staging: comedi: simplify driver module counting

For a legacy device attachment with the `COMEDI_DEVCONFIG` ioctl,
`do_devconfig_ioctl()` calls `comedi_device_attach()` to find a matching
device driver and attach the device.  It then tries to increment the
device driver's module count and if that fails it detaches the device.
So on successful attachment of a device by the `COMEDI_DEVCONFIG` ioctl,
the device driver's module count will have been incremented.

`comedi_device_attach()` is called from nowhere else.  It already
increments the device driver's module count temporarily and decrements
it again; if it gets as far as calling `comedi_device_postconfig()` the
module count is decremented within that function.

Simplify the above by removing the decrement of the device driver module
count from `comedi_device_postconfig()`.  If the call to
`comedi_device_postconfig()` succeeds, `comedi_device_attach()` will
return with the module count still incremented, otherwise decrement the
module count before returning the error.  Don't try and increment the
module count in `do_devconfig_ioctl()` after a successful return from
`comedi_device_attach()` as the module count has now already been
incremented.

`comedi_device_postconfig()` is also called by `comedi_auto_config()`
which currently has to increment the device driver's module count
temporarily so that `comedi_device_postconfig()` can decrement it, but
always returns with no overall change to the module count.  Remove all
the module count manipulations from `comedi_device_postconfig()`.  There
is no other reason for `comedi_auto_config()` to increment the device
driver's module count temporarily, since it is only called (indirectly)
from the device driver itself (usually via one of the wrappers
`comedi_pci_auto_config()` or `comedi_usb_auto_config()`).
Signed-off-by: NIan Abbott <abbotti@mev.co.uk>
Reviewed-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -520,7 +520,6 @@ static int do_devconfig_ioctl(struct comedi_device *dev,
 			      struct comedi_devconfig __user *arg)
 {
 	struct comedi_devconfig it;
-	int ret;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
@@ -551,15 +550,8 @@ static int do_devconfig_ioctl(struct comedi_device *dev,
 		/* don't re-use dynamically allocated comedi devices */
 		return -EBUSY;
 
-	ret = comedi_device_attach(dev, &it);
-	if (ret == 0) {
-		if (!try_module_get(dev->driver->module)) {
-			comedi_device_detach(dev);
-			ret = -ENOSYS;
-		}
-	}
-
-	return ret;
+	/* This increments the driver module count on success. */
+	return comedi_device_attach(dev, &it);
 }
 
 /*

drivers/staging/comedi/drivers.c

@@ -277,11 +277,11 @@ static int __comedi_device_postconfig(struct comedi_device *dev)
 }
 
 /* do a little post-config cleanup */
-/* called with module refcount incremented, decrements it */
 static int comedi_device_postconfig(struct comedi_device *dev)
 {
-	int ret = __comedi_device_postconfig(dev);
-	module_put(dev->driver->module);
+	int ret;
+
+	ret = __comedi_device_postconfig(dev);
 	if (ret < 0) {
 		__comedi_device_detach(dev);
 		return ret;
@@ -400,7 +400,11 @@ int comedi_device_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 		__comedi_device_detach(dev);
 		return ret;
 	}
-	return comedi_device_postconfig(dev);
+	ret = comedi_device_postconfig(dev);
+	if (ret < 0)
+		module_put(dev->driver->module);
+	/* On success, the driver module count has been incremented. */
+	return ret;
 }
 
 int comedi_auto_config(struct device *hardware_device,
@@ -431,19 +435,13 @@ int comedi_auto_config(struct device *hardware_device,
 		return PTR_ERR(comedi_dev);
 	/* Note: comedi_alloc_board_minor() locked comedi_dev->mutex. */
 
-	if (!try_module_get(driver->module))
-		ret = -EIO;
-	else {
-		comedi_set_hw_dev(comedi_dev, hardware_device);
-		comedi_dev->driver = driver;
-		ret = driver->auto_attach(comedi_dev, context);
-		if (ret < 0) {
-			module_put(driver->module);
-			__comedi_device_detach(comedi_dev);
-		} else {
-			ret = comedi_device_postconfig(comedi_dev);
-		}
-	}
+	comedi_set_hw_dev(comedi_dev, hardware_device);
+	comedi_dev->driver = driver;
+	ret = driver->auto_attach(comedi_dev, context);
+	if (ret < 0)
+		__comedi_device_detach(comedi_dev);
+	else
+		ret = comedi_device_postconfig(comedi_dev);
 	mutex_unlock(&comedi_dev->mutex);
 
 	if (ret < 0)