From: Andries Brouwer <aebr@win.tue.nl>

I see an Oops in the SCSI code, caused by the fact that sdkp is NULL
in sd_shutdown. "How can that be?", you will ask - dev->driver_data was set
in sd_probe. But in my case sd_probe never finished. An insmod usb-storage
hangs forever, or at least for more than six hours, giving ample opportunity
to observe this race between sd_probe and sd_shutdown.
(Of course sd_probe hangs in sd_revalidate disk.)

Perhaps the obvious test is a good idea.
Locking seems meaningless - sd_probe will never finish.