staging/comedi: bug fix for module usage count on device removal

staging/comedi: bug fix for module usage count on device removal

When a dynamically created comedi device is being automatically removed
by a call to `comedi_auto_unconfig()` from the lower level driver,
`comedi_device_cleanup()` is called to perform the detachment from the
lower level driver.  If the comedi device is open at the time,
`dev->use_count` will be the the number of outstanding opens.  The
function currently decrements the the module counts of the "comedi"
module and the low-level driver module by this amount and reduces
`dev->use_count` to zero.  There are various problems with this as the
`release` file operation handler `comedi_close()` also decrements
`dev->use_count` and decrements the module usage counts.  This means
that `dev->use_count` and the module counts can end up negative.

Also, the assumed one-to-one relationship between the file open count
and the low-level module usage count is invalid and can get screwed up.
We only want to stop the low-level module being unloaded while a comedi
device using the module has an open file object.

Also, there is no need to manipulate the module count of the core
"comedi" module at all since the comedi module is the owner of the file
operations structure and the system will not unload the module while
there are open file objects using it.

Correct the bugs and simplify as follows:

1. Get rid of the module count manipulations of the core "comedi" module
(`THIS_MODULE`) altogether.

2. Don't alter `dev->use_count` in `comedi_device_cleanup()` as it
should only be altered by the `open` and `release` file operation
handlers `comedi_open()` and `comedi_close()`.

3. Increment the low-level module count for the following reasons:

  a) In `comedi_open()` if the open count was zero and the comedi device
     is attached to the low-level driver.
  b) When the `COMEDI_DEVCONFIG` ioctl is used to manually attach an
     unattached comedi device to a low-level driver.  The open count
     will be greater than zero at this time.  The actual increment of
     the low-level module count is already done by
     `comedi_device_attach()`.

4. Decrement the low-level module count for the following reasons:

  a) In `comedi_close()` if the open count was 1 and the comedi device
     is attached to the low-level driver.
  b) In `comedi_device_cleanup()` (called via `comedi_auto_unconfig()`
     --> `comedi_release_hardware_device()` -->
     `comedi_free_board_dev()` when the comedi device is automatically
     unconfigured due to action by the low-level driver) if the device
     was attached (which it should be) and open count was non-zero
     (greater than zero).
  c) When the `COMEDI_DEVCONFIG` ioctl is used to manually detach the
     comedi device from the low-level driver.  The open count will be
     greater than zero at this time.

The open count should never go negative.  Parts 3 and 4 ensure that the
low-level module usage count is incremented on entering the state where
the comedi device is attached to the low-level driver AND the open count
is greater than zero, and is decremented on leaving that state.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>