linux-kernel - [PATCH v2 2/3] gpiolib: replace the GPIO device mutex with a read-write semaphore

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Date: Tue,  2 Jan 2024 16:59:48 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: linux-gpio@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: [PATCH v2 2/3] gpiolib: replace the GPIO device mutex with a read-write semaphore

From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>

There are only two spots where we modify (add to or remove objects from)
the GPIO device list. Readers should be able to access it concurrently.
Replace the mutex with a read-write semaphore and adjust the locking
operations accordingly.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
---
 drivers/gpio/gpiolib-sysfs.c |  2 +-
 drivers/gpio/gpiolib.c       | 18 +++++++++---------
 drivers/gpio/gpiolib.h       |  2 +-
 3 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/drivers/gpio/gpiolib-sysfs.c b/drivers/gpio/gpiolib-sysfs.c
index 2d29ae37d953..4dbf298bb5dd 100644
--- a/drivers/gpio/gpiolib-sysfs.c
+++ b/drivers/gpio/gpiolib-sysfs.c
@@ -773,7 +773,7 @@ int gpiochip_sysfs_register_all(void)
 	struct gpio_device *gdev;
 	int ret;
 
-	guard(mutex)(&gpio_devices_lock);
+	guard(rwsem_read)(&gpio_devices_sem);
 
 	list_for_each_entry(gdev, &gpio_devices, list) {
 		if (gdev->mockdev)
diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index e019c4243809..4c93cf73a826 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -85,7 +85,7 @@ static DEFINE_MUTEX(gpio_lookup_lock);
 static LIST_HEAD(gpio_lookup_list);
 
 LIST_HEAD(gpio_devices);
-DEFINE_MUTEX(gpio_devices_lock);
+DECLARE_RWSEM(gpio_devices_sem);
 
 static DEFINE_MUTEX(gpio_machine_hogs_mutex);
 static LIST_HEAD(gpio_machine_hogs);
@@ -118,7 +118,7 @@ struct gpio_desc *gpio_to_desc(unsigned gpio)
 {
 	struct gpio_device *gdev;
 
-	scoped_guard(mutex, &gpio_devices_lock) {
+	scoped_guard(rwsem_read, &gpio_devices_sem) {
 		list_for_each_entry(gdev, &gpio_devices, list) {
 			if (gdev->base <= gpio &&
 			    gdev->base + gdev->ngpio > gpio)
@@ -402,7 +402,7 @@ static struct gpio_desc *gpio_name_to_desc(const char * const name)
 	if (!name)
 		return NULL;
 
-	guard(mutex)(&gpio_devices_lock);
+	guard(rwsem_read)(&gpio_devices_sem);
 
 	list_for_each_entry(gdev, &gpio_devices, list) {
 		struct gpio_desc *desc;
@@ -871,7 +871,7 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 
 	gdev->ngpio = gc->ngpio;
 
-	scoped_guard(mutex, &gpio_devices_lock) {
+	scoped_guard(rwsem_write, &gpio_devices_sem) {
 		/*
 		 * TODO: this allocates a Linux GPIO number base in the global
 		 * GPIO numberspace for this chip. In the long run we want to
@@ -1001,7 +1001,7 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 		goto err_print_message;
 	}
 err_remove_from_list:
-	scoped_guard(mutex, &gpio_devices_lock)
+	scoped_guard(rwsem_write, &gpio_devices_sem)
 		list_del(&gdev->list);
 err_free_label:
 	kfree_const(gdev->label);
@@ -1065,7 +1065,7 @@ void gpiochip_remove(struct gpio_chip *gc)
 		dev_crit(&gdev->dev,
 			 "REMOVING GPIOCHIP WITH GPIOS STILL REQUESTED\n");
 
-	scoped_guard(mutex, &gpio_devices_lock)
+	scoped_guard(rwsem_write, &gpio_devices_sem)
 		list_del(&gdev->list);
 
 	/*
@@ -1114,7 +1114,7 @@ struct gpio_device *gpio_device_find(void *data,
 	 */
 	might_sleep();
 
-	guard(mutex)(&gpio_devices_lock);
+	guard(rwsem_read)(&gpio_devices_sem);
 
 	list_for_each_entry(gdev, &gpio_devices, list) {
 		if (gdev->chip && match(gdev->chip, data))
@@ -4730,7 +4730,7 @@ static void *gpiolib_seq_start(struct seq_file *s, loff_t *pos)
 
 	s->private = "";
 
-	guard(mutex)(&gpio_devices_lock);
+	guard(rwsem_read)(&gpio_devices_sem);
 
 	list_for_each_entry(gdev, &gpio_devices, list) {
 		if (index-- == 0)
@@ -4745,7 +4745,7 @@ static void *gpiolib_seq_next(struct seq_file *s, void *v, loff_t *pos)
 	struct gpio_device *gdev = v;
 	void *ret = NULL;
 
-	scoped_guard(mutex, &gpio_devices_lock) {
+	scoped_guard(rwsem_read, &gpio_devices_sem) {
 		if (list_is_last(&gdev->list, &gpio_devices))
 			ret = NULL;
 		else
diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
index 0ce7451a6b24..97df54abf57a 100644
--- a/drivers/gpio/gpiolib.h
+++ b/drivers/gpio/gpiolib.h
@@ -137,7 +137,7 @@ int gpiod_set_transitory(struct gpio_desc *desc, bool transitory);
 
 extern spinlock_t gpio_lock;
 extern struct list_head gpio_devices;
-extern struct mutex gpio_devices_lock;
+extern struct rw_semaphore gpio_devices_sem;
 
 void gpiod_line_state_notify(struct gpio_desc *desc, unsigned long action);
 
-- 
2.40.1