diff --git a/src/jrd/dls.cpp b/src/jrd/dls.cpp
index 875ed3f627..2504263fe0 100644
--- a/src/jrd/dls.cpp
+++ b/src/jrd/dls.cpp
@@ -73,14 +73,8 @@ bool DLS_get_temp_space(ULONG size, sort_work_file* sfb)
 
 	mutexed_dir_list* ptr = DLS_get_access();
 
-#ifdef V4_THREADING
-	if (!ptr->mdls_mutex_init) {
-		V4_MUTEX_INIT(ptr->mdls_mutex);
-		ptr->mdls_mutex_init = true;
-	}
+	Firebird::MutexLockGuard guard(ptr->mdls_mutex);
 
-	V4_MUTEX_LOCK(ptr->mdls_mutex);
-#endif
 	if (!sfb->sfb_dls) {
 		/* allocate temp. space starting search from the begining of the dir_list */
 		for (sfb->sfb_dls = ptr->mdls_dls;
@@ -101,11 +95,8 @@ bool DLS_get_temp_space(ULONG size, sort_work_file* sfb)
 			result = true;
 		}
 	}
-#ifdef V4_THREADING
-	V4_MUTEX_UNLOCK(ptr->mdls_mutex);
-#endif
 
-	return (result);
+	return result;
 }
 
 
@@ -123,18 +114,12 @@ void DLS_put_temp_space(sort_work_file* sfb)
  **************************************/
 	if (sfb && sfb->sfb_dls) {
 		mutexed_dir_list* ptr = DLS_get_access();
-#ifdef V4_THREADING
-		fb_assert(ptr->mdls_mutex_init);
-		V4_MUTEX_LOCK(ptr->mdls_mutex);
-#endif
+		Firebird::MutexLockGuard guard(ptr->mdls_mutex);
 		fb_assert(sfb->sfb_dls->dls_inuse >= sfb->sfb_file_size);
 		if (sfb->sfb_dls->dls_inuse > sfb->sfb_file_size)
 			sfb->sfb_dls->dls_inuse -= sfb->sfb_file_size;
 		else
 			sfb->sfb_dls->dls_inuse = 0;
-#ifdef V4_THREADING
-		V4_MUTEX_UNLOCK(ptr->mdls_mutex);
-#endif
 	}
 }
 
@@ -168,16 +153,7 @@ bool DLS_add_dir(ULONG size, const TEXT* dir_name)
 
 	mutexed_dir_list* mdls = DLS_get_access();
 
-#ifdef V4_THREADING
-/* lock mutex, initialize it in case of the first access */
-
-	if (!mdls->mdls_mutex_init) {
-		V4_MUTEX_INIT(mdls->mdls_mutex);
-		mdls->mdls_mutex_init = true;
-	}
-
-	V4_MUTEX_LOCK(mdls->mdls_mutex);
-#endif
+	Firebird::MutexLockGuard guard(mdls->mdls_mutex);
 
 /* add new entry to the end of list */
 
@@ -191,12 +167,6 @@ bool DLS_add_dir(ULONG size, const TEXT* dir_name)
 		dls_iterator->dls_next = new_dls;
 	}
 
-#ifdef V4_THREADING
-/* release lock */
-
-	V4_MUTEX_UNLOCK(mdls->mdls_mutex);
-#endif
-
 	return true;
 }
 
diff --git a/src/jrd/fil.h b/src/jrd/fil.h
index 33b546d7d5..009341cb1f 100644
--- a/src/jrd/fil.h
+++ b/src/jrd/fil.h
@@ -47,11 +47,10 @@ struct dir_list {
 
 struct mutexed_dir_list {
 	dir_list* mdls_dls;				/* Pointer to the directory list */
-	bool mdls_mutex_init;
-	MUTX_T mdls_mutex[1];		/* Mutex for directory list. Must
+	MUTX_T mdls_mutex;				/* Mutex for directory list. Must
 								   be locked before list operations */
 	mutexed_dir_list()
-		: mdls_dls(0), mdls_mutex_init(false) {}
+		: mdls_dls(NULL) {}
 };
 
 /* external function directory list */
diff --git a/src/jrd/thd.h b/src/jrd/thd.h
index 8ac0bba570..94e403823f 100644
--- a/src/jrd/thd.h
+++ b/src/jrd/thd.h
@@ -199,19 +199,15 @@ struct wlck_t {
 int		THD_wlck_lock(wlck_t*, enum WLCK_type);
 int		THD_wlck_unlock(wlck_t*);
 
-#define V4_MUTEX_LOCK(mutx)		THD_mutex_lock (mutx)
-#define V4_MUTEX_UNLOCK(mutx)		THD_mutex_unlock (mutx)
-#define V4_GLOBAL_MUTEX_LOCK		THD_mutex_lock_global()
-#define V4_GLOBAL_MUTEX_UNLOCK		THD_mutex_unlock_global()
-#define V4_RW_LOCK_LOCK(wlck, type)	THD_wlck_lock (wlck, type)
-#define V4_RW_LOCK_UNLOCK(wlck)		THD_wlck_unlock (wlck)
+#define V4_RW_LOCK_LOCK(wlck, type)	THD_wlck_lock(wlck, type)
+#define V4_RW_LOCK_UNLOCK(wlck)		THD_wlck_unlock(wlck)
 #endif // V4_THREADING
 
 #ifdef ANY_THREADING
 #define THD_GLOBAL_MUTEX_LOCK		THD_mutex_lock_global()
 #define THD_GLOBAL_MUTEX_UNLOCK		THD_mutex_unlock_global()
-#define THD_MUTEX_LOCK(mutx)		THD_mutex_lock (mutx)
-#define THD_MUTEX_UNLOCK(mutx)		THD_mutex_unlock (mutx)
+#define THD_MUTEX_LOCK(mutx)		THD_mutex_lock(mutx)
+#define THD_MUTEX_UNLOCK(mutx)		THD_mutex_unlock(mutx)
 #else
 #define THD_GLOBAL_MUTEX_LOCK
 #define THD_GLOBAL_MUTEX_UNLOCK
diff --git a/src/jrd/vio.cpp b/src/jrd/vio.cpp
index 3a35e3cf59..730edaa922 100644
--- a/src/jrd/vio.cpp
+++ b/src/jrd/vio.cpp
@@ -1620,10 +1620,6 @@ Record* VIO_gc_record(thread_db* tdbb, jrd_rel* relation)
 	Database* dbb = tdbb->tdbb_database;
 	CHECK_DBB(dbb);
 
-/* This will require mutex synchronization for pre-emptive multithreading. */
-
-/* V4_MUTEX_LOCK (&relation->rel_mutex); */
-
 /* Allocate a vector of garbage collect record blocks for relation. */
 	vec<Record*>* vector = relation->rel_gc_rec;
 	if (!vector) {
@@ -1638,7 +1634,6 @@ Record* VIO_gc_record(thread_db* tdbb, jrd_rel* relation)
 		Record* record = *rec_ptr;
 		if (record && !(record->rec_flags & REC_gc_active)) {
 			record->rec_flags |= REC_gc_active;
-/*	V4_MUTEX_UNLOCK (&relation->rel_mutex); */
 			return record;
 		}
 	}
@@ -1659,7 +1654,6 @@ Record* VIO_gc_record(thread_db* tdbb, jrd_rel* relation)
 	}
 	(*vector)[slot] = record;
 
-/* V4_MUTEX_UNLOCK (&relation->rel_mutex); */
 	return record;
 }
 
@@ -4624,8 +4618,6 @@ static Record* replace_gc_record(jrd_rel* relation, Record** gc_record, USHORT l
  *
  **************************************/
 
-/* V4_MUTEX_LOCK (&relation->rel_mutex); */
-
 	vec<Record*>* vector = relation->rel_gc_rec;
 	vec<Record*>::iterator rec_ptr, end;
 	for (rec_ptr = vector->begin(), end = vector->end(); rec_ptr < end;
@@ -4635,12 +4627,10 @@ static Record* replace_gc_record(jrd_rel* relation, Record** gc_record, USHORT l
 			// 26 Sep 2002, SKIDDER: Failure to do so (*gc_record = ...) causes nasty memory corruption in 
 			// some cases.
 			*gc_record = realloc_record(*rec_ptr, length);
-/*	V4_MUTEX_UNLOCK (&relation->rel_mutex); */
 			return *rec_ptr;
 		}
 	}
 
-/* V4_MUTEX_UNLOCK (&relation->rel_mutex); */
 	BUGCHECK(288);				/* msg 288 garbage collect record disappeared */
 	return NULL;			/* Added to remove compiler warnings */
 }