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Message-Id: <20250311-gpiolib-line-state-raw-notifier-v2-1-138374581e1e@linaro.org>
Date: Tue, 11 Mar 2025 15:31:43 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Bartosz Golaszewski <brgl@...ev.pl>, Kent Gibson <warthog618@...il.com>,
Linus Walleij <linus.walleij@...aro.org>, David Jander <david@...tonic.nl>
Cc: linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: [PATCH RFT v2] gpio: cdev: use raw notifier for line state events
From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
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@...tonic.nl>
Closes: https://lore.kernel.org/all/20250311110034.53959031@erd003.prtnl/
Tested-by: David Jander <david@...tonic.nl>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
---
Changes in v2:
- improve the commit message
- collect tags
- Link to v1: https://lore.kernel.org/r/20250311-gpiolib-line-state-raw-notifier-v1-1-d0fa44fd67cc@linaro.org
---
drivers/gpio/gpiolib-cdev.c | 15 +++++++++------
drivers/gpio/gpiolib.c | 8 +++++---
drivers/gpio/gpiolib.h | 5 ++++-
3 files changed, 18 insertions(+), 10 deletions(-)
diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index 40f76a90fd7d..107d75558b5a 100644
--- 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 @@ static int gpio_chrdev_open(struct inode *inode, struct file *file)
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 e5eb3f0ee071..b8197502a5ac 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -1075,7 +1075,8 @@ 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);
@@ -4361,8 +4362,9 @@ 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 a738e6c647d8..58f64056de77 100644
--- 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>
@@ -47,6 +48,7 @@
* @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
@@ -75,7 +77,8 @@ 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;
---
base-commit: 0a2f889128969dab41861b6e40111aa03dc57014
change-id: 20250311-gpiolib-line-state-raw-notifier-70c1ad3e99eb
Best regards,
--
Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
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