staging:iio:core set the iio_dev.info pointer to null on unregister under lock.

This prevents use of provider callbacks after it has been unregistered.
Note that all code using this that can be called from a consumer *must*
check the pointer before using and hold the info_exist_lock throughout
the usage of the callbacks in info.
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/iio/iio.h

@@ -310,6 +310,7 @@ struct iio_buffer_setup_ops {
  * @chan_attr_group:	[INTERN] group for all attrs in base directory
  * @name:		[DRIVER] name of the device.
  * @info:		[DRIVER] callbacks and constant info from driver
+ * @info_exist_lock:	[INTERN] lock to prevent use during removal
  * @chrdev:		[INTERN] associated character device
  * @groups:		[INTERN] attribute groups
  * @groupcounter:	[INTERN] index of next attribute group
@@ -340,6 +341,7 @@ struct iio_dev {
 	struct attribute_group		chan_attr_group;
 	const char			*name;
 	const struct iio_info		*info;
+	struct mutex			info_exist_lock;
 	const struct iio_buffer_setup_ops	*setup_ops;
 	struct cdev			chrdev;
 #define IIO_MAX_GROUPS 6

drivers/staging/iio/industrialio-core.c

@@ -594,6 +594,7 @@ struct iio_dev *iio_allocate_device(int sizeof_priv)
 		device_initialize(&dev->dev);
 		dev_set_drvdata(&dev->dev, (void *)dev);
 		mutex_init(&dev->mlock);
+		mutex_init(&dev->info_exist_lock);
 
 		dev->id = ida_simple_get(&iio_ida, 0, 0, GFP_KERNEL);
 		if (dev->id < 0) {
@@ -718,6 +719,9 @@ EXPORT_SYMBOL(iio_device_register);
 
 void iio_device_unregister(struct iio_dev *indio_dev)
 {
+	mutex_lock(&indio_dev->info_exist_lock);
+	indio_dev->info = NULL;
+	mutex_unlock(&indio_dev->info_exist_lock);
 	device_unregister(&indio_dev->dev);
 }
 EXPORT_SYMBOL(iio_device_unregister);