[PATCH v4 1/7] gpiolib: wait for pending label frees before cleaning up desc_srcu

[Adrian Ambrozewicz <aambrozewicz@nvidia.com>]

desc_set_label() frees the old label with call_srcu() on
gdev->desc_srcu, and gpiodev_release() calls synchronize_srcu() before
cleanup_srcu_struct(). synchronize_srcu() waits for a grace period, not
for queued callbacks. Once the srcu_struct uses per-CPU callback lists,
a label freed by gpiochip_free_hogs() can still be pending, and
cleanup_srcu_struct() warns:

  WARNING: kernel/rcu/srcutree.c:706 at cleanup_srcu_struct+0x239/0x330, CPU#0: init/1
  Call Trace:
   gpiodev_release
   device_release
   kobject_put
   devres_release_all
   device_release_driver_internal
   platform_device_unregister
   gpio_sim_device_deactivate

Use srcu_barrier(), which waits for them.

Reproduced with a hogged gpio-sim line, setting the chip live and back
on a 4-CPU guest with srcutree.convert_to_big=1: it warns within 60
cycles; with this change, 1000 cycles pass.

Fixes: a86d27693066 ("gpiolib: fix the speed of descriptor label setting with SRCU")
Cc: stable@vger.kernel.org
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Assisted-by: LLM
Signed-off-by: Adrian Ambrozewicz <aambrozewicz@nvidia.com>
---
 drivers/gpio/gpiolib.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 90731a547ed6..faf4a8507dd6 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -895,7 +895,7 @@ static void gpiodev_release(struct device *dev)
 	struct gpio_device *gdev = to_gpio_device(dev);
 
 	/* Call pending kfree()s for descriptor labels. */
-	synchronize_srcu(&gdev->desc_srcu);
+	srcu_barrier(&gdev->desc_srcu);
 	cleanup_srcu_struct(&gdev->desc_srcu);
 
 	ida_free(&gpio_ida, gdev->id);
-- 
2.43.0