[PATCH v2 2/2] reset: Fix circular locking dependency in reset_controller_unregister()

[Tzung-Bi Shih]

reset_control_acquire() and reset_control_put_internal() acquire
`rcdev->lock` inside the `rstc->srcu` read-side critical section
(`rstc->srcu` -> `rcdev->lock`).

However, reset_controller_unregister() holds `rcdev->lock` while calling
synchronize_srcu(&rstc->srcu) (`rcdev->lock` -> `rstc->srcu`), causing a
lockdep circular locking dependency warning and a potential ABBA
deadlock.

Run synchronize_srcu() outside `rcdev->lock`.

Fixes: 44a0acb2caca ("reset: protect struct reset_controller_dev with its own mutex")
Signed-off-by: Tzung-Bi Shih <tzungbi@xxxxxxxxxx>
---
Test:
With "[PATCH v3] reset: Add KUnit test cases"[1] applied:
$ ./tools/testing/kunit/kunit.py run \
        --kconfig_add CONFIG_RESET_CONTROLLER=y \
        --kconfig_add CONFIG_RESET_KUNIT_TEST=y \
        --kconfig_add CONFIG_PROVE_LOCKING=y \
        --kconfig_add CONFIG_DEBUG_KERNEL=y \
        --kconfig_add CONFIG_KASAN=y \
        --arch=x86_64 \
        --raw_output=all \
        reset_test

[1] https://lore.kernel.org/all/20261104160123.6784-1-tzungbi@xxxxxxxxxx
---
v2:
- Use list_move() to make the intent clear.
- Defer reset_controller_remove() after synchronize_srcu() to avoid race.
- Drop lockdep_assert_held() and add comment for reset_controller_remove().

v1: https://lore.kernel.org/all/20261102155840.20376-3-tzungbi@xxxxxxxxxx
---
 drivers/reset/core.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/drivers/reset/core.c b/drivers/reset/core.c
index f0ab09795ae1..4a582721782f 100644
--- a/drivers/reset/core.c
+++ b/drivers/reset/core.c
@@ -163,11 +163,15 @@ EXPORT_SYMBOL_GPL(reset_controller_register);
 static void __reset_control_release(struct kref *kref);
 static void __reset_control_free(struct reset_control *rstc);
 
+/*
+ * Removes rstc from its list and drops the references on rcdev's owner module
+ * and device.  The caller must ensure rstc->list access is serialized (e.g.,
+ * by holding rcdev->lock or moving rstc to a private list) and that no
+ * operations on rcdev via rstc are in flight.
+ */
 static void reset_controller_remove(struct reset_controller_dev *rcdev,
 				    struct reset_control *rstc)
 {
-	lockdep_assert_held(&rcdev->lock);
-
 	list_del(&rstc->list);
 	module_put(rcdev->owner);
 	put_device(rcdev->dev);
@@ -180,6 +184,7 @@ static void reset_controller_remove(struct reset_controller_dev *rcdev,
 void reset_controller_unregister(struct reset_controller_dev *rcdev)
 {
 	struct reset_control *rstc, *pos;
+	LIST_HEAD(reset_control_head);
 
 	scoped_guard(mutex, &reset_list_mutex)
 		list_del(&rcdev->list);
@@ -196,13 +201,22 @@ void reset_controller_unregister(struct reset_controller_dev *rcdev)
 		list_for_each_entry_safe(rstc, pos, &rcdev->reset_control_head, list) {
 			kref_get(&rstc->refcnt);
 			rcu_assign_pointer(rstc->rcdev, NULL);
-			synchronize_srcu(&rstc->srcu);
-			reset_controller_remove(rcdev, rstc);
-			if (kref_put(&rstc->refcnt, __reset_control_release))
-				__reset_control_free(rstc);
+			list_move(&rstc->list, &reset_control_head);
 		}
 	}
 
+	/*
+	 * Run synchronize_srcu() outside rcdev->lock to avoid a circular
+	 * locking dependency with paths that acquire rcdev->lock inside
+	 * the rstc->srcu read-side critical section.
+	 */
+	list_for_each_entry_safe(rstc, pos, &reset_control_head, list) {
+		synchronize_srcu(&rstc->srcu);
+		reset_controller_remove(rcdev, rstc);
+		if (kref_put(&rstc->refcnt, __reset_control_release))
+			__reset_control_free(rstc);
+	}
+
 	mutex_destroy(&rcdev->lock);
 }
 EXPORT_SYMBOL_GPL(reset_controller_unregister);
-- 
2.53.0