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gpiolib: fix the speed of descriptor label setting with SRCU

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Commit 1f2bcb8c8c ("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: Neil Armstrong <neil.armstrong@linaro.org>
Closes: https://lore.kernel.org/linux-gpio/CAMRc=Mfig2oooDQYTqo23W3PXSdzhVO4p=G4+P8y1ppBOrkrJQ@mail.gmail.com/
Fixes: 1f2bcb8c8c ("gpio: protect the descriptor label with SRCU")
Suggested-by: "Paul E. McKenney" <paulmck@kernel.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8650-QRD
Acked-by: "Paul E. McKenney" <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20240507121346.16969-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
This commit is contained in:
Bartosz Golaszewski 2024-05-07 14:13:46 +02:00
parent e67572cd22
commit a86d276930
2 changed files with 30 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

31
drivers/gpio/gpiolib.c
View File

@ -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);

7
drivers/gpio/gpiolib.h
View File

@ -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