gpio: protect the descriptor label with SRCU

In order to ensure that the label is not freed while it's being
accessed, let's protect it with SRCU and synchronize it everytime it's
changed.

Let's modify desc_set_label() to manage the memory used for the label as
it can only be freed once synchronize_srcu() returns.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

1 files changed, 30 insertions, 17 deletions

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 78b9e9dc268a..c72f3ffbe6b4 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -114,12 +114,26 @@ const char *gpiod_get_label(struct gpio_desc *desc)
 	    !test_bit(FLAG_REQUESTED, &flags))
 		return "interrupt";
 
-	return test_bit(FLAG_REQUESTED, &flags) ? desc->label : NULL;
+	return test_bit(FLAG_REQUESTED, &flags) ?
+			rcu_dereference(desc->label) : NULL;
 }
 
-static inline void desc_set_label(struct gpio_desc *d, const char *label)
+static int desc_set_label(struct gpio_desc *desc, const char *label)
 {
-	d->label = label;
+	const char *new = NULL, *old;
+
+	if (label) {
+		/* FIXME: make this GFP_KERNEL once the spinlock is out. */
+		new = kstrdup_const(label, GFP_ATOMIC);
+		if (!new)
+			return -ENOMEM;
+	}
+
+	old = rcu_replace_pointer(desc->label, new, 1);
+	synchronize_srcu(&desc->srcu);
+	kfree_const(old);
+
+	return 0;
 }
 
 /**
@@ -2229,9 +2243,7 @@ static int gpiod_request_commit(struct gpio_desc *desc, const char *label)
 	 * before IRQs are enabled, for non-sleeping (SOC) GPIOs.
 	 */
 
-	if (test_and_set_bit(FLAG_REQUESTED, &desc->flags) == 0) {
-		desc_set_label(desc, label ? : "?");
-	} else {
+	if (test_and_set_bit(FLAG_REQUESTED, &desc->flags)) {
 		ret = -EBUSY;
 		goto out_free_unlock;
 	}
@@ -2259,6 +2271,13 @@ static int gpiod_request_commit(struct gpio_desc *desc, const char *label)
 		spin_lock_irqsave(&gpio_lock, flags);
 	}
 	spin_unlock_irqrestore(&gpio_lock, flags);
+
+	ret = desc_set_label(desc, label ? : "?");
+	if (ret) {
+		clear_bit(FLAG_REQUESTED, &desc->flags);
+		return ret;
+	}
+
 	return 0;
 
 out_free_unlock:
@@ -2343,8 +2362,6 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
 			gc->free(gc, gpio_chip_hwgpio(desc));
 			spin_lock_irqsave(&gpio_lock, flags);
 		}
-		kfree_const(desc->label);
-		desc_set_label(desc, NULL);
 		clear_bit(FLAG_ACTIVE_LOW, &desc->flags);
 		clear_bit(FLAG_REQUESTED, &desc->flags);
 		clear_bit(FLAG_OPEN_DRAIN, &desc->flags);
@@ -2362,6 +2379,7 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
 	}
 
 	spin_unlock_irqrestore(&gpio_lock, flags);
+	desc_set_label(desc, NULL);
 	gpiod_line_state_notify(desc, GPIOLINE_CHANGED_RELEASED);
 
 	return ret;
@@ -2409,6 +2427,8 @@ 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);
+
 	/*
 	 * FIXME: Once we mark gpiod_direction_input/output() and
 	 * gpiod_get_direction() with might_sleep(), we'll be able to protect
@@ -3520,16 +3540,8 @@ EXPORT_SYMBOL_GPL(gpiod_cansleep);
 int gpiod_set_consumer_name(struct gpio_desc *desc, const char *name)
 {
 	VALIDATE_DESC(desc);
-	if (name) {
-		name = kstrdup_const(name, GFP_KERNEL);
-		if (!name)
-			return -ENOMEM;
-	}
 
-	kfree_const(desc->label);
-	desc_set_label(desc, name);
-
-	return 0;
+	return desc_set_label(desc, name);
 }
 EXPORT_SYMBOL_GPL(gpiod_set_consumer_name);
 
@@ -4739,6 +4751,7 @@ static void gpiolib_dbg_show(struct seq_file *s, struct gpio_device *gdev)
 	int value;
 
 	for_each_gpio_desc(gc, desc) {
+		guard(srcu)(&desc->srcu);
 		if (test_bit(FLAG_REQUESTED, &desc->flags)) {
 			gpiod_get_direction(desc);
 			is_out = test_bit(FLAG_IS_OUT, &desc->flags);