[RFC PATCH 1/2] kernel/locking/ww_mutex: Add per-lock lock-check helpers

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

Code using ww_mutexes typically by design have a number of
such mutexes sharing the same ww_class, and within a ww transaction
they are all lockdep annotated using a nest_lock which means
that multiple ww_mutexes of the same lockdep class may be locked at
the same time. That means that lock_is_held() returns true and
lockdep_assert_held() doesn't fire as long as there is a *single*
ww_mutex held of the same class. IOW within a WW transaction.

Code using these mutexes typically want to assert that individual
ww_mutexes are held. Not that any ww_mutex of the same class is
held.

Introduce functions that can be used for that.

RFC: Placement of the functions? lockdep.c? Are the #ifdefs testing for
the correct config?

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 include/linux/ww_mutex.h | 18 ++++++++++++++++++
 kernel/locking/mutex.c   | 10 ++++++++++
 2 files changed, 28 insertions(+)

diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
index 45ff6f7a872b..7bc0f533dea6 100644
--- a/include/linux/ww_mutex.h
+++ b/include/linux/ww_mutex.h
@@ -380,4 +380,22 @@ static inline bool ww_mutex_is_locked(struct ww_mutex *lock)
 	return ww_mutex_base_is_locked(&lock->base);
 }
 
+#ifdef CONFIG_PROVE_LOCKING
+
+bool ww_mutex_held(struct ww_mutex *lock);
+
+#else /* CONFIG_PROVE_LOCKING */
+
+static inline bool ww_mutex_held(struct ww_mutex *lock)
+{
+	return true;
+}
+
+#endif /* CONFIG_PROVE_LOCKING */
+
+static inline void ww_mutex_assert_held(struct ww_mutex *lock)
+{
+	lockdep_assert(ww_mutex_held(lock));
+}
+
 #endif
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index de7d6702cd96..37868b739efd 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -1174,3 +1174,13 @@ int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
 	return 1;
 }
 EXPORT_SYMBOL(atomic_dec_and_mutex_lock);
+
+#ifdef CONFIG_PROVE_LOCKING
+
+bool ww_mutex_held(struct ww_mutex *lock)
+{
+	return __ww_mutex_owner(&lock->base) == current;
+}
+EXPORT_SYMBOL(ww_mutex_held);
+
+#endif /* CONFIG_PROVE_LOCKING */
-- 
2.51.1