gpiolib: use a single SRCU struct for all GPIO descriptors

We used a per-descriptor SRCU struct in order to not impose a wait with
synchronize_srcu() for descriptor X on read-only operations of
descriptor Y. Now that we no longer call synchronize_srcu() on
descriptor label change but only when releasing descriptor resources, we
can use a single SRCU structure for all GPIO descriptors in a given chip.

Suggested-by: "Paul E. McKenney" <paulmck@kernel.org>
Acked-by: "Paul E. McKenney" <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20240507172414.28513-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Author: Bartosz Golaszewski

drivers/gpio/gpiolib-cdev.c

@@ -2351,7 +2351,7 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
 
 	dflags = READ_ONCE(desc->flags);
 
-	scoped_guard(srcu, &desc->srcu) {
+	scoped_guard(srcu, &desc->gdev->desc_srcu) {
 		label = gpiod_get_label(desc);
 		if (label && test_bit(FLAG_REQUESTED, &dflags))
 			strscpy(info->consumer, label,

drivers/gpio/gpiolib.c

@@ -112,8 +112,8 @@ const char *gpiod_get_label(struct gpio_desc *desc)
 	if (!test_bit(FLAG_REQUESTED, &flags))
 		return NULL;
 
-	label = srcu_dereference_check(desc->label, &desc->srcu,
-				       srcu_read_lock_held(&desc->srcu));
+	label = srcu_dereference_check(desc->label, &desc->gdev->desc_srcu,
+				srcu_read_lock_held(&desc->gdev->desc_srcu));
 
 	return label->str;
 }
@@ -138,7 +138,7 @@ static int desc_set_label(struct gpio_desc *desc, const char *label)
 
 	old = rcu_replace_pointer(desc->label, new, 1);
 	if (old)
-		call_srcu(&desc->srcu, &old->rh, desc_free_label);
+		call_srcu(&desc->gdev->desc_srcu, &old->rh, desc_free_label);
 
 	return 0;
 }
@@ -709,13 +709,10 @@ EXPORT_SYMBOL_GPL(gpiochip_line_is_valid);
 static void gpiodev_release(struct device *dev)
 {
 	struct gpio_device *gdev = to_gpio_device(dev);
-	unsigned int i;
 
-	for (i = 0; i < gdev->ngpio; i++) {
-		/* Free pending label. */
-		synchronize_srcu(&gdev->descs[i].srcu);
-		cleanup_srcu_struct(&gdev->descs[i].srcu);
-	}
+	/* Call pending kfree()s for descriptor labels. */
+	synchronize_srcu(&gdev->desc_srcu);
+	cleanup_srcu_struct(&gdev->desc_srcu);
 
 	ida_free(&gpio_ida, gdev->id);
 	kfree_const(gdev->label);
@@ -992,30 +989,30 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 	if (ret)
 		goto err_remove_from_list;
 
+	ret = init_srcu_struct(&gdev->desc_srcu);
+	if (ret)
+		goto err_cleanup_gdev_srcu;
+
 #ifdef CONFIG_PINCTRL
 	INIT_LIST_HEAD(&gdev->pin_ranges);
 #endif
 
 	if (gc->names) {
 		ret = gpiochip_set_desc_names(gc);
 		if (ret)
-			goto err_cleanup_gdev_srcu;
+			goto err_cleanup_desc_srcu;
 	}
 	ret = gpiochip_set_names(gc);
 	if (ret)
-		goto err_cleanup_gdev_srcu;
+		goto err_cleanup_desc_srcu;
 
 	ret = gpiochip_init_valid_mask(gc);
 	if (ret)
-		goto err_cleanup_gdev_srcu;
+		goto err_cleanup_desc_srcu;
 
 	for (desc_index = 0; desc_index < gc->ngpio; desc_index++) {
 		struct gpio_desc *desc = &gdev->descs[desc_index];
 
-		ret = init_srcu_struct(&desc->srcu);
-		if (ret)
-			goto err_cleanup_desc_srcu;
-
 		if (gc->get_direction && gpiochip_line_is_valid(gc, desc_index)) {
 			assign_bit(FLAG_IS_OUT,
 				   &desc->flags, !gc->get_direction(gc, desc_index));
@@ -1027,7 +1024,7 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 
 	ret = of_gpiochip_add(gc);
 	if (ret)
-		goto err_cleanup_desc_srcu;
+		goto err_free_valid_mask;
 
 	ret = gpiochip_add_pin_ranges(gc);
 	if (ret)
@@ -1074,10 +1071,10 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 	gpiochip_remove_pin_ranges(gc);
 err_remove_of_chip:
 	of_gpiochip_remove(gc);
-err_cleanup_desc_srcu:
-	while (desc_index--)
-		cleanup_srcu_struct(&gdev->descs[desc_index].srcu);
+err_free_valid_mask:
 	gpiochip_free_valid_mask(gc);
+err_cleanup_desc_srcu:
+	cleanup_srcu_struct(&gdev->desc_srcu);
 err_cleanup_gdev_srcu:
 	cleanup_srcu_struct(&gdev->srcu);
 err_remove_from_list:
@@ -2407,7 +2404,7 @@ char *gpiochip_dup_line_label(struct gpio_chip *gc, unsigned int offset)
 	if (!test_bit(FLAG_REQUESTED, &desc->flags))
 		return NULL;
 
-	guard(srcu)(&desc->srcu);
+	guard(srcu)(&desc->gdev->desc_srcu);
 
 	label = kstrdup(gpiod_get_label(desc), GFP_KERNEL);
 	if (!label)
@@ -4798,7 +4795,7 @@ static void gpiolib_dbg_show(struct seq_file *s, struct gpio_device *gdev)
 	}
 
 	for_each_gpio_desc(gc, desc) {
-		guard(srcu)(&desc->srcu);
+		guard(srcu)(&desc->gdev->desc_srcu);
 		if (test_bit(FLAG_REQUESTED, &desc->flags)) {
 			gpiod_get_direction(desc);
 			is_out = test_bit(FLAG_IS_OUT, &desc->flags);

drivers/gpio/gpiolib.h

@@ -31,6 +31,7 @@
  * @chip: pointer to the corresponding gpiochip, holding static
  * data for this device
  * @descs: array of ngpio descriptors.
+ * @desc_srcu: ensures consistent state of GPIO descriptors exposed to users
  * @ngpio: the number of GPIO lines on this GPIO device, equal to the size
  * of the @descs array.
  * @can_sleep: indicate whether the GPIO chip driver's callbacks can sleep
@@ -61,6 +62,7 @@ struct gpio_device {
 	struct module		*owner;
 	struct gpio_chip __rcu	*chip;
 	struct gpio_desc	*descs;
+	struct srcu_struct	desc_srcu;
 	int			base;
 	u16			ngpio;
 	bool			can_sleep;
@@ -150,7 +152,6 @@ struct gpio_desc_label {
  * @label:		Name of the consumer
  * @name:		Line name
  * @hog:		Pointer to the device node that hogs this line (if any)
- * @srcu:		SRCU struct protecting the label pointer.
  *
  * These are obtained using gpiod_get() and are preferable to the old
  * integer-based handles.
@@ -188,7 +189,6 @@ struct gpio_desc {
 #ifdef CONFIG_OF_DYNAMIC
 	struct device_node	*hog;
 #endif
-	struct srcu_struct	srcu;
 };
 
 #define gpiod_not_found(desc)		(IS_ERR(desc) && PTR_ERR(desc) == -ENOENT)
@@ -256,23 +256,23 @@ static inline int gpio_chip_hwgpio(const struct gpio_desc *desc)
 
 #define gpiod_err(desc, fmt, ...) \
 do { \
-	scoped_guard(srcu, &desc->srcu) { \
+	scoped_guard(srcu, &desc->gdev->desc_srcu) { \
 		pr_err("gpio-%d (%s): " fmt, desc_to_gpio(desc), \
 		       gpiod_get_label(desc) ? : "?", ##__VA_ARGS__); \
 	} \
 } while (0)
 
 #define gpiod_warn(desc, fmt, ...) \
 do { \
-	scoped_guard(srcu, &desc->srcu) { \
+	scoped_guard(srcu, &desc->gdev->desc_srcu) { \
 		pr_warn("gpio-%d (%s): " fmt, desc_to_gpio(desc), \
 			gpiod_get_label(desc) ? : "?", ##__VA_ARGS__); \
 	} \
 } while (0)
 
 #define gpiod_dbg(desc, fmt, ...) \
 do { \
-	scoped_guard(srcu, &desc->srcu) { \
+	scoped_guard(srcu, &desc->gdev->desc_srcu) { \
 		pr_debug("gpio-%d (%s): " fmt, desc_to_gpio(desc), \
 			 gpiod_get_label(desc) ? : "?", ##__VA_ARGS__); \
 	} \