[PATCH v2 2/3] gpio: shared: fix a race condition

[Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>]

When matching the reset-gpio reference with the actual firmware node
consuming the GPIO, we also need to lock the structure associated with
the latter as it can change while we're doing it.

Due to triggering lockdep false-positives, we need to use a per-reference
lockdep class but accidentally, this also allows us to remove the
previous lockdep workaround for cleaner code.

Fixes: 49416483a953 ("gpio: shared: allow sharing a reset-gpios pin between reset-gpio and gpiolib")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Closes: https://lore.kernel.org/all/00107523-7737-4b92-a785-14ce4e93b8cb@samsung.com/
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
---
 drivers/gpio/gpiolib-shared.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/drivers/gpio/gpiolib-shared.c b/drivers/gpio/gpiolib-shared.c
index a68af06a6cc4e1e33946d7f200cecd4d3dc066af..4c57b0928760c3f9f251ca88cfc54ee5887d50c9 100644
--- a/drivers/gpio/gpiolib-shared.c
+++ b/drivers/gpio/gpiolib-shared.c
@@ -38,6 +38,7 @@ struct gpio_shared_ref {
 	int dev_id;
 	/* Protects the auxiliary device struct and the lookup table. */
 	struct mutex lock;
+	struct lock_class_key lock_key;
 	struct auxiliary_device adev;
 	struct gpiod_lookup_table *lookup;
 };
@@ -99,7 +100,8 @@ static struct gpio_shared_ref *gpio_shared_make_ref(struct fwnode_handle *fwnode
 	ref->flags = flags;
 	ref->con_id = no_free_ptr(con_id_cpy);
 	ref->fwnode = fwnode;
-	mutex_init(&ref->lock);
+	lockdep_register_key(&ref->lock_key);
+	mutex_init_with_key(&ref->lock, &ref->lock_key);
 
 	return no_free_ptr(ref);
 }
@@ -378,6 +380,11 @@ static bool gpio_shared_dev_is_reset_gpio(struct device *consumer,
 	 * arguments match the ones from this consumer's node.
 	 */
 	list_for_each_entry(real_ref, &entry->refs, list) {
+		if (real_ref == ref)
+			continue;
+
+		guard(mutex)(&real_ref->lock);
+
 		if (!real_ref->fwnode)
 			continue;
 
@@ -568,15 +575,6 @@ void gpio_device_teardown_shared(struct gpio_device *gdev)
 		if (!device_match_fwnode(&gdev->dev, entry->fwnode))
 			continue;
 
-		/*
-		 * For some reason if we call synchronize_srcu() in GPIO core,
-		 * descent here and take this mutex and then recursively call
-		 * synchronize_srcu() again from gpiochip_remove() (which is
-		 * totally fine) called after gpio_shared_remove_adev(),
-		 * lockdep prints a false positive deadlock splat. Disable
-		 * lockdep here.
-		 */
-		lockdep_off();
 		list_for_each_entry(ref, &entry->refs, list) {
 			guard(mutex)(&ref->lock);
 
@@ -589,7 +587,6 @@ void gpio_device_teardown_shared(struct gpio_device *gdev)
 
 			gpio_shared_remove_adev(&ref->adev);
 		}
-		lockdep_on();
 	}
 }
 
@@ -685,6 +682,7 @@ static void gpio_shared_drop_ref(struct gpio_shared_ref *ref)
 {
 	list_del(&ref->list);
 	mutex_destroy(&ref->lock);
+	lockdep_unregister_key(&ref->lock_key);
 	kfree(ref->con_id);
 	ida_free(&gpio_shared_ida, ref->dev_id);
 	fwnode_handle_put(ref->fwnode);

-- 
2.47.3