[PATCH net-next 6/7] lockdep: provide dummy forward declaration of *_is_held() helpers

From: Jakub Kicinski
Date: Wed Sep 16 2020 - 14:47:45 EST

When CONFIG_LOCKDEP is not set, lock_is_held() and lockdep_is_held()
are not declared or defined. This forces all callers to use ifdefs
around these checks.

Recent RCU changes added a lot of lockdep_is_held() calls inside
rcu_dereference_protected(). rcu_dereference_protected() hides
its argument on !LOCKDEP builds, but this may lead to unused variable

Provide forward declarations of lock_is_held() and lockdep_is_held()
but never define them. This way callers can keep them visible to
the compiler on !LOCKDEP builds and instead depend on dead code
elimination to remove the references before the linker barfs.

We need lock_is_held() for RCU.

Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
CC: peterz@xxxxxxxxxxxxx
CC: mingo@xxxxxxxxxx
CC: will@xxxxxxxxxx
include/linux/lockdep.h | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 6a584b3e5c74..6b5bbc536bf6 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -371,6 +371,12 @@ static inline void lockdep_unregister_key(struct lock_class_key *key)

#define lockdep_depth(tsk) (0)

+ * Dummy forward declarations, allow users to write less ifdef-y code
+ * and depend on dead code elimination.
+ */
+int lock_is_held(const void *);
+int lockdep_is_held(const void *);
#define lockdep_is_held_type(l, r) (1)

#define lockdep_assert_held(l) do { (void)(l); } while (0)