Re: open sockets preventing unregister_netdevice from completing in linux-next (next-20120724)

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2012-07-25 at 15:45 +0200, Bjørn Mork wrote:
> I am currently researching several power management regressions in
> linux-next as of next-20120724, spread over the PCI, USB and net
> subsystems. This one I believe belongs to the net subsystem, although I
> definitely may be wrong, mixing these together.
> 
> My test case is:
> 
> - open a ssh connection over a USB network device (qmi_wwan - which is why
>   I am looking at this, but I really don't think it's the driver this time)
> - suspend laptop with netdev and ssh connection up
> - attempt to resume
> 
> The USB device will be gone on resume because it is power cycled, so the
> drivers need to clean up, to let the device be rediscovered and bound
> again.  But this does not happen anymore in linux-next as long as some
> socket is open.  Instead we have these messages:
> 
>  Jul 25 15:13:11 nemi kernel: [ 7704.560306] unregister_netdevice: waiting for wwan0 to become free. Usage count = 1
>  Jul 25 15:13:21 nemi kernel: [ 7714.800308] unregister_netdevice: waiting for wwan0 to become free. Usage count = 1
>  Jul 25 15:13:31 nemi kernel: [ 7725.040316] unregister_netdevice: waiting for wwan0 to become free. Usage count = 1
> 
> 
> There are quite a few problems with the system in this state.  Any write
> to the power/control associated with that USB device will hang,
> presumably because the USB device does not exist anymore.  This will
> also make new attempts to suspend fail.  And the USB device is of course
> not functional.  The driver has not yet had a chance to clean up any of
> the other devices associated with the dead USB device (wwan1,
> /dev/cdc-wdm0 and /dev/cdc-wdm1), so these ghost devices will appear as
> non- functional.  And new devices cannot be registered until the
> previous USB device is deleted and a new one created.
> 
> Killing the ssh session let the unregister_netdevice continue and
> everything will be cleaned up and go back to normal.
> 
> This is a regression compared to 3.5, where unregister_netdevice would
> succeed regardless of any open sockets. Or maybe the sockets were
> auto-reaped?  I don't know the inner details - just observing the
> results.
> 
> The test case above is quite normal operational mode for me.  I often
> leave open sessions while suspending (because I intend to continue using
> them after resuming).  And I always forget that this won't work for the
> USB modem case.  I don't really care either.  I expect the netdev to be
> removed, routes deleted and any sockets referencing either should just
> die or live on as zombies or whatever.  The important part is that they
> should not prevent deletion of a netdev when e.g. the physical device is
> gone. That's the way things used to work.
> 
> 

Yes, we miss what was done with rt_cache_flush() : find all cached
routes and release all dev references...