staging:iio: Make sure a device is only opened once at a time

Our buffer implementation does not support multiple concurrent readers. So we
have to ensure that a device is only opened once at a time. So do the same thing
we do for the event fd and introduce a per device busy flag. The flag gets set
when opening the device and gets cleared when closing the device. If a open is
attempted while the busy flag is set we return -EBUSY.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/iio.h

@@ -313,6 +313,7 @@ struct iio_buffer_setup_ops {
  * @chrdev:		[INTERN] associated character device
  * @groups:		[INTERN] attribute groups
  * @groupcounter:	[INTERN] index of next attribute group
+ * @flags:		[INTERN] file ops related flags including busy flag.
  **/
 struct iio_dev {
 	int				id;
@@ -344,6 +345,8 @@ struct iio_dev {
 #define IIO_MAX_GROUPS 6
 	const struct attribute_group	*groups[IIO_MAX_GROUPS + 1];
 	int				groupcounter;
+
+	unsigned long			flags;
 };
 
 /**

drivers/staging/iio/industrialio-core.c

@@ -1083,9 +1083,18 @@ static int iio_chrdev_open(struct inode *inode, struct file *filp)
 {
 	struct iio_dev *indio_dev = container_of(inode->i_cdev,
 						struct iio_dev, chrdev);
+	unsigned int ret;
+
+	if (test_and_set_bit(IIO_BUSY_BIT_POS, &indio_dev->flags))
+		return -EBUSY;
+
 	filp->private_data = indio_dev;
 
-	return iio_chrdev_buffer_open(indio_dev);
+	ret = iio_chrdev_buffer_open(indio_dev);
+	if (ret < 0)
+		clear_bit(IIO_BUSY_BIT_POS, &indio_dev->flags);
+
+	return ret;
 }
 
 /**
@@ -1093,8 +1102,10 @@ static int iio_chrdev_open(struct inode *inode, struct file *filp)
  **/
 static int iio_chrdev_release(struct inode *inode, struct file *filp)
 {
-	iio_chrdev_buffer_release(container_of(inode->i_cdev,
-					     struct iio_dev, chrdev));
+	struct iio_dev *indio_dev = container_of(inode->i_cdev,
+						struct iio_dev, chrdev);
+	iio_chrdev_buffer_release(indio_dev);
+	clear_bit(IIO_BUSY_BIT_POS, &indio_dev->flags);
 	return 0;
 }