Fix a bug in the scheduler

sys/kern/thread.c

@@ -226,6 +226,14 @@ Thread_Scheduler()
 
     // Select next thread
     next = TAILQ_FIRST(&runnableQueue);
+    if (!next) {
+	/*
+	 * There may be no other runnable processes on this core.  This is a 
+	 * good opportunity to migrate threads.
+	 */
+	Spinlock_Unlock(&threadLock);
+	return;
+    }
     TAILQ_REMOVE(&runnableQueue, next, schedQueue);
 
     prev = curProc;