gpiolib: fix the speed of descriptor label setting with SRCU

Commit 1f2bcb8 ("gpio: protect the descriptor label with SRCU")
caused a massive drop in performance of requesting GPIO lines due to the
call to synchronize_srcu() on each label change. Rework the code to not
wait until all read-only users are done with reading the label but
instead atomically replace the label pointer and schedule its release
after all read-only critical sections are done.

To that end wrap the descriptor label in a struct that also contains the
rcu_head struct required for deferring tasks using call_srcu() and stop
using kstrdup_const() as we're required to allocate memory anyway. Just
allocate enough for the label string and rcu_head in one go.

Reported-by: Neil Armstrong <neil.armstrong@linaro.org>
Closes: https://lore.kernel.org/linux-gpio/CAMRc=Mfig2oooDQYTqo23W3PXSdzhVO4p=G4+P8y1ppBOrkrJQ@mail.gmail.com/
Fixes: 1f2bcb8 ("gpio: protect the descriptor label with SRCU")
Suggested-by: "Paul E. McKenney" <paulmck@kernel.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8650-QRD
Acked-by: "Paul E. McKenney" <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20240507121346.16969-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Author: Bartosz Golaszewski

drivers/gpio/gpiolib.c

@@ -101,30 +101,44 @@ static bool gpiolib_initialized;
 
 const char *gpiod_get_label(struct gpio_desc *desc)
 {
+	struct gpio_desc_label *label;
 	unsigned long flags;
 
 	flags = READ_ONCE(desc->flags);
 	if (test_bit(FLAG_USED_AS_IRQ, &flags) &&
 	    !test_bit(FLAG_REQUESTED, &flags))
 		return "interrupt";
 
-	return test_bit(FLAG_REQUESTED, &flags) ?
-			srcu_dereference(desc->label, &desc->srcu) : NULL;
+	if (!test_bit(FLAG_REQUESTED, &flags))
+		return NULL;
+
+	label = srcu_dereference_check(desc->label, &desc->srcu,
+				       srcu_read_lock_held(&desc->srcu));
+
+	return label->str;
+}
+
+static void desc_free_label(struct rcu_head *rh)
+{
+	kfree(container_of(rh, struct gpio_desc_label, rh));
 }
 
 static int desc_set_label(struct gpio_desc *desc, const char *label)
 {
-	const char *new = NULL, *old;
+	struct gpio_desc_label *new = NULL, *old;
 
 	if (label) {
-		new = kstrdup_const(label, GFP_KERNEL);
+		new = kzalloc(struct_size(new, str, strlen(label) + 1),
+			      GFP_KERNEL);
 		if (!new)
 			return -ENOMEM;
+
+		strcpy(new->str, label);
 	}
 
 	old = rcu_replace_pointer(desc->label, new, 1);
-	synchronize_srcu(&desc->srcu);
-	kfree_const(old);
+	if (old)
+		call_srcu(&desc->srcu, &old->rh, desc_free_label);
 
 	return 0;
 }
@@ -697,8 +711,11 @@ 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++)
+	for (i = 0; i < gdev->ngpio; i++) {
+		/* Free pending label. */
+		synchronize_srcu(&gdev->descs[i].srcu);
 		cleanup_srcu_struct(&gdev->descs[i].srcu);
+	}
 
 	ida_free(&gpio_ida, gdev->id);
 	kfree_const(gdev->label);

drivers/gpio/gpiolib.h

@@ -137,6 +137,11 @@ int gpiod_set_transitory(struct gpio_desc *desc, bool transitory);
 
 void gpiod_line_state_notify(struct gpio_desc *desc, unsigned long action);
 
+struct gpio_desc_label {
+	struct rcu_head rh;
+	char str[];
+};
+
 /**
  * struct gpio_desc - Opaque descriptor for a GPIO
  *
@@ -177,7 +182,7 @@ struct gpio_desc {
 #define FLAG_EVENT_CLOCK_HTE		19 /* GPIO CDEV reports hardware timestamps in events */
 
 	/* Connection label */
-	const char __rcu	*label;
+	struct gpio_desc_label __rcu *label;
 	/* Name of the GPIO */
 	const char		*name;
 #ifdef CONFIG_OF_DYNAMIC