[PATCH v2 1/6] 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.
gpiodev_release() calls synchronize_srcu() and then cleanup_srcu_struct()
on it, meaning to run the pending frees first, but synchronize_srcu()
only waits for a grace period, not for callbacks queued before it to be
invoked. Once the srcu_struct uses per-CPU callback lists, a label freed
on one CPU, for instance by gpiochip_free_hogs() at the start of
gpiochip_remove(), can still be waiting for its invocation timer there
when synchronize_srcu() returns on another, and cleanup_srcu_struct()
warns:

  WARNING: kernel/rcu/srcutree.c:706 at cleanup_srcu_struct
  Call Trace:
   gpiodev_release
   device_release
   kobject_put
   devres_release_all
   device_release_driver_internal
   platform_device_unregister
   gpio_sim_device_deactivate

cleanup_srcu_struct() then flushes the callbacks itself, so the label is
still freed, but the warning is what the comment above that check says
srcu_barrier() is for. Use it.

A gpio-sim chip with one hogged line, set live and back in a loop, warns
within 4 to 60 cycles in three runs on a 4-CPU guest booted with
srcutree.convert_to_big=1; with this change, three runs of 1000 cycles
pass. With the default srcutree parameters the srcu_struct only goes
per-CPU under contention and the loop did not hit it in 3000 cycles; the
warning was first seen that way, once, on a KCSAN arm64 guest unbinding
a USB-attached GPIO controller whose firmware node hogs a line, and
since then on real hardware with v7.3-rc5 arm64 KCSAN kernels, when
unbinding a USB GPIO expander whose driver is not yet upstream, built
both as an out-of-tree module and into the kernel.

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>
---
Changes in v2:
- Backtrace cut down to the warning and the frames that matter (Andy).
- Paul's Reviewed-by.
- Assisted-by tag.

 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 772c6f04f489..1984f3416437 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