) -> hci_release_dev -> destroy_workqueue
> 
> [...]

Here is the summary with links:
  - [v2] Bluetooth: 6lowpan: fix cyclic locking warning on netdev unregister
    https://git.kernel.org/bluetooth/bluetooth-next/c/87151e363b7e

You are awesome, thank you!
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