Provide a function to downgrade a cglock from a write lock to an intermediate variant.

miner.h

@@ -843,7 +843,7 @@ static inline void cglock_init(cglock_t *lock)
 	rwlock_init(&lock->rwlock);
 }
 
-/* Read lock variant of cglock */
+/* Read lock variant of cglock. Cannot be promoted. */
 static inline void cg_rlock(cglock_t *lock)
 {
 	mutex_lock(&lock->mutex);
@@ -851,7 +851,8 @@ static inline void cg_rlock(cglock_t *lock)
 	mutex_unlock_noyield(&lock->mutex);
 }
 
-/* Intermediate variant of cglock */
+/* Intermediate variant of cglock - behaves as a read lock but can be promoted
+ * to a write lock or demoted to read lock. */
 static inline void cg_ilock(cglock_t *lock)
 {
 	mutex_lock(&lock->mutex);
@@ -878,6 +879,12 @@ static inline void cg_dwlock(cglock_t *lock)
 	mutex_unlock_noyield(&lock->mutex);
 }
 
+/* Demote a write variant to an intermediate variant */
+static inline void cg_dwilock(cglock_t *lock)
+{
+	wr_unlock(&lock->rwlock);
+}
+
 /* Downgrade intermediate variant to a read lock */
 static inline void cg_dlock(cglock_t *lock)
 {