[PATCH] counter: interrupt-cnt: Protect enable/disable OPs with mutex

[A. Sverdlin]

From: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxxxx>

Enable/disable seems to be racy on SMP, consider the following scenario:

CPU0					CPU1

interrupt_cnt_enable_write(true)
{
	if (priv->enabled == enable)
		return 0;

	if (enable) {
		priv->enabled = true;
					interrupt_cnt_enable_write(false)
					{
						if (priv->enabled == enable)
							return 0;

						if (enable) {
							priv->enabled = true;
							enable_irq(priv->irq);
						} else {
							disable_irq(priv->irq)
							priv->enabled = false;
						}
		enable_irq(priv->irq);
	} else {
		disable_irq(priv->irq);
		priv->enabled = false;
	}

The above would result in priv->enabled == false, but IRQ left enabled.
Protect both write (above race) and read (to propagate the value on SMP)
callbacks with a mutex.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxxxx>
---
 drivers/counter/interrupt-cnt.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/counter/interrupt-cnt.c b/drivers/counter/interrupt-cnt.c
index 949598d51575a..d83848d0fe2af 100644
--- a/drivers/counter/interrupt-cnt.c
+++ b/drivers/counter/interrupt-cnt.c
@@ -3,12 +3,14 @@
  * Copyright (c) 2021 Pengutronix, Oleksij Rempel <kernel@xxxxxxxxxxxxxx>
  */
 
+#include <linux/cleanup.h>
 #include <linux/counter.h>
 #include <linux/gpio/consumer.h>
 #include <linux/interrupt.h>
 #include <linux/irq.h>
 #include <linux/mod_devicetable.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/platform_device.h>
 #include <linux/types.h>
 
@@ -19,6 +21,7 @@ struct interrupt_cnt_priv {
 	struct gpio_desc *gpio;
 	int irq;
 	bool enabled;
+	struct mutex lock;
 	struct counter_signal signals;
 	struct counter_synapse synapses;
 	struct counter_count cnts;
@@ -41,6 +44,8 @@ static int interrupt_cnt_enable_read(struct counter_device *counter,
 {
 	struct interrupt_cnt_priv *priv = counter_priv(counter);
 
+	guard(mutex)(&priv->lock);
+
 	*enable = priv->enabled;
 
 	return 0;
@@ -51,6 +56,8 @@ static int interrupt_cnt_enable_write(struct counter_device *counter,
 {
 	struct interrupt_cnt_priv *priv = counter_priv(counter);
 
+	guard(mutex)(&priv->lock);
+
 	if (priv->enabled == enable)
 		return 0;
 
@@ -227,6 +234,8 @@ static int interrupt_cnt_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
+	mutex_init(&priv->lock);
+
 	ret = devm_counter_add(dev, counter);
 	if (ret < 0)
 		return dev_err_probe(dev, ret, "Failed to add counter\n");
-- 
2.49.0