iio: adc: exynos: do not rely on 'users' counter in ISR

The order in which 'users' counter is decremented vs calling drivers'
close() method is implementation specific, and we should not rely on
it. Let's introduce driver private flag and use it to signal ISR
to exit when device is being closed.

This has a side-effect of fixing issue of accessing inut->users
outside of input->mutex protection.
Reported-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201006215509.GA2556081@dtor-wsSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/exynos_adc.c

@@ -7,6 +7,7 @@
  *  Copyright (C) 2013 Naveen Krishna Chatradhi <ch.naveen@samsung.com>
  */
 
+#include <linux/compiler.h>
 #include <linux/module.h>
 #include <linux/platform_device.h>
 #include <linux/interrupt.h>
@@ -135,6 +136,8 @@ struct exynos_adc {
 	u32			value;
 	unsigned int            version;
 
+	bool			ts_enabled;
+
 	bool			read_ts;
 	u32			ts_x;
 	u32			ts_y;
@@ -651,7 +654,7 @@ static irqreturn_t exynos_ts_isr(int irq, void *dev_id)
 	bool pressed;
 	int ret;
 
-	while (info->input->users) {
+	while (READ_ONCE(info->ts_enabled)) {
 		ret = exynos_read_s3c64xx_ts(dev, &x, &y);
 		if (ret == -ETIMEDOUT)
 			break;
@@ -731,6 +734,7 @@ static int exynos_adc_ts_open(struct input_dev *dev)
 {
 	struct exynos_adc *info = input_get_drvdata(dev);
 
+	WRITE_ONCE(info->ts_enabled, true);
 	enable_irq(info->tsirq);
 
 	return 0;
@@ -740,6 +744,7 @@ static void exynos_adc_ts_close(struct input_dev *dev)
 {
 	struct exynos_adc *info = input_get_drvdata(dev);
 
+	WRITE_ONCE(info->ts_enabled, false);
 	disable_irq(info->tsirq);
 }