gpio: cdev: use raw notifier for line state events

We use a notifier to implement the mechanism of informing the user-space
about changes in GPIO line status. We register with the notifier when
the GPIO character device file is opened and unregister when the last
reference to the associated file descriptor is dropped.

Since commit fcc8b637c542 ("gpiolib: switch the line state notifier to
atomic") we use the atomic notifier variant. Atomic notifiers call
rcu_synchronize in atomic_notifier_chain_unregister() which caused a
significant performance regression in some circumstances, observed by
user-space when calling close() on the GPIO device file descriptor.

Replace the atomic notifier with the raw variant and provide
synchronization with a read-write spinlock.

Fixes: fcc8b637c542 ("gpiolib: switch the line state notifier to atomic")
Reported-by: David Jander <david@protonic.nl>
Closes: https://lore.kernel.org/all/20250311110034.53959031@erd003.prtnl/
Tested-by: David Jander <david@protonic.nl>
Tested-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20250311-gpiolib-line-state-raw-notifier-v2-1-138374581e1e@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index 40f76a90fd7db9de7f3ecf3c3f733fb214e8e38e..107d75558b5a80db138c361d2a56bcebed5cd20a 100644 (file)
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -2729,8 +2729,9 @@ static int gpio_chrdev_open(struct inode *inode, struct file *file)
        cdev->gdev = gpio_device_get(gdev);
 
        cdev->lineinfo_changed_nb.notifier_call = lineinfo_changed_notify;
-       ret = atomic_notifier_chain_register(&gdev->line_state_notifier,
-                                            &cdev->lineinfo_changed_nb);
+       scoped_guard(write_lock_irqsave, &gdev->line_state_lock)
+               ret = raw_notifier_chain_register(&gdev->line_state_notifier,
+                                                 &cdev->lineinfo_changed_nb);
        if (ret)
                goto out_free_bitmap;
 
@@ -2754,8 +2755,9 @@ out_unregister_device_notifier:
        blocking_notifier_chain_unregister(&gdev->device_notifier,
                                           &cdev->device_unregistered_nb);
 out_unregister_line_notifier:
-       atomic_notifier_chain_unregister(&gdev->line_state_notifier,
-                                        &cdev->lineinfo_changed_nb);
+       scoped_guard(write_lock_irqsave, &gdev->line_state_lock)
+               raw_notifier_chain_unregister(&gdev->line_state_notifier,
+                                             &cdev->lineinfo_changed_nb);
 out_free_bitmap:
        gpio_device_put(gdev);
        bitmap_free(cdev->watched_lines);
@@ -2779,8 +2781,9 @@ static int gpio_chrdev_release(struct inode *inode, struct file *file)
 
        blocking_notifier_chain_unregister(&gdev->device_notifier,
                                           &cdev->device_unregistered_nb);
-       atomic_notifier_chain_unregister(&gdev->line_state_notifier,
-                                        &cdev->lineinfo_changed_nb);
+       scoped_guard(write_lock_irqsave, &gdev->line_state_lock)
+               raw_notifier_chain_unregister(&gdev->line_state_notifier,
+                                             &cdev->lineinfo_changed_nb);
        bitmap_free(cdev->watched_lines);
        gpio_device_put(gdev);
        kfree(cdev);

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index de708d0818581174130ec9a581a28b0d92610f67..0c00ed2ab4315030f7e0bfb127c74a0e976cd17a 100644 (file)
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -1025,7 +1025,8 @@ int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
                }
        }
 
-       ATOMIC_INIT_NOTIFIER_HEAD(&gdev->line_state_notifier);
+       rwlock_init(&gdev->line_state_lock);
+       RAW_INIT_NOTIFIER_HEAD(&gdev->line_state_notifier);
        BLOCKING_INIT_NOTIFIER_HEAD(&gdev->device_notifier);
 
        ret = init_srcu_struct(&gdev->srcu);
@@ -4188,8 +4189,9 @@ EXPORT_SYMBOL_GPL(gpiod_set_array_value_cansleep);
 
 void gpiod_line_state_notify(struct gpio_desc *desc, unsigned long action)
 {
-       atomic_notifier_call_chain(&desc->gdev->line_state_notifier,
-                                  action, desc);
+       guard(read_lock_irqsave)(&desc->gdev->line_state_lock);
+
+       raw_notifier_call_chain(&desc->gdev->line_state_notifier, action, desc);
 }
 
 /**

diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
index 147156ec502b29e7c298c013b2112f22a065a5ed..c129a03e2040872440c13406fda74d3ed9d45592 100644 (file)
--- a/drivers/gpio/gpiolib.h
+++ b/drivers/gpio/gpiolib.h
@@ -16,6 +16,7 @@
 #include <linux/gpio/driver.h>
 #include <linux/module.h>
 #include <linux/notifier.h>
+#include <linux/spinlock.h>
 #include <linux/srcu.h>
 #include <linux/workqueue.h>
 
@@ -45,6 +46,7 @@
  * @list: links gpio_device:s together for traversal
  * @line_state_notifier: used to notify subscribers about lines being
  *                       requested, released or reconfigured
+ * @line_state_lock: RW-spinlock protecting the line state notifier
  * @line_state_wq: used to emit line state events from a separate thread in
  *                 process context
  * @device_notifier: used to notify character device wait queues about the GPIO
@@ -72,7 +74,8 @@ struct gpio_device {
        const char              *label;
        void                    *data;
        struct list_head        list;
-       struct atomic_notifier_head line_state_notifier;
+       struct raw_notifier_head line_state_notifier;
+       rwlock_t                line_state_lock;
        struct workqueue_struct *line_state_wq;
        struct blocking_notifier_head device_notifier;
        struct srcu_struct      srcu;