Commit a86d2769 authored by Bartosz Golaszewski's avatar Bartosz Golaszewski
Browse files

gpiolib: fix the speed of descriptor label setting with SRCU 



Commit 1f2bcb8c ("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: default avatarNeil Armstrong <neil.armstrong@linaro.org>
Closes: https://lore.kernel.org/linux-gpio/CAMRc=Mfig2oooDQYTqo23W3PXSdzhVO4p=G4+P8y1ppBOrkrJQ@mail.gmail.com/


Fixes: 1f2bcb8c ("gpio: protect the descriptor label with SRCU")
Suggested-by: default avatar"Paul E. McKenney" <paulmck@kernel.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8650-QRD
Acked-by: default avatar"Paul E. McKenney" <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20240507121346.16969-1-brgl@bgdev.pl


Signed-off-by: default avatarBartosz Golaszewski <bartosz.golaszewski@linaro.org>
parent e67572cd

drivers/gpio/gpiolib.c

+24 −7
Original line number Diff line number Diff line
@@ -101,6 +101,7 @@ 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);
@@ -108,23 +109,36 @@ const char *gpiod_get_label(struct gpio_desc *desc) 
	    !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

+6 −1
Original line number Diff line number Diff line
@@ -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