staging: comedi: kcomedilib: protect against device detachment

The functions in "kcomedilib" need to prevent the comedi device being
detached during their operation.  This can be done by acquiring either
the main mutex or the "attach lock" semaphore in the `struct
comedi_device`.  Use the attach lock when merely checking whether the
device is attached.  Use the mutex when processing a comedi instruction.

Also, don't bother trying to manipulate the module use count of
low-level comedi driver in `comedi_open()` and `comedi_close()`.  If the
device gets detached while it is "open", we wouldn't be able to
decrement the module use count anyway.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/kcomedilib/kcomedilib_main.c

@@ -50,10 +50,12 @@ struct comedi_device *comedi_open(const char *filename)
 	if (!dev)
 		return NULL;
 
-	if (dev->attached && try_module_get(dev->driver->module))
+	down_read(&dev->attach_lock);
+	if (dev->attached)
 		retval = dev;
 	else
 		retval = NULL;
+	up_read(&dev->attach_lock);
 
 	if (retval == NULL)
 		comedi_dev_put(dev);
@@ -62,13 +64,9 @@ struct comedi_device *comedi_open(const char *filename)
 }
 EXPORT_SYMBOL_GPL(comedi_open);
 
-int comedi_close(struct comedi_device *d)
+int comedi_close(struct comedi_device *dev)
 {
-	struct comedi_device *dev = (struct comedi_device *)d;
-
-	module_put(dev->driver->module);
 	comedi_dev_put(dev);
-
 	return 0;
 }
 EXPORT_SYMBOL_GPL(comedi_close);
@@ -78,7 +76,14 @@ static int comedi_do_insn(struct comedi_device *dev,
 			  unsigned int *data)
 {
 	struct comedi_subdevice *s;
-	int ret = 0;
+	int ret;
+
+	mutex_lock(&dev->mutex);
+
+	if (!dev->attached) {
+		ret = -EINVAL;
+		goto error;
+	}
 
 	/* a subdevice instruction */
 	if (insn->subdev >= dev->n_subdevices) {
@@ -125,6 +130,7 @@ static int comedi_do_insn(struct comedi_device *dev,
 	s->busy = NULL;
 error:
 
+	mutex_unlock(&dev->mutex);
 	return ret;
 }
 
@@ -174,9 +180,6 @@ int comedi_dio_bitfield2(struct comedi_device *dev, unsigned int subdev,
 	unsigned int shift;
 	int ret;
 
-	if (subdev >= dev->n_subdevices)
-		return -EINVAL;
-
 	base_channel = CR_CHAN(base_channel);
 	n_chan = comedi_get_n_channels(dev, subdev);
 	if (base_channel >= n_chan)
@@ -216,23 +219,33 @@ int comedi_find_subdevice_by_type(struct comedi_device *dev, int type,
 				  unsigned int subd)
 {
 	struct comedi_subdevice *s;
-
-	if (subd > dev->n_subdevices)
-		return -ENODEV;
-
-	for (; subd < dev->n_subdevices; subd++) {
-		s = &dev->subdevices[subd];
-		if (s->type == type)
-			return subd;
-	}
-	return -1;
+	int ret = -ENODEV;
+
+	down_read(&dev->attach_lock);
+	if (dev->attached)
+		for (; subd < dev->n_subdevices; subd++) {
+			s = &dev->subdevices[subd];
+			if (s->type == type) {
+				ret = subd;
+				break;
+			}
+		}
+	up_read(&dev->attach_lock);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(comedi_find_subdevice_by_type);
 
 int comedi_get_n_channels(struct comedi_device *dev, unsigned int subdevice)
 {
-	struct comedi_subdevice *s = &dev->subdevices[subdevice];
+	int n;
+
+	down_read(&dev->attach_lock);
+	if (!dev->attached || subdevice >= dev->n_subdevices)
+		n = 0;
+	else
+		n = dev->subdevices[subdevice].n_chan;
+	up_read(&dev->attach_lock);
 
-	return s->n_chan;
+	return n;
 }
 EXPORT_SYMBOL_GPL(comedi_get_n_channels);