Re: [PATCH] ethtool: don't touch the parent device of a net device being unregistered

[Alexander Popov]

On 3/22/26 17:39, Andrew Lunn wrote:
> On Sun, Mar 22, 2026 at 10:59:15AM +0300, Alexander Popov wrote:
> > The ethtool operations may be performed on a net device that is currently
> > being unregistered. This is also described in the commit dde91ccfa25fd58f
> > ("ethtool: do not perform operations on net devices being unregistered").
> > Moreover, such a device may have a parent that has already been freed.
>
> I don't know the device core too well, but can you explain in the
> example stack trace you gave how the parent can already be free? I
> would of expected devices are arranged in a tree, and free happens
> from the leaves towards the root? In order to do a clean shutdown, you
> might need to perform operations on the parent.
>
> I just want to make sure this is not a broken parent device which
> should be fixed...

Hello Andrew, let me describe the scenario that I see:

 - The netdev_run_todo() function handles the net devices in net_todo_list
   in a loop and moves each of them into the NETREG_UNREGISTERED state:
     netdev_lock(dev);
     WRITE_ONCE(dev->reg_state, NETREG_UNREGISTERED);
     netdev_unlock(dev);

 - Then netdev_run_todo() frees these net devices in another loop.
   On each iteration, it chooses a device for freeing:
     dev = netdev_wait_allrefs_any(&list);

 - At the same time, the ethnl_set_features() function calls
   ethnl_parse_header_dev_get() for the child net device.

 - If the race condition succeeds, ethnl_set_features() takes the reference
   to the child net device being unregistered. That makes netdev_run_todo()
   free the parent first.

Best regards,
Alexander