X-Git-Url: http://plrg.eecs.uci.edu/git/?p=c11tester.git;a=blobdiff_plain;f=model.cc;h=0283911b6eea07e57c7b7266fe3c80fc2d755595;hp=cc464a2bcc5b76b933d8769ba450486b4804edbf;hb=c7cd65f78d1c7956a02ac1e1cbd833e3dd17a3f1;hpb=a0562e0739fd643fdb2108f784f4722482ea5a4f

diff --git a/model.cc b/model.cc
index cc464a2b..0283911b 100644
--- a/model.cc
+++ b/model.cc
@@ -11,6 +11,7 @@
 #include "cyclegraph.h"
 #include "promise.h"
 #include "datarace.h"
+#include "mutex.h"
 
 #define INITIAL_THREAD_ID	0
 
@@ -21,14 +22,17 @@ ModelChecker::ModelChecker(struct model_params params) :
 	/* Initialize default scheduler */
 	scheduler(new Scheduler()),
 	num_executions(0),
+	num_feasible_executions(0),
 	params(params),
 	diverge(NULL),
 	action_trace(new action_list_t()),
 	thread_map(new HashTable<int, Thread *, int>()),
 	obj_map(new HashTable<const void *, action_list_t, uintptr_t, 4>()),
+	lock_waiters_map(new HashTable<const void *, action_list_t, uintptr_t, 4>()),
 	obj_thrd_map(new HashTable<void *, std::vector<action_list_t>, uintptr_t, 4 >()),
 	promises(new std::vector<Promise *>()),
-	lazy_sync_with_release(new HashTable<void *, std::list<ModelAction *>, uintptr_t, 4>()),
+	futurevalues(new std::vector<struct PendingFutureValue>()),
+	lazy_sync_with_release(new HashTable<void *, action_list_t, uintptr_t, 4>()),
 	thrd_last_action(new std::vector<ModelAction *>(1)),
 	node_stack(new NodeStack()),
 	mo_graph(new CycleGraph()),
@@ -53,6 +57,7 @@ ModelChecker::~ModelChecker()
 
 	delete obj_thrd_map;
 	delete obj_map;
+	delete lock_waiters_map;
 	delete action_trace;
 
 	for (unsigned int i = 0; i < promises->size(); i++)
@@ -81,19 +86,19 @@ void ModelChecker::reset_to_initial_state()
 	snapshotObject->backTrackBeforeStep(0);
 }
 
-/** @returns a thread ID for a new Thread */
+/** @return a thread ID for a new Thread */
 thread_id_t ModelChecker::get_next_id()
 {
 	return priv->next_thread_id++;
 }
 
-/** @returns the number of user threads created during this execution */
+/** @return the number of user threads created during this execution */
 int ModelChecker::get_num_threads()
 {
 	return priv->next_thread_id;
 }
 
-/** @returns a sequence number for a new ModelAction */
+/** @return a sequence number for a new ModelAction */
 modelclock_t ModelChecker::get_next_seq_num()
 {
 	return ++priv->used_sequence_numbers;
@@ -116,12 +121,14 @@ Thread * ModelChecker::get_next_thread(ModelAction *curr)
 {
 	thread_id_t tid;
 
-	/* Do not split atomic actions. */
-	if (curr->is_rmwr())
-		return thread_current();
-	/* The THREAD_CREATE action points to the created Thread */
-	else if (curr->get_type() == THREAD_CREATE)
-		return (Thread *)curr->get_location();
+	if (curr!=NULL) {
+		/* Do not split atomic actions. */
+		if (curr->is_rmwr())
+			return thread_current();
+		/* The THREAD_CREATE action points to the created Thread */
+		else if (curr->get_type() == THREAD_CREATE)
+			return (Thread *)curr->get_location();
+	}
 
 	/* Have we completed exploring the preselected path? */
 	if (diverge == NULL)
@@ -173,6 +180,8 @@ bool ModelChecker::next_execution()
 	DBG();
 
 	num_executions++;
+	if (isfinalfeasible())
+		num_feasible_executions++;
 
 	if (isfinalfeasible() || DBG_ENABLED())
 		print_summary();
@@ -191,58 +200,96 @@ bool ModelChecker::next_execution()
 
 ModelAction * ModelChecker::get_last_conflict(ModelAction *act)
 {
-	action_type type = act->get_type();
-
-	switch (type) {
-		case ATOMIC_READ:
-		case ATOMIC_WRITE:
-		case ATOMIC_RMW:
-			break;
-		default:
-			return NULL;
+	switch (act->get_type()) {
+	case ATOMIC_READ:
+	case ATOMIC_WRITE:
+	case ATOMIC_RMW: {
+		/* linear search: from most recent to oldest */
+		action_list_t *list = obj_map->get_safe_ptr(act->get_location());
+		action_list_t::reverse_iterator rit;
+		for (rit = list->rbegin(); rit != list->rend(); rit++) {
+			ModelAction *prev = *rit;
+			if (act->is_synchronizing(prev))
+				return prev;
+		}
+		break;
 	}
-	/* linear search: from most recent to oldest */
-	action_list_t *list = obj_map->get_safe_ptr(act->get_location());
-	action_list_t::reverse_iterator rit;
-	for (rit = list->rbegin(); rit != list->rend(); rit++) {
-		ModelAction *prev = *rit;
-		if (act->is_synchronizing(prev))
-			return prev;
+	case ATOMIC_LOCK:
+	case ATOMIC_TRYLOCK: {
+		/* linear search: from most recent to oldest */
+		action_list_t *list = obj_map->get_safe_ptr(act->get_location());
+		action_list_t::reverse_iterator rit;
+		for (rit = list->rbegin(); rit != list->rend(); rit++) {
+			ModelAction *prev = *rit;
+			if (act->is_conflicting_lock(prev))
+				return prev;
+		}
+		break;
+	}
+	case ATOMIC_UNLOCK: {
+		/* linear search: from most recent to oldest */
+		action_list_t *list = obj_map->get_safe_ptr(act->get_location());
+		action_list_t::reverse_iterator rit;
+		for (rit = list->rbegin(); rit != list->rend(); rit++) {
+			ModelAction *prev = *rit;
+			if (!act->same_thread(prev)&&prev->is_failed_trylock())
+				return prev;
+		}
+		break;
+	}
+	default:
+		break;
 	}
 	return NULL;
 }
 
+/** This method find backtracking points where we should try to
+ * reorder the parameter ModelAction against.
+ *
+ * @param the ModelAction to find backtracking points for.
+ */
+
+
 void ModelChecker::set_backtracking(ModelAction *act)
 {
-	ModelAction *prev;
-	Node *node;
 	Thread *t = get_thread(act);
-
-	prev = get_last_conflict(act);
+	ModelAction * prev = get_last_conflict(act);
 	if (prev == NULL)
 		return;
 
-	node = prev->get_node()->get_parent();
+	Node * node = prev->get_node()->get_parent();
+
+	int low_tid, high_tid;
+	if (node->is_enabled(t)) {
+		low_tid = id_to_int(act->get_tid());
+		high_tid = low_tid+1;
+	} else {
+		low_tid = 0;
+		high_tid = get_num_threads();
+	}
 
-	while (!node->is_enabled(t))
-		t = t->get_parent();
+	for(int i = low_tid; i < high_tid; i++) {
+		thread_id_t tid = int_to_id(i);
+		if (!node->is_enabled(tid))
+			continue;
 
-	/* Check if this has been explored already */
-	if (node->has_been_explored(t->get_id()))
-		return;
+		/* Check if this has been explored already */
+		if (node->has_been_explored(tid))
+			continue;
 
-	/* Cache the latest backtracking point */
-	if (!priv->next_backtrack || *prev > *priv->next_backtrack)
-		priv->next_backtrack = prev;
+		/* Cache the latest backtracking point */
+		if (!priv->next_backtrack || *prev > *priv->next_backtrack)
+			priv->next_backtrack = prev;
 
-	/* If this is a new backtracking point, mark the tree */
-	if (!node->set_backtrack(t->get_id()))
-		return;
-	DEBUG("Setting backtrack: conflict = %d, instead tid = %d\n",
-			prev->get_tid(), t->get_id());
-	if (DBG_ENABLED()) {
-		prev->print();
-		act->print();
+		/* If this is a new backtracking point, mark the tree */
+		if (!node->set_backtrack(tid))
+			continue;
+		DEBUG("Setting backtrack: conflict = %d, instead tid = %d\n",
+					prev->get_tid(), t->get_id());
+		if (DBG_ENABLED()) {
+			prev->print();
+			act->print();
+		}
 	}
 }
 
@@ -261,37 +308,37 @@ ModelAction * ModelChecker::get_next_backtrack()
 /**
  * Processes a read or rmw model action.
  * @param curr is the read model action to process.
- * @param th is the thread
  * @param second_part_of_rmw is boolean that is true is this is the second action of a rmw.
  * @return True if processing this read updates the mo_graph.
  */
-
-bool ModelChecker::process_read(ModelAction *curr, Thread * th, bool second_part_of_rmw) {
+bool ModelChecker::process_read(ModelAction *curr, bool second_part_of_rmw)
+{
 	uint64_t value;
-	bool updated=false;
-	while(true) {
+	bool updated = false;
+	while (true) {
 		const ModelAction *reads_from = curr->get_node()->get_read_from();
 		if (reads_from != NULL) {
 			mo_graph->startChanges();
 
 			value = reads_from->get_value();
-				/* Assign reads_from, perform release/acquire synchronization */
-			curr->read_from(reads_from);
+			bool r_status = false;
+
 			if (!second_part_of_rmw) {
-				check_recency(curr,false);
+				check_recency(curr);
+				r_status = r_modification_order(curr, reads_from);
 			}
 
-			bool r_status=r_modification_order(curr,reads_from);
 
-			if (!second_part_of_rmw&&!isfeasible()&&(curr->get_node()->increment_read_from()||!curr->get_node()->future_value_empty())) {
+			if (!second_part_of_rmw&&!isfeasible()&&(curr->get_node()->increment_read_from()||curr->get_node()->increment_future_value())) {
 				mo_graph->rollbackChanges();
-				too_many_reads=false;
+				too_many_reads = false;
 				continue;
 			}
 
+			curr->read_from(reads_from);
 			mo_graph->commitChanges();
 			updated |= r_status;
-		} else {
+		} else if (!second_part_of_rmw) {
 			/* Read from future value */
 			value = curr->get_node()->get_future_value();
 			modelclock_t expiration = curr->get_node()->get_future_value_expiration();
@@ -299,11 +346,165 @@ bool ModelChecker::process_read(ModelAction *curr, Thread * th, bool second_part
 			Promise *valuepromise = new Promise(curr, value, expiration);
 			promises->push_back(valuepromise);
 		}
-		th->set_return_value(value);
+		get_thread(curr)->set_return_value(value);
 		return updated;
 	}
 }
 
+/**
+ * Processes a lock, trylock, or unlock model action.  @param curr is
+ * the read model action to process.
+ *
+ * The try lock operation checks whether the lock is taken.  If not,
+ * it falls to the normal lock operation case.  If so, it returns
+ * fail.
+ *
+ * The lock operation has already been checked that it is enabled, so
+ * it just grabs the lock and synchronizes with the previous unlock.
+ *
+ * The unlock operation has to re-enable all of the threads that are
+ * waiting on the lock.
+ */
+void ModelChecker::process_mutex(ModelAction *curr) {
+	std::mutex *mutex = (std::mutex *)curr->get_location();
+	struct std::mutex_state *state = mutex->get_state();
+	switch (curr->get_type()) {
+	case ATOMIC_TRYLOCK: {
+		bool success = !state->islocked;
+		curr->set_try_lock(success);
+		if (!success) {
+			get_thread(curr)->set_return_value(0);
+			break;
+		}
+		get_thread(curr)->set_return_value(1);
+	}
+		//otherwise fall into the lock case
+	case ATOMIC_LOCK: {
+		if (curr->get_cv()->getClock(state->alloc_tid) <= state->alloc_clock) {
+			printf("Lock access before initialization\n");
+			set_assert();
+		}
+		state->islocked = true;
+		ModelAction *unlock = get_last_unlock(curr);
+		//synchronize with the previous unlock statement
+		if (unlock != NULL)
+			curr->synchronize_with(unlock);
+		break;
+	}
+	case ATOMIC_UNLOCK: {
+		//unlock the lock
+		state->islocked = false;
+		//wake up the other threads
+		action_list_t *waiters = lock_waiters_map->get_safe_ptr(curr->get_location());
+		//activate all the waiting threads
+		for (action_list_t::iterator rit = waiters->begin(); rit != waiters->end(); rit++) {
+			scheduler->add_thread(get_thread((*rit)->get_tid()));
+		}
+		waiters->clear();
+		break;
+	}
+	default:
+		ASSERT(0);
+	}
+}
+
+/**
+ * Process a write ModelAction
+ * @param curr The ModelAction to process
+ * @return True if the mo_graph was updated or promises were resolved
+ */
+bool ModelChecker::process_write(ModelAction *curr)
+{
+	bool updated_mod_order = w_modification_order(curr);
+	bool updated_promises = resolve_promises(curr);
+
+	if (promises->size() == 0) {
+		for (unsigned int i = 0; i < futurevalues->size(); i++) {
+			struct PendingFutureValue pfv = (*futurevalues)[i];
+			if (pfv.act->get_node()->add_future_value(pfv.value, pfv.expiration) &&
+					(!priv->next_backtrack || *pfv.act > *priv->next_backtrack))
+				priv->next_backtrack = pfv.act;
+		}
+		futurevalues->resize(0);
+	}
+
+	mo_graph->commitChanges();
+	get_thread(curr)->set_return_value(VALUE_NONE);
+	return updated_mod_order || updated_promises;
+}
+
+/**
+ * Initialize the current action by performing one or more of the following
+ * actions, as appropriate: merging RMWR and RMWC/RMW actions, stepping forward
+ * in the NodeStack, manipulating backtracking sets, allocating and
+ * initializing clock vectors, and computing the promises to fulfill.
+ *
+ * @param curr The current action, as passed from the user context; may be
+ * freed/invalidated after the execution of this function
+ * @return The current action, as processed by the ModelChecker. Is only the
+ * same as the parameter @a curr if this is a newly-explored action.
+ */
+ModelAction * ModelChecker::initialize_curr_action(ModelAction *curr)
+{
+	ModelAction *newcurr;
+
+	if (curr->is_rmwc() || curr->is_rmw()) {
+		newcurr = process_rmw(curr);
+		delete curr;
+		compute_promises(newcurr);
+		return newcurr;
+	}
+
+	newcurr = node_stack->explore_action(curr, scheduler->get_enabled());
+	if (newcurr) {
+		/* First restore type and order in case of RMW operation */
+		if (curr->is_rmwr())
+			newcurr->copy_typeandorder(curr);
+
+		ASSERT(curr->get_location() == newcurr->get_location());
+		newcurr->copy_from_new(curr);
+
+		/* Discard duplicate ModelAction; use action from NodeStack */
+		delete curr;
+
+		/* If we have diverged, we need to reset the clock vector. */
+		if (diverge == NULL)
+			newcurr->create_cv(get_parent_action(newcurr->get_tid()));
+	} else {
+		newcurr = curr;
+		/*
+		 * Perform one-time actions when pushing new ModelAction onto
+		 * NodeStack
+		 */
+		curr->create_cv(get_parent_action(curr->get_tid()));
+		if (curr->is_write())
+			compute_promises(curr);
+	}
+	return newcurr;
+}
+
+/**
+ * This method checks whether a model action is enabled at the given point.
+ * At this point, it checks whether a lock operation would be successful at this point.
+ * If not, it puts the thread in a waiter list.
+ * @param curr is the ModelAction to check whether it is enabled.
+ * @return a bool that indicates whether the action is enabled.
+ */
+
+bool ModelChecker::check_action_enabled(ModelAction *curr) {
+	if (curr->is_lock()) {
+		std::mutex * lock = (std::mutex *)curr->get_location();
+		struct std::mutex_state * state = lock->get_state();
+		if (state->islocked) {
+			//Stick the action in the appropriate waiting queue
+			lock_waiters_map->get_safe_ptr(curr->get_location())->push_back(curr);
+			return false;
+		}
+	}
+
+	return true;
+}
+
 /**
  * This is the heart of the model checker routine. It performs model-checking
  * actions corresponding to a given "current action." Among other processes, it
@@ -318,46 +519,31 @@ bool ModelChecker::process_read(ModelAction *curr, Thread * th, bool second_part
  */
 Thread * ModelChecker::check_current_action(ModelAction *curr)
 {
-	bool second_part_of_rmw = false;
-
 	ASSERT(curr);
 
-	if (curr->is_rmwc() || curr->is_rmw()) {
-		ModelAction *tmp = process_rmw(curr);
-		second_part_of_rmw = true;
-		delete curr;
-		curr = tmp;
-		compute_promises(curr);
-	} else {
-		ModelAction *tmp = node_stack->explore_action(curr);
-		if (tmp) {
-			/* Discard duplicate ModelAction; use action from NodeStack */
-			/* First restore type and order in case of RMW operation */
-			if (curr->is_rmwr())
-				tmp->copy_typeandorder(curr);
-
-			/* If we have diverged, we need to reset the clock vector. */
-			if (diverge == NULL)
-				tmp->create_cv(get_parent_action(tmp->get_tid()));
-
-			delete curr;
-			curr = tmp;
-		} else {
-			/*
-			 * Perform one-time actions when pushing new ModelAction onto
-			 * NodeStack
-			 */
-			curr->create_cv(get_parent_action(curr->get_tid()));
-			/* Build may_read_from set */
-			if (curr->is_read())
-				build_reads_from_past(curr);
-			if (curr->is_write())
-				compute_promises(curr);
-		}
+	bool second_part_of_rmw = curr->is_rmwc() || curr->is_rmw();
+
+	if (!check_action_enabled(curr)) {
+		//we'll make the execution look like we chose to run this action
+		//much later...when a lock is actually available to relese
+		get_current_thread()->set_pending(curr);
+		remove_thread(get_current_thread());
+		return get_next_thread(NULL);
 	}
 
+	ModelAction *newcurr = initialize_curr_action(curr);
+
+	/* Add the action to lists before any other model-checking tasks */
+	if (!second_part_of_rmw)
+		add_action_to_lists(newcurr);
+
+	/* Build may_read_from set for newly-created actions */
+	if (curr == newcurr && curr->is_read())
+		build_reads_from_past(curr);
+	curr = newcurr;
+
 	/* Thread specific actions */
-	switch(curr->get_type()) {
+	switch (curr->get_type()) {
 	case THREAD_CREATE: {
 		Thread *th = (Thread *)curr->get_location();
 		th->set_creation(curr);
@@ -370,6 +556,8 @@ Thread * ModelChecker::check_current_action(ModelAction *curr)
 		if (!blocking->is_complete()) {
 			blocking->push_wait_list(curr);
 			scheduler->sleep(waiting);
+		} else {
+			do_complete_join(curr);
 		}
 		break;
 	}
@@ -379,6 +567,7 @@ Thread * ModelChecker::check_current_action(ModelAction *curr)
 			ModelAction *act = th->pop_wait_list();
 			Thread *wake = get_thread(act);
 			scheduler->wake(wake);
+			do_complete_join(act);
 		}
 		th->complete();
 		break;
@@ -391,40 +580,80 @@ Thread * ModelChecker::check_current_action(ModelAction *curr)
 		break;
 	}
 
-	Thread *th = get_thread(curr);
+	work_queue_t work_queue(1, CheckCurrWorkEntry(curr));
 
-	bool updated = false;
-	if (curr->is_read()) {
-		updated=process_read(curr, th, second_part_of_rmw);
-	}
+	while (!work_queue.empty()) {
+		WorkQueueEntry work = work_queue.front();
+		work_queue.pop_front();
 
-	if (curr->is_write()) {
-		bool updated_mod_order=w_modification_order(curr);
-		bool updated_promises=resolve_promises(curr);
-		updated=updated_mod_order|updated_promises;
+		switch (work.type) {
+		case WORK_CHECK_CURR_ACTION: {
+			ModelAction *act = work.action;
+			bool updated = false;
+			if (act->is_read() && process_read(act, second_part_of_rmw))
+				updated = true;
 
-		mo_graph->commitChanges();
-		th->set_return_value(VALUE_NONE);
-	}
+			if (act->is_write() && process_write(act))
+				updated = true;
 
-	if (updated)
-		resolve_release_sequences(curr->get_location());
+			if (act->is_mutex_op())
+				process_mutex(act);
 
-	/* Add action to list.  */
-	if (!second_part_of_rmw)
-		add_action_to_lists(curr);
+			if (updated)
+				work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
+			break;
+		}
+		case WORK_CHECK_RELEASE_SEQ:
+			resolve_release_sequences(work.location, &work_queue);
+			break;
+		case WORK_CHECK_MO_EDGES: {
+			/** @todo Complete verification of work_queue */
+			ModelAction *act = work.action;
+			bool updated = false;
+
+			if (act->is_read()) {
+				if (r_modification_order(act, act->get_reads_from()))
+					updated = true;
+			}
+			if (act->is_write()) {
+				if (w_modification_order(act))
+					updated = true;
+			}
+
+			if (updated)
+				work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
+			break;
+		}
+		default:
+			ASSERT(false);
+			break;
+		}
+	}
 
 	check_curr_backtracking(curr);
-	
+
 	set_backtracking(curr);
 
 	return get_next_thread(curr);
 }
 
+/**
+ * Complete a THREAD_JOIN operation, by synchronizing with the THREAD_FINISH
+ * operation from the Thread it is joining with. Must be called after the
+ * completion of the Thread in question.
+ * @param join The THREAD_JOIN action
+ */
+void ModelChecker::do_complete_join(ModelAction *join)
+{
+	Thread *blocking = (Thread *)join->get_location();
+	ModelAction *act = get_last_action(blocking->get_id());
+	join->synchronize_with(act);
+}
+
 void ModelChecker::check_curr_backtracking(ModelAction * curr) {
 	Node *currnode = curr->get_node();
 	Node *parnode = currnode->get_parent();
-	
+
 	if ((!parnode->backtrack_empty() ||
 			 !currnode->read_from_empty() ||
 			 !currnode->future_value_empty() ||
@@ -435,7 +664,6 @@ void ModelChecker::check_curr_backtracking(ModelAction * curr) {
 	}
 }
 
-
 bool ModelChecker::promises_expired() {
 	for (unsigned int promise_index = 0; promise_index < promises->size(); promise_index++) {
 		Promise *promise = (*promises)[promise_index];
@@ -446,19 +674,38 @@ bool ModelChecker::promises_expired() {
 	return false;
 }
 
-/** @returns whether the current partial trace must be a prefix of a
+/** @return whether the current partial trace must be a prefix of a
  * feasible trace. */
 bool ModelChecker::isfeasibleprefix() {
 	return promises->size() == 0 && *lazy_sync_size == 0;
 }
 
-/** @returns whether the current partial trace is feasible. */
+/** @return whether the current partial trace is feasible. */
 bool ModelChecker::isfeasible() {
+	return !mo_graph->checkForRMWViolation() && isfeasibleotherthanRMW();
+}
+
+/** @return whether the current partial trace is feasible other than
+ * multiple RMW reading from the same store. */
+bool ModelChecker::isfeasibleotherthanRMW() {
+	if (DBG_ENABLED()) {
+		if (mo_graph->checkForCycles())
+			DEBUG("Infeasible: modification order cycles\n");
+		if (failed_promise)
+			DEBUG("Infeasible: failed promise\n");
+		if (too_many_reads)
+			DEBUG("Infeasible: too many reads\n");
+		if (promises_expired())
+			DEBUG("Infeasible: promises expired\n");
+	}
 	return !mo_graph->checkForCycles() && !failed_promise && !too_many_reads && !promises_expired();
 }
 
 /** Returns whether the current completed trace is feasible. */
 bool ModelChecker::isfinalfeasible() {
+	if (DBG_ENABLED() && promises->size() != 0)
+		DEBUG("Infeasible: unrevolved promises\n");
+
 	return isfeasible() && promises->size() == 0;
 }
 
@@ -485,7 +732,7 @@ ModelAction * ModelChecker::process_rmw(ModelAction *act) {
  *
  * If so, we decide that the execution is no longer feasible.
  */
-void ModelChecker::check_recency(ModelAction *curr, bool already_added) {
+void ModelChecker::check_recency(ModelAction *curr) {
 	if (params.maxreads != 0) {
 		if (curr->get_node()->get_read_from_size() <= 1)
 			return;
@@ -506,16 +753,14 @@ void ModelChecker::check_recency(ModelAction *curr, bool already_added) {
 
 		action_list_t::reverse_iterator rit = list->rbegin();
 		/* Skip past curr */
-		if (already_added) {
-			for (; (*rit) != curr; rit++)
-				;
-			/* go past curr now */
-			rit++;
-		}
+		for (; (*rit) != curr; rit++)
+			;
+		/* go past curr now */
+		rit++;
 
-		action_list_t::reverse_iterator ritcopy=rit;
+		action_list_t::reverse_iterator ritcopy = rit;
 		//See if we have enough reads from the same value
-		int count=0;
+		int count = 0;
 		for (; count < params.maxreads; rit++,count++) {
 			if (rit==list->rend())
 				return;
@@ -528,9 +773,9 @@ void ModelChecker::check_recency(ModelAction *curr, bool already_added) {
 				return;
 		}
 
-		for (int i=0;i<curr->get_node()->get_read_from_size();i++) {
+		for (int i = 0; i<curr->get_node()->get_read_from_size(); i++) {
 			//Get write
-			const ModelAction * write=curr->get_node()->get_read_from_at(i);
+			const ModelAction * write = curr->get_node()->get_read_from_at(i);
 			//Need a different write
 			if (write==curr->get_reads_from())
 				continue;
@@ -538,27 +783,27 @@ void ModelChecker::check_recency(ModelAction *curr, bool already_added) {
 			/* Test to see whether this is a feasible write to read from*/
 			mo_graph->startChanges();
 			r_modification_order(curr, write);
-			bool feasiblereadfrom=isfeasible();
+			bool feasiblereadfrom = isfeasible();
 			mo_graph->rollbackChanges();
 
 			if (!feasiblereadfrom)
 				continue;
-			rit=ritcopy;
+			rit = ritcopy;
 
-			bool feasiblewrite=true;
+			bool feasiblewrite = true;
 			//new we need to see if this write works for everyone
 
-			for (int loop=count;loop>0;loop--,rit++) {
+			for (int loop = count; loop>0; loop--,rit++) {
 				ModelAction *act=*rit;
-				bool foundvalue=false;
-				for(int j=0;j<act->get_node()->get_read_from_size();j++) {
+				bool foundvalue = false;
+				for (int j = 0; j<act->get_node()->get_read_from_size(); j++) {
 					if (act->get_node()->get_read_from_at(i)==write) {
-						foundvalue=true;
+						foundvalue = true;
 						break;
 					}
 				}
 				if (!foundvalue) {
-					feasiblewrite=false;
+					feasiblewrite = false;
 					break;
 				}
 			}
@@ -571,7 +816,18 @@ void ModelChecker::check_recency(ModelAction *curr, bool already_added) {
 }
 
 /**
- * Updates the mo_graph with the constraints imposed from the current read.
+ * Updates the mo_graph with the constraints imposed from the current
+ * read.
+ *
+ * Basic idea is the following: Go through each other thread and find
+ * the lastest action that happened before our read.  Two cases:
+ *
+ * (1) The action is a write => that write must either occur before
+ * the write we read from or be the write we read from.
+ *
+ * (2) The action is a read => the write that that action read from
+ * must occur before the write we read from or be the same write.
+ *
  * @param curr The current action. Must be a read.
  * @param rf The action that curr reads from. Must be a write.
  * @return True if modification order edges were added; false otherwise
@@ -591,17 +847,22 @@ bool ModelChecker::r_modification_order(ModelAction *curr, const ModelAction *rf
 		for (rit = list->rbegin(); rit != list->rend(); rit++) {
 			ModelAction *act = *rit;
 
-			/* Include at most one act per-thread that "happens before" curr */
-			if (act->happens_before(curr)) {
-				if (act->is_read()) {
+			/*
+			 * Include at most one act per-thread that "happens
+			 * before" curr. Don't consider reflexively.
+			 */
+			if (act->happens_before(curr) && act != curr) {
+				if (act->is_write()) {
+					if (rf != act) {
+						mo_graph->addEdge(act, rf);
+						added = true;
+					}
+				} else {
 					const ModelAction *prevreadfrom = act->get_reads_from();
 					if (prevreadfrom != NULL && rf != prevreadfrom) {
 						mo_graph->addEdge(prevreadfrom, rf);
 						added = true;
 					}
-				} else if (rf != act) {
-					mo_graph->addEdge(act, rf);
-					added = true;
 				}
 				break;
 			}
@@ -611,7 +872,22 @@ bool ModelChecker::r_modification_order(ModelAction *curr, const ModelAction *rf
 	return added;
 }
 
-/** Updates the mo_graph with the constraints imposed from the current read. */
+/** This method fixes up the modification order when we resolve a
+ *  promises.  The basic problem is that actions that occur after the
+ *  read curr could not property add items to the modification order
+ *  for our read.
+ *
+ *  So for each thread, we find the earliest item that happens after
+ *  the read curr.  This is the item we have to fix up with additional
+ *  constraints.  If that action is write, we add a MO edge between
+ *  the Action rf and that action.  If the action is a read, we add a
+ *  MO edge between the Action rf, and whatever the read accessed.
+ *
+ * @param curr is the read ModelAction that we are fixing up MO edges for.
+ * @param rf is the write ModelAction that curr reads from.
+ *
+ */
+
 void ModelChecker::post_r_modification_order(ModelAction *curr, const ModelAction *rf)
 {
 	std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
@@ -650,6 +926,23 @@ void ModelChecker::post_r_modification_order(ModelAction *curr, const ModelActio
 
 /**
  * Updates the mo_graph with the constraints imposed from the current write.
+ *
+ * Basic idea is the following: Go through each other thread and find
+ * the lastest action that happened before our write.  Two cases:
+ *
+ * (1) The action is a write => that write must occur before
+ * the current write
+ *
+ * (2) The action is a read => the write that that action read from
+ * must occur before the current write.
+ *
+ * This method also handles two other issues:
+ *
+ * (I) Sequential Consistency: Making sure that if the current write is
+ * seq_cst, that it occurs after the previous seq_cst write.
+ *
+ * (II) Sending the write back to non-synchronizing reads.
+ *
  * @param curr The current action. Must be a write.
  * @return True if modification order edges were added; false otherwise
  */
@@ -663,7 +956,7 @@ bool ModelChecker::w_modification_order(ModelAction *curr)
 	if (curr->is_seqcst()) {
 		/* We have to at least see the last sequentially consistent write,
 			 so we are initialized. */
-		ModelAction *last_seq_cst = get_last_seq_cst(curr->get_location());
+		ModelAction *last_seq_cst = get_last_seq_cst(curr);
 		if (last_seq_cst != NULL) {
 			mo_graph->addEdge(last_seq_cst, curr);
 			added = true;
@@ -677,8 +970,23 @@ bool ModelChecker::w_modification_order(ModelAction *curr)
 		action_list_t::reverse_iterator rit;
 		for (rit = list->rbegin(); rit != list->rend(); rit++) {
 			ModelAction *act = *rit;
+			if (act == curr) {
+				/*
+				 * If RMW, we already have all relevant edges,
+				 * so just skip to next thread.
+				 * If normal write, we need to look at earlier
+				 * actions, so continue processing list.
+				 */
+				if (curr->is_rmw())
+					break;
+				else
+					continue;
+			}
 
-			/* Include at most one act per-thread that "happens before" curr */
+			/*
+			 * Include at most one act per-thread that "happens
+			 * before" curr
+			 */
 			if (act->happens_before(curr)) {
 				/*
 				 * Note: if act is RMW, just add edge:
@@ -686,11 +994,9 @@ bool ModelChecker::w_modification_order(ModelAction *curr)
 				 * The following edge should be handled elsewhere:
 				 *   readfrom(act) --mo--> act
 				 */
-				if (act->is_write()) {
-					//RMW shouldn't have an edge to themselves
-					if (act!=curr)
-						mo_graph->addEdge(act, curr);
-				} else if (act->is_read() && act->get_reads_from() != NULL)
+				if (act->is_write())
+					mo_graph->addEdge(act, curr);
+				else if (act->is_read() && act->get_reads_from() != NULL)
 					mo_graph->addEdge(act->get_reads_from(), curr);
 				added = true;
 				break;
@@ -704,9 +1010,13 @@ bool ModelChecker::w_modification_order(ModelAction *curr)
 				   =>
 				   that read could potentially read from our write.
 				 */
-				if (isfeasible() && act->get_node()->add_future_value(curr->get_value(), curr->get_seq_number()+params.maxfuturedelay) &&
-						(!priv->next_backtrack || *act > *priv->next_backtrack))
-					priv->next_backtrack = act;
+				if (thin_air_constraint_may_allow(curr, act)) {
+					if (isfeasible() ||
+							(curr->is_rmw() && act->is_rmw() && curr->get_reads_from() == act->get_reads_from() && isfeasibleotherthanRMW())) {
+						struct PendingFutureValue pfv = {curr->get_value(),curr->get_seq_number()+params.maxfuturedelay,act};
+						futurevalues->push_back(pfv);
+					}
+				}
 			}
 		}
 	}
@@ -714,6 +1024,28 @@ bool ModelChecker::w_modification_order(ModelAction *curr)
 	return added;
 }
 
+/** Arbitrary reads from the future are not allowed.  Section 29.3
+ * part 9 places some constraints.  This method checks one result of constraint
+ * constraint.  Others require compiler support. */
+
+bool ModelChecker::thin_air_constraint_may_allow(const ModelAction * writer, const ModelAction *reader) {
+	if (!writer->is_rmw())
+		return true;
+
+	if (!reader->is_rmw())
+		return true;
+
+	for (const ModelAction *search = writer->get_reads_from(); search != NULL; search = search->get_reads_from()) {
+		if (search == reader)
+			return false;
+		if (search->get_tid() == reader->get_tid() &&
+				search->happens_before(reader))
+			break;
+	}
+
+	return true;
+}
+
 /**
  * Finds the head(s) of the release sequence(s) containing a given ModelAction.
  * The ModelAction under consideration is expected to be taking part in
@@ -735,14 +1067,15 @@ bool ModelChecker::w_modification_order(ModelAction *curr)
  * @return true, if the ModelChecker is certain that release_heads is complete;
  * false otherwise
  */
-bool ModelChecker::release_seq_head(const ModelAction *rf,
-                std::vector<const ModelAction *> *release_heads) const
+bool ModelChecker::release_seq_head(const ModelAction *rf, rel_heads_list_t *release_heads) const
 {
-	ASSERT(rf->is_write());
 	if (!rf) {
 		/* read from future: need to settle this later */
 		return false; /* incomplete */
 	}
+
+	ASSERT(rf->is_write());
+
 	if (rf->is_release())
 		release_heads->push_back(rf);
 	if (rf->is_rmw()) {
@@ -790,10 +1123,12 @@ bool ModelChecker::release_seq_head(const ModelAction *rf,
 		 * the release seq? */
 		bool future_ordered = false;
 
+		ModelAction *last = get_last_action(int_to_id(i));
+		if (last && rf->happens_before(last))
+			future_ordered = true;
+
 		for (rit = list->rbegin(); rit != list->rend(); rit++) {
 			const ModelAction *act = *rit;
-			if (!act->is_write())
-				continue;
 			/* Reach synchronization -> this thread is complete */
 			if (act->happens_before(release))
 				break;
@@ -802,6 +1137,10 @@ bool ModelChecker::release_seq_head(const ModelAction *rf,
 				continue;
 			}
 
+			/* Only writes can break release sequences */
+			if (!act->is_write())
+				continue;
+
 			/* Check modification order */
 			if (mo_graph->checkReachable(rf, act)) {
 				/* rf --mo--> act */
@@ -839,15 +1178,14 @@ bool ModelChecker::release_seq_head(const ModelAction *rf,
  * with the head(s) of the release sequence(s), if they exists with certainty.
  * @see ModelChecker::release_seq_head
  */
-void ModelChecker::get_release_seq_heads(ModelAction *act,
-                std::vector<const ModelAction *> *release_heads)
+void ModelChecker::get_release_seq_heads(ModelAction *act, rel_heads_list_t *release_heads)
 {
 	const ModelAction *rf = act->get_reads_from();
 	bool complete;
 	complete = release_seq_head(rf, release_heads);
 	if (!complete) {
 		/* add act to 'lazy checking' list */
-		std::list<ModelAction *> *list;
+		action_list_t *list;
 		list = lazy_sync_with_release->get_safe_ptr(act->get_location());
 		list->push_back(act);
 		(*lazy_sync_size)++;
@@ -862,21 +1200,24 @@ void ModelChecker::get_release_seq_heads(ModelAction *act,
  *
  * @param location The location/object that should be checked for release
  * sequence resolutions
- * @return True if any updates occurred (new synchronization, new mo_graph edges)
+ * @param work_queue The work queue to which to add work items as they are
+ * generated
+ * @return True if any updates occurred (new synchronization, new mo_graph
+ * edges)
  */
-bool ModelChecker::resolve_release_sequences(void *location)
+bool ModelChecker::resolve_release_sequences(void *location, work_queue_t *work_queue)
 {
-	std::list<ModelAction *> *list;
+	action_list_t *list;
 	list = lazy_sync_with_release->getptr(location);
 	if (!list)
 		return false;
 
 	bool updated = false;
-	std::list<ModelAction *>::iterator it = list->begin();
+	action_list_t::iterator it = list->begin();
 	while (it != list->end()) {
 		ModelAction *act = *it;
 		const ModelAction *rf = act->get_reads_from();
-		std::vector<const ModelAction *> release_heads;
+		rel_heads_list_t release_heads;
 		bool complete;
 		complete = release_seq_head(rf, &release_heads);
 		for (unsigned int i = 0; i < release_heads.size(); i++) {
@@ -887,14 +1228,18 @@ bool ModelChecker::resolve_release_sequences(void *location)
 		}
 
 		if (updated) {
+			/* Re-check act for mo_graph edges */
+			work_queue->push_back(MOEdgeWorkEntry(act));
+
 			/* propagate synchronization to later actions */
 			action_list_t::reverse_iterator it = action_trace->rbegin();
-			while ((*it) != act) {
+			for (; (*it) != act; it++) {
 				ModelAction *propagate = *it;
-				if (act->happens_before(propagate))
-					/** @todo new mo_graph edges along with
-					 * this synchronization? */
+				if (act->happens_before(propagate)) {
 					propagate->synchronize_with(act);
+					/* Re-check 'propagate' for mo_graph edges */
+					work_queue->push_back(MOEdgeWorkEntry(propagate));
+				}
 			}
 		}
 		if (complete) {
@@ -936,27 +1281,56 @@ void ModelChecker::add_action_to_lists(ModelAction *act)
 	(*thrd_last_action)[tid] = act;
 }
 
-ModelAction * ModelChecker::get_last_action(thread_id_t tid)
+/**
+ * @brief Get the last action performed by a particular Thread
+ * @param tid The thread ID of the Thread in question
+ * @return The last action in the thread
+ */
+ModelAction * ModelChecker::get_last_action(thread_id_t tid) const
 {
-	int nthreads = get_num_threads();
-	if ((int)thrd_last_action->size() < nthreads)
-		thrd_last_action->resize(nthreads);
-	return (*thrd_last_action)[id_to_int(tid)];
+	int threadid = id_to_int(tid);
+	if (threadid < (int)thrd_last_action->size())
+		return (*thrd_last_action)[id_to_int(tid)];
+	else
+		return NULL;
 }
 
 /**
- * Gets the last memory_order_seq_cst action (in the total global sequence)
- * performed on a particular object (i.e., memory location).
- * @param location The object location to check
- * @return The last seq_cst action performed
+ * Gets the last memory_order_seq_cst write (in the total global sequence)
+ * performed on a particular object (i.e., memory location), not including the
+ * current action.
+ * @param curr The current ModelAction; also denotes the object location to
+ * check
+ * @return The last seq_cst write
  */
-ModelAction * ModelChecker::get_last_seq_cst(const void *location)
+ModelAction * ModelChecker::get_last_seq_cst(ModelAction *curr) const
 {
+	void *location = curr->get_location();
 	action_list_t *list = obj_map->get_safe_ptr(location);
 	/* Find: max({i in dom(S) | seq_cst(t_i) && isWrite(t_i) && samevar(t_i, t)}) */
 	action_list_t::reverse_iterator rit;
 	for (rit = list->rbegin(); rit != list->rend(); rit++)
-		if ((*rit)->is_write() && (*rit)->is_seqcst())
+		if ((*rit)->is_write() && (*rit)->is_seqcst() && (*rit) != curr)
+			return *rit;
+	return NULL;
+}
+
+/**
+ * Gets the last unlock operation performed on a particular mutex (i.e., memory
+ * location). This function identifies the mutex according to the current
+ * action, which is presumed to perform on the same mutex.
+ * @param curr The current ModelAction; also denotes the object location to
+ * check
+ * @return The last unlock operation
+ */
+ModelAction * ModelChecker::get_last_unlock(ModelAction *curr) const
+{
+	void *location = curr->get_location();
+	action_list_t *list = obj_map->get_safe_ptr(location);
+	/* Find: max({i in dom(S) | isUnlock(t_i) && samevar(t_i, t)}) */
+	action_list_t::reverse_iterator rit;
+	for (rit = list->rbegin(); rit != list->rend(); rit++)
+		if ((*rit)->is_unlock())
 			return *rit;
 	return NULL;
 }
@@ -988,6 +1362,7 @@ ClockVector * ModelChecker::get_cv(thread_id_t tid)
 bool ModelChecker::resolve_promises(ModelAction *write)
 {
 	bool resolved = false;
+
 	for (unsigned int i = 0, promise_index = 0; promise_index < promises->size(); i++) {
 		Promise *promise = (*promises)[promise_index];
 		if (write->get_node()->get_promise(i)) {
@@ -996,14 +1371,17 @@ bool ModelChecker::resolve_promises(ModelAction *write)
 			if (read->is_rmw()) {
 				mo_graph->addRMWEdge(write, read);
 			}
+			//First fix up the modification order for actions that happened
+			//before the read
 			r_modification_order(read, write);
+			//Next fix up the modification order for actions that happened
+			//after the read.
 			post_r_modification_order(read, write);
 			promises->erase(promises->begin() + promise_index);
 			resolved = true;
 		} else
 			promise_index++;
 	}
-
 	return resolved;
 }
 
@@ -1066,7 +1444,7 @@ void ModelChecker::build_reads_from_past(ModelAction *curr)
 	bool initialized = false;
 
 	if (curr->is_seqcst()) {
-		last_seq_cst = get_last_seq_cst(curr->get_location());
+		last_seq_cst = get_last_seq_cst(curr);
 		/* We have to at least see the last sequentially consistent write,
 			 so we are initialized. */
 		if (last_seq_cst != NULL)
@@ -1082,11 +1460,11 @@ void ModelChecker::build_reads_from_past(ModelAction *curr)
 			ModelAction *act = *rit;
 
 			/* Only consider 'write' actions */
-			if (!act->is_write())
+			if (!act->is_write() || act == curr)
 				continue;
 
 			/* Don't consider more than one seq_cst write if we are a seq_cst read. */
-			if (!curr->is_seqcst()|| (!act->is_seqcst() && (last_seq_cst==NULL||!act->happens_before(last_seq_cst))) || act == last_seq_cst) {
+			if (!curr->is_seqcst() || (!act->is_seqcst() && (last_seq_cst == NULL || !act->happens_before(last_seq_cst))) || act == last_seq_cst) {
 				DEBUG("Adding action to may_read_from:\n");
 				if (DBG_ENABLED()) {
 					act->print();
@@ -1136,9 +1514,15 @@ void ModelChecker::print_summary()
 {
 	printf("\n");
 	printf("Number of executions: %d\n", num_executions);
+	printf("Number of feasible executions: %d\n", num_feasible_executions);
 	printf("Total nodes created: %d\n", node_stack->get_total_nodes());
 
+#if SUPPORT_MOD_ORDER_DUMP
 	scheduler->print();
+	char buffername[100];
+	sprintf(buffername, "exec%04u", num_executions);
+	mo_graph->dumpGraphToFile(buffername);
+#endif
 
 	if (!isfinalfeasible())
 		printf("INFEASIBLE EXECUTION!\n");
@@ -1157,6 +1541,11 @@ void ModelChecker::add_thread(Thread *t)
 	scheduler->add_thread(t);
 }
 
+/**
+ * Removes a thread from the scheduler. 
+ * @param the thread to remove.
+ */
+
 void ModelChecker::remove_thread(Thread *t)
 {
 	scheduler->remove_thread(t);
@@ -1184,24 +1573,23 @@ int ModelChecker::switch_to_master(ModelAction *act)
  * @return Returns true (success) if a step was taken and false otherwise.
  */
 bool ModelChecker::take_step() {
-	Thread *curr, *next;
-
 	if (has_asserted())
 		return false;
 
-	curr = thread_current();
+	Thread * curr = thread_current();
 	if (curr) {
 		if (curr->get_state() == THREAD_READY) {
 			ASSERT(priv->current_action);
+
 			priv->nextThread = check_current_action(priv->current_action);
 			priv->current_action = NULL;
-			if (!curr->is_blocked() && !curr->is_complete())
-				scheduler->add_thread(curr);
+			if (curr->is_blocked() || curr->is_complete())
+				scheduler->remove_thread(curr);
 		} else {
 			ASSERT(false);
 		}
 	}
-	next = scheduler->next_thread(priv->nextThread);
+	Thread * next = scheduler->next_thread(priv->nextThread);
 
 	/* Infeasible -> don't take any more steps */
 	if (!isfeasible())
@@ -1214,6 +1602,15 @@ bool ModelChecker::take_step() {
 	/* next == NULL -> don't take any more steps */
 	if (!next)
 		return false;
+
+	if ( next->get_pending() != NULL ) {
+		//restart a pending action
+		set_current_action(next->get_pending());
+		next->set_pending(NULL);
+		next->set_state(THREAD_READY);
+		return true;
+	}
+
 	/* Return false only if swap fails with an error */
 	return (Thread::swap(&system_context, next) == 0);
 }