iio: adc: ina2xx: avoid kthread_stop() with stale task_struct

When the buffer is enabled for ina2xx driver, a dedicated kthread is
invoked to capture mesurement data.  When the buffer is disabled, the
kthread is stopped.

However if the kthread gets register access errors, it immediately exits
and when the malfunctional buffer is disabled, the stale task_struct
pointer is accessed as there is no kthread to be stopped.

A similar issue in the usbip driver is prevented by kthread_get_run and
kthread_stop_put helpers by increasing usage count of the task_struct.
This change applies the same solution.

Cc: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Fixes: c43a102e ("iio: ina2xx: add support for TI INA2xx Power Monitors")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ina2xx-adc.c

@@ -30,6 +30,7 @@
 #include <linux/module.h>
 #include <linux/of_device.h>
 #include <linux/regmap.h>
+#include <linux/sched/task.h>
 #include <linux/util_macros.h>
 
 #include <linux/platform_data/ina2xx.h>
@@ -826,6 +827,7 @@ static int ina2xx_buffer_enable(struct iio_dev *indio_dev)
 {
 	struct ina2xx_chip_info *chip = iio_priv(indio_dev);
 	unsigned int sampling_us = SAMPLING_PERIOD(chip);
+	struct task_struct *task;
 
 	dev_dbg(&indio_dev->dev, "Enabling buffer w/ scan_mask %02x, freq = %d, avg =%u\n",
 		(unsigned int)(*indio_dev->active_scan_mask),
@@ -835,11 +837,17 @@ static int ina2xx_buffer_enable(struct iio_dev *indio_dev)
 	dev_dbg(&indio_dev->dev, "Async readout mode: %d\n",
 		chip->allow_async_readout);
 
-	chip->task = kthread_run(ina2xx_capture_thread, (void *)indio_dev,
-				 "%s:%d-%uus", indio_dev->name, indio_dev->id,
-				 sampling_us);
+	task = kthread_create(ina2xx_capture_thread, (void *)indio_dev,
+			      "%s:%d-%uus", indio_dev->name, indio_dev->id,
+			      sampling_us);
+	if (IS_ERR(task))
+		return PTR_ERR(task);
+
+	get_task_struct(task);
+	wake_up_process(task);
+	chip->task = task;
 
-	return PTR_ERR_OR_ZERO(chip->task);
+	return 0;
 }
 
 static int ina2xx_buffer_disable(struct iio_dev *indio_dev)
@@ -848,6 +856,7 @@ static int ina2xx_buffer_disable(struct iio_dev *indio_dev)
 
 	if (chip->task) {
 		kthread_stop(chip->task);
+		put_task_struct(chip->task);
 		chip->task = NULL;
 	}