[PATCH-tip v2] locking/lockdep: Cleanup reinit_class()

[Waiman Long]

Replace the duplicated WARN_ON_ONCE() tests in reinit_class() with
BUILD_BUG_ON() which have the same effect of making sure that those
lists aren't cleared accidentally because of their placements in the
lock_class structure.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 include/linux/lockdep_types.h | 4 ++++
 kernel/locking/lockdep.c      | 8 +++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/linux/lockdep_types.h b/include/linux/lockdep_types.h
index d22430840b53..446d981ae3a2 100644
--- a/include/linux/lockdep_types.h
+++ b/include/linux/lockdep_types.h
@@ -108,6 +108,10 @@ struct lock_class {
 	 */
 	struct list_head		locks_after, locks_before;
 
+	/*
+	 * All the fields that require re-initialization after being zapped
+	 * must be put after "key".
+	 */
 	const struct lockdep_subclass_key *key;
 	unsigned int			subclass;
 	unsigned int			dep_gen_id;
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 2e6892ec3756..5fb015dc1686 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -6011,13 +6011,15 @@ static void zap_class(struct pending_free *pf, struct lock_class *class)
 
 static void reinit_class(struct lock_class *class)
 {
+	const unsigned int key_offset = offsetof(struct lock_class, key);
+
 	WARN_ON_ONCE(!class->lock_entry.next);
 	WARN_ON_ONCE(!list_empty(&class->locks_after));
 	WARN_ON_ONCE(!list_empty(&class->locks_before));
+	BUILD_BUG_ON(offsetof(struct lock_class, lock_entry) > key_offset);
+	BUILD_BUG_ON(offsetof(struct lock_class, locks_after) > key_offset);
+	BUILD_BUG_ON(offsetof(struct lock_class, locks_before) > key_offset);
 	memset_startat(class, 0, key);
-	WARN_ON_ONCE(!class->lock_entry.next);
-	WARN_ON_ONCE(!list_empty(&class->locks_after));
-	WARN_ON_ONCE(!list_empty(&class->locks_before));
 }
 
 static inline int within(const void *addr, void *start, unsigned long size)
-- 
2.27.0