usbnet: fix kernel crash after disconnect

["Jan Klötzke" <jan@kloetzke.net>]

Hi David,

On Thu, Apr 18, 2019 at 04:35:44PM -0700, David Miller wrote:
> From: Kloetzke Jan <Jan.Kloetzke@preh.de>
> Date: Thu, 18 Apr 2019 08:02:59 +0000
> 
> > I think this assumption is not correct. As far as I understand the
> > networking code it is still possible that the ndo_start_xmit callback
> > is called while ndo_stop is running and even after ndo_stop has
> > returned. You can only be sure after unregister_netdev() has returned.
> > Maybe some networking folks can comment on that.
> 
> The kernel loops over the devices being unregistered, and first it clears
> the __LINK_STATE_START on all of them, then it invokes ->ndo_stop() on
> all of them.
> 
> __LINK_STATE_START controls what netif_running() returns.
> 
> All calls to ->ndo_start_xmit() are guarded by netif_running() checks.
> 
> So when ndo_stop is invoked you should get no more ndo_start_xmit
> invocations on that device.  Otherwise how could you shut down DMA
> resources and turn off the TX engine properly?

But you could still race with another CPU that is past the
netif_running() check, can you? So the driver has to make sure that it
gracefully handles concurrent ->ndo_start_xmit() and ->ndo_stop() calls.

Or are there any locks/barriers involved that make sure all
->ndo_start_xmit() calls have returned before invoking ->ndo_stop()?

Regards,
Jan