Index: src/libcfa/concurrency/kernel
===================================================================
--- src/libcfa/concurrency/kernel	(revision a1a17a744695781ffbd73827d771f9063f9c11ba)
+++ src/libcfa/concurrency/kernel	(revision 09800e9b5b749cee9e6635061d104d0b3b63c527)
@@ -43,15 +43,45 @@
 extern struct cluster * mainCluster;
 
-enum FinishOpCode { No_Action, Release, Schedule, Release_Schedule, Release_Multi, Release_Multi_Schedule };
+enum FinishOpCode { No_Action, Release, Schedule, Release_Schedule, Release_Multi, Release_Multi_Schedule, Callback };
+
+typedef void (*__finish_callback_fptr_t)(void);
 
 //TODO use union, many of these fields are mutually exclusive (i.e. MULTI vs NOMULTI)
 struct FinishAction {
 	FinishOpCode action_code;
+	/*
+	// Union of possible actions
+	union {
+		// Option 1 : locks and threads
+		struct {
+			// 1 thread or N thread
+			union {
+				thread_desc * thrd;
+				struct {
+					thread_desc ** thrds;
+					unsigned short thrd_count;
+				};
+			};
+			// 1 lock or N lock
+			union {
+				__spinlock_t * lock;
+				struct {
+					__spinlock_t ** locks;
+					unsigned short lock_count;
+				};
+			};
+		};
+		// Option 2 : action pointer
+		__finish_callback_fptr_t callback;
+	};
+	/*/
 	thread_desc * thrd;
+	thread_desc ** thrds;
+	unsigned short thrd_count;
 	__spinlock_t * lock;
 	__spinlock_t ** locks;
 	unsigned short lock_count;
-	thread_desc ** thrds;
-	unsigned short thrd_count;
+	__finish_callback_fptr_t callback;
+	//*/
 };
 static inline void ?{}(FinishAction & this) {