staging: comedi: add comedi_clear_board_dev()

Add local function `comedi_clear_board_dev()` as a safer alternative to
`comedi_clear_board_minor()` when we already have a pointer to a `struct
comedi_device`.  It uses the board minor device number stored in the
`struct comedi_device` (which must have already been initialized) and
only clears the entry in `comedi_board_minor_table[]` if it points to
the specified `struct comedi_device`.  Rather than returning the old
table entry, it returns `true` if the entry matched (and so has just
been cleared) and returns `false` otherwise.

Call `comedi_clear_board_dev()` instead of `comedi_clear_board_minor()`
in `comedi_unlocked_ioctl()` (in the code that frees a dynamically
allocated comedi device detached by the `COMEDI_DEVCONFIG` ioctl).  That
ought to return `true` but check it just in case before freeing the
device.  There is still a race condition here which needs to be dealt
with once we've implemented reference counting for `struct
comedi_device`s.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index b5e4bf1..dec04b9 100644 (file)
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -120,6 +120,20 @@ static void comedi_device_cleanup(struct comedi_device *dev)
        mutex_destroy(&dev->mutex);
 }
 
+static bool comedi_clear_board_dev(struct comedi_device *dev)
+{
+       unsigned int i = dev->minor;
+       bool cleared = false;
+
+       mutex_lock(&comedi_board_minor_table_lock);
+       if (dev == comedi_board_minor_table[i]) {
+               comedi_board_minor_table[i] = NULL;
+               cleared = true;
+       }
+       mutex_unlock(&comedi_board_minor_table_lock);
+       return cleared;
+}
+
 static struct comedi_device *comedi_clear_board_minor(unsigned minor)
 {
        struct comedi_device *dev;
@@ -1766,9 +1780,7 @@ static long comedi_unlocked_ioctl(struct file *file, unsigned int cmd,
                            dev->minor >= comedi_num_legacy_minors) {
                                /* Successfully unconfigured a dynamically
                                 * allocated device.  Try and remove it. */
-                               struct comedi_device *devr;
-                               devr = comedi_clear_board_minor(dev->minor);
-                               if (dev == devr) {
+                               if (comedi_clear_board_dev(dev)) {
                                        mutex_unlock(&dev->mutex);
                                        comedi_free_board_dev(dev);
                                        return rc;