Unbinding an I/O subchannel with a child-CCW device in disconnected
state sometimes causes a kernel-panic. The race condition was seen
mostly during testing, when setting all the CHPIDs of a device to
offline and at the same time, the unbinding the I/O subchannel driver.

The kernel-panic occurs because of double delete, the I/O subchannel
driver calls device_del on the CCW device while another device_del
invocation for the same device is in-flight.  For instance, disabling
all the CHPIDs will trigger the ccw_device_remove function, which will
call a ccw_device_unregister(), which ends up calling the device_del()
which is asynchronous via cdev's todo workqueue. And unbinding the I/O
subchannel driver calls io_subchannel_remove() function which calls the
ccw_device_unregister() and device_del().

This double delete can be prevented by serializing all CCW device
registration/unregistration calls into the driver core. This patch
introduces a mutex which will be used for this purpose.
Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reported-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>