[PATCHv2 1/2] refcount: introduce __refcount_dec_and_lock macro

[Alexander Aring]

This patch adds the __refcount_dec_and_lock macro to generate code for a
lock specific refcount_dec_and_lock implementation. Existing
refcount_dec_and_lock implementation are updated to use the new
__refcount_dec_and_lock macro. In future other lock implementation can
added to use the refcount_dec_and_lock trick to only hold the lock when
the refcount is going to be zero. Per subsystem own lock implementation
can use the macro as well to provide such implementation for their own
locking type.

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Co-developed: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Alexander Aring <aahringo@xxxxxxxxxx>
---
changes since v2:
 - drop kdoc move to header, it need to be at the implementation
 - add Acked-by from Peter Zijlstra
 - fix Peter Zijlstra in Co-developed

 include/linux/refcount.h | 26 ++++++++++++++++++++++++++
 lib/refcount.c           | 36 ++++++------------------------------
 2 files changed, 32 insertions(+), 30 deletions(-)

diff --git a/include/linux/refcount.h b/include/linux/refcount.h
index a62fcca97486..0db03df681fe 100644
--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -361,6 +361,32 @@ static inline void refcount_dec(refcount_t *r)
 
 extern __must_check bool refcount_dec_if_one(refcount_t *r);
 extern __must_check bool refcount_dec_not_one(refcount_t *r);
+
+/**
+ * __refcount_dec_and_lock - macro to create code to holding a lock if being
+ *                           able to decremnt refcount to 0
+ * @_ref: the refcount
+ * @_lock: lock function call code
+ * @_unlock: unlock function call code
+ *
+ * The result will be directly returned as a right operand operation. Uusally
+ * the caller use it directly after a return statement.
+ */
+#define __refcount_dec_and_lock(_ref, _lock, _unlock)	\
+({							\
+	bool _ret = false;				\
+							\
+	if (!refcount_dec_not_one(_ref)) {		\
+		_lock;					\
+		if (!refcount_dec_and_test(_ref))	\
+			_unlock;			\
+		else					\
+			_ret = true;			\
+	}						\
+							\
+	_ret;						\
+})
+
 extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) __cond_acquires(lock);
 extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) __cond_acquires(lock);
 extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
diff --git a/lib/refcount.c b/lib/refcount.c
index a207a8f22b3c..2afe279cfb5a 100644
--- a/lib/refcount.c
+++ b/lib/refcount.c
@@ -112,16 +112,8 @@ EXPORT_SYMBOL(refcount_dec_not_one);
  */
 bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
 {
-	if (refcount_dec_not_one(r))
-		return false;
-
-	mutex_lock(lock);
-	if (!refcount_dec_and_test(r)) {
-		mutex_unlock(lock);
-		return false;
-	}
-
-	return true;
+	return __refcount_dec_and_lock(r, mutex_lock(lock),
+				       mutex_unlock(lock));
 }
 EXPORT_SYMBOL(refcount_dec_and_mutex_lock);
 
@@ -143,16 +135,8 @@ EXPORT_SYMBOL(refcount_dec_and_mutex_lock);
  */
 bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
 {
-	if (refcount_dec_not_one(r))
-		return false;
-
-	spin_lock(lock);
-	if (!refcount_dec_and_test(r)) {
-		spin_unlock(lock);
-		return false;
-	}
-
-	return true;
+	return __refcount_dec_and_lock(r, spin_lock(lock),
+				       spin_unlock(lock));
 }
 EXPORT_SYMBOL(refcount_dec_and_lock);
 
@@ -172,15 +156,7 @@ EXPORT_SYMBOL(refcount_dec_and_lock);
 bool refcount_dec_and_lock_irqsave(refcount_t *r, spinlock_t *lock,
 				   unsigned long *flags)
 {
-	if (refcount_dec_not_one(r))
-		return false;
-
-	spin_lock_irqsave(lock, *flags);
-	if (!refcount_dec_and_test(r)) {
-		spin_unlock_irqrestore(lock, *flags);
-		return false;
-	}
-
-	return true;
+	return __refcount_dec_and_lock(r, spin_lock_irqsave(lock, *flags),
+				       spin_unlock_irqrestore(lock, *flags));
 }
 EXPORT_SYMBOL(refcount_dec_and_lock_irqsave);
-- 
2.39.3