[PATCH 01/22] gpio: protect the list of GPIO devices with SRCU

[Bartosz Golaszewski <brgl@xxxxxxxx>]

From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>

We're working towards removing the "multi-function" GPIO spinlock that's
implemented terribly wrong. We tried using an RW-semaphore to protect
the list of GPIO devices but it turned out that we still have old code
using legacy GPIO calls that need to translate the global GPIO number to
the address of the associated descriptor and - to that end - traverse
the list while holding the lock. If we change the spinlock to a sleeping
lock then we'll end up with "scheduling while atomic" bugs.

Let's allow lockless traversal of the list using SRCU and only use the
mutex when modyfing the list.

While at it: let's protect the period between when we start the lookup
and when we finally request the descriptor (increasing the reference
count of the GPIO device) with the SRCU read lock.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
---
 drivers/gpio/gpiolib.c | 203 +++++++++++++++++++++--------------------
 1 file changed, 104 insertions(+), 99 deletions(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index d50a786f8176..69979de76485 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -2,6 +2,7 @@
 
 #include <linux/acpi.h>
 #include <linux/bitmap.h>
+#include <linux/cleanup.h>
 #include <linux/compat.h>
 #include <linux/debugfs.h>
 #include <linux/device.h>
@@ -14,12 +15,14 @@
 #include <linux/irq.h>
 #include <linux/kernel.h>
 #include <linux/list.h>
+#include <linux/lockdep.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/pinctrl/consumer.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <linux/srcu.h>
 #include <linux/string.h>
 
 #include <linux/gpio.h>
@@ -81,7 +84,12 @@ DEFINE_SPINLOCK(gpio_lock);
 
 static DEFINE_MUTEX(gpio_lookup_lock);
 static LIST_HEAD(gpio_lookup_list);
+
 LIST_HEAD(gpio_devices);
+/* Protects the GPIO device list against concurrent modifications. */
+static DEFINE_MUTEX(gpio_devices_lock);
+/* Ensures coherence during read-only accesses to the list of GPIO devices. */
+DEFINE_STATIC_SRCU(gpio_devices_srcu);
 
 static DEFINE_MUTEX(gpio_machine_hogs_mutex);
 static LIST_HEAD(gpio_machine_hogs);
@@ -113,20 +121,16 @@ static inline void desc_set_label(struct gpio_desc *d, const char *label)
 struct gpio_desc *gpio_to_desc(unsigned gpio)
 {
 	struct gpio_device *gdev;
-	unsigned long flags;
 
-	spin_lock_irqsave(&gpio_lock, flags);
-
-	list_for_each_entry(gdev, &gpio_devices, list) {
-		if (gdev->base <= gpio &&
-		    gdev->base + gdev->ngpio > gpio) {
-			spin_unlock_irqrestore(&gpio_lock, flags);
-			return &gdev->descs[gpio - gdev->base];
+	scoped_guard(srcu, &gpio_devices_srcu) {
+		list_for_each_entry_srcu(gdev, &gpio_devices, list,
+				srcu_read_lock_held(&gpio_devices_srcu)) {
+			if (gdev->base <= gpio &&
+			    gdev->base + gdev->ngpio > gpio)
+				return &gdev->descs[gpio - gdev->base];
 		}
 	}
 
-	spin_unlock_irqrestore(&gpio_lock, flags);
-
 	if (!gpio_is_valid(gpio))
 		pr_warn("invalid GPIO %d\n", gpio);
 
@@ -282,7 +286,8 @@ static int gpiochip_find_base_unlocked(int ngpio)
 	struct gpio_device *gdev;
 	int base = GPIO_DYNAMIC_BASE;
 
-	list_for_each_entry(gdev, &gpio_devices, list) {
+	list_for_each_entry_srcu(gdev, &gpio_devices, list,
+				 lockdep_is_held(&gpio_devices_lock)) {
 		/* found a free space? */
 		if (gdev->base >= base + ngpio)
 			break;
@@ -354,23 +359,25 @@ static int gpiodev_add_to_list_unlocked(struct gpio_device *gdev)
 {
 	struct gpio_device *prev, *next;
 
+	lockdep_assert_held(&gpio_devices_lock);
+
 	if (list_empty(&gpio_devices)) {
 		/* initial entry in list */
-		list_add_tail(&gdev->list, &gpio_devices);
+		list_add_tail_rcu(&gdev->list, &gpio_devices);
 		return 0;
 	}
 
 	next = list_first_entry(&gpio_devices, struct gpio_device, list);
 	if (gdev->base + gdev->ngpio <= next->base) {
 		/* add before first entry */
-		list_add(&gdev->list, &gpio_devices);
+		list_add_rcu(&gdev->list, &gpio_devices);
 		return 0;
 	}
 
 	prev = list_last_entry(&gpio_devices, struct gpio_device, list);
 	if (prev->base + prev->ngpio <= gdev->base) {
 		/* add behind last entry */
-		list_add_tail(&gdev->list, &gpio_devices);
+		list_add_tail_rcu(&gdev->list, &gpio_devices);
 		return 0;
 	}
 
@@ -382,11 +389,13 @@ static int gpiodev_add_to_list_unlocked(struct gpio_device *gdev)
 		/* add between prev and next */
 		if (prev->base + prev->ngpio <= gdev->base
 				&& gdev->base + gdev->ngpio <= next->base) {
-			list_add(&gdev->list, &prev->list);
+			list_add_rcu(&gdev->list, &prev->list);
 			return 0;
 		}
 	}
 
+	synchronize_srcu(&gpio_devices_srcu);
+
 	return -EBUSY;
 }
 
@@ -399,26 +408,21 @@ static int gpiodev_add_to_list_unlocked(struct gpio_device *gdev)
 static struct gpio_desc *gpio_name_to_desc(const char * const name)
 {
 	struct gpio_device *gdev;
-	unsigned long flags;
+	struct gpio_desc *desc;
 
 	if (!name)
 		return NULL;
 
-	spin_lock_irqsave(&gpio_lock, flags);
-
-	list_for_each_entry(gdev, &gpio_devices, list) {
-		struct gpio_desc *desc;
+	guard(srcu)(&gpio_devices_srcu);
 
+	list_for_each_entry_srcu(gdev, &gpio_devices, list,
+				 srcu_read_lock_held(&gpio_devices_srcu)) {
 		for_each_gpio_desc(gdev->chip, desc) {
-			if (desc->name && !strcmp(desc->name, name)) {
-				spin_unlock_irqrestore(&gpio_lock, flags);
+			if (desc->name && !strcmp(desc->name, name))
 				return desc;
-			}
 		}
 	}
 
-	spin_unlock_irqrestore(&gpio_lock, flags);
-
 	return NULL;
 }
 
@@ -813,7 +817,6 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 			       struct lock_class_key *request_key)
 {
 	struct gpio_device *gdev;
-	unsigned long flags;
 	unsigned int i;
 	int base = 0;
 	int ret = 0;
@@ -878,49 +881,47 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 
 	gdev->ngpio = gc->ngpio;
 
-	spin_lock_irqsave(&gpio_lock, flags);
-
-	/*
-	 * TODO: this allocates a Linux GPIO number base in the global
-	 * GPIO numberspace for this chip. In the long run we want to
-	 * get *rid* of this numberspace and use only descriptors, but
-	 * it may be a pipe dream. It will not happen before we get rid
-	 * of the sysfs interface anyways.
-	 */
-	base = gc->base;
-	if (base < 0) {
-		base = gpiochip_find_base_unlocked(gc->ngpio);
+	scoped_guard(mutex, &gpio_devices_lock) {
+		/*
+		 * TODO: this allocates a Linux GPIO number base in the global
+		 * GPIO numberspace for this chip. In the long run we want to
+		 * get *rid* of this numberspace and use only descriptors, but
+		 * it may be a pipe dream. It will not happen before we get rid
+		 * of the sysfs interface anyways.
+		 */
+		base = gc->base;
 		if (base < 0) {
-			spin_unlock_irqrestore(&gpio_lock, flags);
-			ret = base;
-			base = 0;
+			base = gpiochip_find_base_unlocked(gc->ngpio);
+			if (base < 0) {
+				ret = base;
+				base = 0;
+				goto err_free_label;
+			}
+
+			/*
+			 * TODO: it should not be necessary to reflect the
+			 * assigned base outside of the GPIO subsystem. Go over
+			 * drivers and see if anyone makes use of this, else
+			 * drop this and assign a poison instead.
+			 */
+			gc->base = base;
+		} else {
+			dev_warn(&gdev->dev,
+				 "Static allocation of GPIO base is deprecated, use dynamic allocation.\n");
+		}
+
+		gdev->base = base;
+
+		ret = gpiodev_add_to_list_unlocked(gdev);
+		if (ret) {
+			chip_err(gc, "GPIO integer space overlap, cannot add chip\n");
 			goto err_free_label;
 		}
-		/*
-		 * TODO: it should not be necessary to reflect the assigned
-		 * base outside of the GPIO subsystem. Go over drivers and
-		 * see if anyone makes use of this, else drop this and assign
-		 * a poison instead.
-		 */
-		gc->base = base;
-	} else {
-		dev_warn(&gdev->dev,
-			 "Static allocation of GPIO base is deprecated, use dynamic allocation.\n");
-	}
-	gdev->base = base;
-
-	ret = gpiodev_add_to_list_unlocked(gdev);
-	if (ret) {
-		spin_unlock_irqrestore(&gpio_lock, flags);
-		chip_err(gc, "GPIO integer space overlap, cannot add chip\n");
-		goto err_free_label;
 	}
 
 	for (i = 0; i < gc->ngpio; i++)
 		gdev->descs[i].gdev = gdev;
 
-	spin_unlock_irqrestore(&gpio_lock, flags);
-
 	BLOCKING_INIT_NOTIFIER_HEAD(&gdev->line_state_notifier);
 	BLOCKING_INIT_NOTIFIER_HEAD(&gdev->device_notifier);
 	init_rwsem(&gdev->sem);
@@ -1011,9 +1012,9 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 		goto err_print_message;
 	}
 err_remove_from_list:
-	spin_lock_irqsave(&gpio_lock, flags);
-	list_del(&gdev->list);
-	spin_unlock_irqrestore(&gpio_lock, flags);
+	scoped_guard(mutex, &gpio_devices_lock)
+		list_del_rcu(&gdev->list);
+	synchronize_srcu(&gpio_devices_srcu);
 err_free_label:
 	kfree_const(gdev->label);
 err_free_descs:
@@ -1076,8 +1077,9 @@ void gpiochip_remove(struct gpio_chip *gc)
 		dev_crit(&gdev->dev,
 			 "REMOVING GPIOCHIP WITH GPIOS STILL REQUESTED\n");
 
-	scoped_guard(spinlock_irqsave, &gpio_lock)
-		list_del(&gdev->list);
+	scoped_guard(mutex, &gpio_devices_lock)
+		list_del_rcu(&gdev->list);
+	synchronize_srcu(&gpio_devices_srcu);
 
 	/*
 	 * The gpiochip side puts its use of the device to rest here:
@@ -1125,7 +1127,7 @@ struct gpio_device *gpio_device_find(void *data,
 	 */
 	might_sleep();
 
-	guard(spinlock_irqsave)(&gpio_lock);
+	guard(srcu)(&gpio_devices_srcu);
 
 	list_for_each_entry(gdev, &gpio_devices, list) {
 		if (gdev->chip && match(gdev->chip, data))
@@ -4136,27 +4138,32 @@ static struct gpio_desc *gpiod_find_and_request(struct device *consumer,
 	struct gpio_desc *desc;
 	int ret;
 
-	desc = gpiod_find_by_fwnode(fwnode, consumer, con_id, idx, &flags, &lookupflags);
-	if (gpiod_not_found(desc) && platform_lookup_allowed) {
+	scoped_guard(srcu, &gpio_devices_srcu) {
+		desc = gpiod_find_by_fwnode(fwnode, consumer, con_id, idx,
+					    &flags, &lookupflags);
+		if (gpiod_not_found(desc) && platform_lookup_allowed) {
+			/*
+			 * Either we are not using DT or ACPI, or their lookup
+			 * did not return a result. In that case, use platform
+			 * lookup as a fallback.
+			 */
+			dev_dbg(consumer,
+				"using lookup tables for GPIO lookup\n");
+			desc = gpiod_find(consumer, con_id, idx, &lookupflags);
+		}
+
+		if (IS_ERR(desc)) {
+			dev_dbg(consumer, "No GPIO consumer %s found\n",
+				con_id);
+			return desc;
+		}
+
 		/*
-		 * Either we are not using DT or ACPI, or their lookup did not
-		 * return a result. In that case, use platform lookup as a
-		 * fallback.
+		 * If a connection label was passed use that, else attempt to use
+		 * the device name as label
 		 */
-		dev_dbg(consumer, "using lookup tables for GPIO lookup\n");
-		desc = gpiod_find(consumer, con_id, idx, &lookupflags);
+		ret = gpiod_request(desc, label);
 	}
-
-	if (IS_ERR(desc)) {
-		dev_dbg(consumer, "No GPIO consumer %s found\n", con_id);
-		return desc;
-	}
-
-	/*
-	 * If a connection label was passed use that, else attempt to use
-	 * the device name as label
-	 */
-	ret = gpiod_request(desc, label);
 	if (ret) {
 		if (!(ret == -EBUSY && flags & GPIOD_FLAGS_BIT_NONEXCLUSIVE))
 			return ERR_PTR(ret);
@@ -4727,35 +4734,33 @@ static void gpiolib_dbg_show(struct seq_file *s, struct gpio_device *gdev)
 
 static void *gpiolib_seq_start(struct seq_file *s, loff_t *pos)
 {
-	unsigned long flags;
 	struct gpio_device *gdev = NULL;
 	loff_t index = *pos;
 
 	s->private = "";
 
-	spin_lock_irqsave(&gpio_lock, flags);
-	list_for_each_entry(gdev, &gpio_devices, list)
-		if (index-- == 0) {
-			spin_unlock_irqrestore(&gpio_lock, flags);
+	guard(srcu)(&gpio_devices_srcu);
+
+	list_for_each_entry(gdev, &gpio_devices, list) {
+		if (index-- == 0)
 			return gdev;
-		}
-	spin_unlock_irqrestore(&gpio_lock, flags);
+	}
 
 	return NULL;
 }
 
 static void *gpiolib_seq_next(struct seq_file *s, void *v, loff_t *pos)
 {
-	unsigned long flags;
 	struct gpio_device *gdev = v;
 	void *ret = NULL;
 
-	spin_lock_irqsave(&gpio_lock, flags);
-	if (list_is_last(&gdev->list, &gpio_devices))
-		ret = NULL;
-	else
-		ret = list_first_entry(&gdev->list, struct gpio_device, list);
-	spin_unlock_irqrestore(&gpio_lock, flags);
+	scoped_guard(srcu, &gpio_devices_srcu) {
+		if (list_is_last(&gdev->list, &gpio_devices))
+			ret = NULL;
+		else
+			ret = list_first_entry(&gdev->list, struct gpio_device,
+					       list);
+	}
 
 	s->private = "\n";
 	++*pos;
-- 
2.40.1