gpiolib: fix the speed of descriptor label setting with SRCU

Commit 1f2bcb8c8ccd ("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: 1f2bcb8c8ccd ("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>

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 94903fc1c1459f9fd26eba62628037492e202620..2fa3756c9073a2572722b228c82734b0209ac3d1 100644 (file)
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -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);

diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
index f67d5991ab1c7a58d375b2f6bd3275b41adb8ade..69a353c789f08fc6cbf42f26000bcb52917c17a0 100644 (file)
--- a/drivers/gpio/gpiolib.h
+++ b/drivers/gpio/gpiolib.h
@@ -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