Re: [BUG] cdc_ncm: soft lockup with netconsole over an AX88179B dock

[Nicolai Buchwitz <nb@tipi-net.de>]

Hi Mikkel

On 19.10.2026 15:48, Mikkel Eskildsen wrote:
> Hi,
> 
> My laptop panicked with a soft lockup on 2026-10-16 and again on 
> 2026-10-19 while netconsole was running over a USB dock NIC that uses 
> cdc_ncm. Both times the laptop was docked and netconsole was on that 
> interface. I have turned netconsole off for now.

I was able to reproduce the issue with a similar device.

Seems like netconsole holds the tx lock while the cdc_ncm driver
lets pending network work run in the middle of the send.
... which then starves on the lock netconsole still holds.

Below is a patch which hopefully fixes the issue.

> 
> The trace below is from pstore. The netconsole thread on CPU15 is in 
> netpoll_send_skb -> usbnet_start_xmit -> cdc_ncm_tx_fixup. The 
> spin_unlock_bh() in cdc_ncm_tx_fixup runs pending softirqs, TCP RX 
> sends a packet and it spins in sch_direct_xmit. My guess is that it 
> waits for the eth0 tx queue lock that netpoll already holds. The other 
> CPUs then wait on the socket lock until the watchdog panics.
> 
> The first panic looks the same, but pstore lost the part with the 
> netconsole thread.
> 

> [...]

> I can test a patch if that helps. Is this a known issue?

https://github.com/nbuchwitz/linux/commit/319cb0933e2eaf984b98906ebddb55befb4e2996

> 
> Mikkel