usbnet: fix kernel crash after disconnect

[Oliver Neukum <oneukum@suse.com>]

On Fr, 2019-04-19 at 09:17 +0200, Jan Klötzke  wrote:
> 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.

Looking at dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
this indeed seems possible. But the documentation says that it is not.

Dave?

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

Jan,

could you make versio of your patch that gives a WARNing if this race
triggers?

	Regards
		Oliver