8 #include "snapshot-interface.h"
10 #include "clockvector.h"
11 #include "cyclegraph.h"
16 #define INITIAL_THREAD_ID 0
20 /** @brief Constructor */
21 ModelChecker::ModelChecker(struct model_params params) :
22 /* Initialize default scheduler */
24 scheduler(new Scheduler()),
26 num_feasible_executions(0),
28 earliest_diverge(NULL),
29 action_trace(new action_list_t()),
30 thread_map(new HashTable<int, Thread *, int>()),
31 obj_map(new HashTable<const void *, action_list_t, uintptr_t, 4>()),
32 lock_waiters_map(new HashTable<const void *, action_list_t, uintptr_t, 4>()),
33 obj_thrd_map(new HashTable<void *, std::vector<action_list_t>, uintptr_t, 4 >()),
34 promises(new std::vector<Promise *>()),
35 futurevalues(new std::vector<struct PendingFutureValue>()),
36 pending_acq_rel_seq(new std::vector<ModelAction *>()),
37 thrd_last_action(new std::vector<ModelAction *>(1)),
38 node_stack(new NodeStack()),
39 mo_graph(new CycleGraph()),
40 failed_promise(false),
41 too_many_reads(false),
43 bad_synchronization(false)
45 /* Allocate this "size" on the snapshotting heap */
46 priv = (struct model_snapshot_members *)calloc(1, sizeof(*priv));
47 /* First thread created will have id INITIAL_THREAD_ID */
48 priv->next_thread_id = INITIAL_THREAD_ID;
51 /** @brief Destructor */
52 ModelChecker::~ModelChecker()
54 for (int i = 0; i < get_num_threads(); i++)
55 delete thread_map->get(i);
60 delete lock_waiters_map;
63 for (unsigned int i = 0; i < promises->size(); i++)
64 delete (*promises)[i];
67 delete pending_acq_rel_seq;
69 delete thrd_last_action;
76 * Restores user program to initial state and resets all model-checker data
79 void ModelChecker::reset_to_initial_state()
81 DEBUG("+++ Resetting to initial state +++\n");
82 node_stack->reset_execution();
83 failed_promise = false;
84 too_many_reads = false;
85 bad_synchronization = false;
87 snapshotObject->backTrackBeforeStep(0);
90 /** @return a thread ID for a new Thread */
91 thread_id_t ModelChecker::get_next_id()
93 return priv->next_thread_id++;
96 /** @return the number of user threads created during this execution */
97 int ModelChecker::get_num_threads()
99 return priv->next_thread_id;
102 /** @return a sequence number for a new ModelAction */
103 modelclock_t ModelChecker::get_next_seq_num()
105 return ++priv->used_sequence_numbers;
109 * @brief Choose the next thread to execute.
111 * This function chooses the next thread that should execute. It can force the
112 * adjacency of read/write portions of a RMW action, force THREAD_CREATE to be
113 * followed by a THREAD_START, or it can enforce execution replay/backtracking.
114 * The model-checker may have no preference regarding the next thread (i.e.,
115 * when exploring a new execution ordering), in which case this will return
117 * @param curr The current ModelAction. This action might guide the choice of
119 * @return The next thread to run. If the model-checker has no preference, NULL.
121 Thread * ModelChecker::get_next_thread(ModelAction *curr)
126 /* Do not split atomic actions. */
128 return thread_current();
129 /* The THREAD_CREATE action points to the created Thread */
130 else if (curr->get_type() == THREAD_CREATE)
131 return (Thread *)curr->get_location();
134 /* Have we completed exploring the preselected path? */
138 /* Else, we are trying to replay an execution */
139 ModelAction *next = node_stack->get_next()->get_action();
141 if (next == diverge) {
142 Node *nextnode = next->get_node();
143 /* Reached divergence point */
144 if (nextnode->increment_promise()) {
145 /* The next node will try to satisfy a different set of promises. */
146 tid = next->get_tid();
147 node_stack->pop_restofstack(2);
148 } else if (nextnode->increment_read_from()) {
149 /* The next node will read from a different value. */
150 tid = next->get_tid();
151 node_stack->pop_restofstack(2);
152 } else if (nextnode->increment_future_value()) {
153 /* The next node will try to read from a different future value. */
154 tid = next->get_tid();
155 node_stack->pop_restofstack(2);
157 /* Make a different thread execute for next step */
158 Node *node = nextnode->get_parent();
159 tid = node->get_next_backtrack();
160 node_stack->pop_restofstack(1);
162 DEBUG("*** Divergence point ***\n");
165 tid = next->get_tid();
167 DEBUG("*** ModelChecker chose next thread = %d ***\n", tid);
168 ASSERT(tid != THREAD_ID_T_NONE);
169 return thread_map->get(id_to_int(tid));
173 * Queries the model-checker for more executions to explore and, if one
174 * exists, resets the model-checker state to execute a new execution.
176 * @return If there are more executions to explore, return true. Otherwise,
179 bool ModelChecker::next_execution()
184 if (isfinalfeasible()) {
185 printf("Earliest divergence point since last feasible execution:\n");
186 if (earliest_diverge)
187 earliest_diverge->print(false);
189 printf("(Not set)\n");
191 earliest_diverge = NULL;
192 num_feasible_executions++;
195 if (isfinalfeasible() || DBG_ENABLED())
198 if ((diverge = get_next_backtrack()) == NULL)
201 if (earliest_diverge == NULL || *diverge < *earliest_diverge)
202 earliest_diverge=diverge;
205 printf("Next execution will diverge at:\n");
209 reset_to_initial_state();
213 ModelAction * ModelChecker::get_last_conflict(ModelAction *act)
215 switch (act->get_type()) {
219 /* linear search: from most recent to oldest */
220 action_list_t *list = obj_map->get_safe_ptr(act->get_location());
221 action_list_t::reverse_iterator rit;
222 for (rit = list->rbegin(); rit != list->rend(); rit++) {
223 ModelAction *prev = *rit;
224 if (act->is_synchronizing(prev))
230 case ATOMIC_TRYLOCK: {
231 /* linear search: from most recent to oldest */
232 action_list_t *list = obj_map->get_safe_ptr(act->get_location());
233 action_list_t::reverse_iterator rit;
234 for (rit = list->rbegin(); rit != list->rend(); rit++) {
235 ModelAction *prev = *rit;
236 if (act->is_conflicting_lock(prev))
241 case ATOMIC_UNLOCK: {
242 /* linear search: from most recent to oldest */
243 action_list_t *list = obj_map->get_safe_ptr(act->get_location());
244 action_list_t::reverse_iterator rit;
245 for (rit = list->rbegin(); rit != list->rend(); rit++) {
246 ModelAction *prev = *rit;
247 if (!act->same_thread(prev)&&prev->is_failed_trylock())
258 /** This method find backtracking points where we should try to
259 * reorder the parameter ModelAction against.
261 * @param the ModelAction to find backtracking points for.
263 void ModelChecker::set_backtracking(ModelAction *act)
265 Thread *t = get_thread(act);
266 ModelAction * prev = get_last_conflict(act);
270 Node * node = prev->get_node()->get_parent();
272 int low_tid, high_tid;
273 if (node->is_enabled(t)) {
274 low_tid = id_to_int(act->get_tid());
275 high_tid = low_tid+1;
278 high_tid = get_num_threads();
281 for(int i = low_tid; i < high_tid; i++) {
282 thread_id_t tid = int_to_id(i);
283 if (!node->is_enabled(tid))
286 /* Check if this has been explored already */
287 if (node->has_been_explored(tid))
290 /* See if fairness allows */
291 if (model->params.fairwindow != 0 && !node->has_priority(tid)) {
293 for(int t=0;t<node->get_num_threads();t++) {
294 thread_id_t tother=int_to_id(t);
295 if (node->is_enabled(tother) && node->has_priority(tother)) {
304 /* Cache the latest backtracking point */
305 if (!priv->next_backtrack || *prev > *priv->next_backtrack)
306 priv->next_backtrack = prev;
308 /* If this is a new backtracking point, mark the tree */
309 if (!node->set_backtrack(tid))
311 DEBUG("Setting backtrack: conflict = %d, instead tid = %d\n",
312 prev->get_tid(), t->get_id());
321 * Returns last backtracking point. The model checker will explore a different
322 * path for this point in the next execution.
323 * @return The ModelAction at which the next execution should diverge.
325 ModelAction * ModelChecker::get_next_backtrack()
327 ModelAction *next = priv->next_backtrack;
328 priv->next_backtrack = NULL;
333 * Processes a read or rmw model action.
334 * @param curr is the read model action to process.
335 * @param second_part_of_rmw is boolean that is true is this is the second action of a rmw.
336 * @return True if processing this read updates the mo_graph.
338 bool ModelChecker::process_read(ModelAction *curr, bool second_part_of_rmw)
341 bool updated = false;
343 const ModelAction *reads_from = curr->get_node()->get_read_from();
344 if (reads_from != NULL) {
345 mo_graph->startChanges();
347 value = reads_from->get_value();
348 bool r_status = false;
350 if (!second_part_of_rmw) {
351 check_recency(curr, reads_from);
352 r_status = r_modification_order(curr, reads_from);
356 if (!second_part_of_rmw&&!isfeasible()&&(curr->get_node()->increment_read_from()||curr->get_node()->increment_future_value())) {
357 mo_graph->rollbackChanges();
358 too_many_reads = false;
362 curr->read_from(reads_from);
363 mo_graph->commitChanges();
365 } else if (!second_part_of_rmw) {
366 /* Read from future value */
367 value = curr->get_node()->get_future_value();
368 modelclock_t expiration = curr->get_node()->get_future_value_expiration();
369 curr->read_from(NULL);
370 Promise *valuepromise = new Promise(curr, value, expiration);
371 promises->push_back(valuepromise);
373 get_thread(curr)->set_return_value(value);
379 * Processes a lock, trylock, or unlock model action. @param curr is
380 * the read model action to process.
382 * The try lock operation checks whether the lock is taken. If not,
383 * it falls to the normal lock operation case. If so, it returns
386 * The lock operation has already been checked that it is enabled, so
387 * it just grabs the lock and synchronizes with the previous unlock.
389 * The unlock operation has to re-enable all of the threads that are
390 * waiting on the lock.
392 * @return True if synchronization was updated; false otherwise
394 bool ModelChecker::process_mutex(ModelAction *curr) {
395 std::mutex *mutex = (std::mutex *)curr->get_location();
396 struct std::mutex_state *state = mutex->get_state();
397 switch (curr->get_type()) {
398 case ATOMIC_TRYLOCK: {
399 bool success = !state->islocked;
400 curr->set_try_lock(success);
402 get_thread(curr)->set_return_value(0);
405 get_thread(curr)->set_return_value(1);
407 //otherwise fall into the lock case
409 if (curr->get_cv()->getClock(state->alloc_tid) <= state->alloc_clock) {
410 printf("Lock access before initialization\n");
413 state->islocked = true;
414 ModelAction *unlock = get_last_unlock(curr);
415 //synchronize with the previous unlock statement
416 if (unlock != NULL) {
417 curr->synchronize_with(unlock);
422 case ATOMIC_UNLOCK: {
424 state->islocked = false;
425 //wake up the other threads
426 action_list_t *waiters = lock_waiters_map->get_safe_ptr(curr->get_location());
427 //activate all the waiting threads
428 for (action_list_t::iterator rit = waiters->begin(); rit != waiters->end(); rit++) {
429 scheduler->wake(get_thread(*rit));
441 * Process a write ModelAction
442 * @param curr The ModelAction to process
443 * @return True if the mo_graph was updated or promises were resolved
445 bool ModelChecker::process_write(ModelAction *curr)
447 bool updated_mod_order = w_modification_order(curr);
448 bool updated_promises = resolve_promises(curr);
450 if (promises->size() == 0) {
451 for (unsigned int i = 0; i < futurevalues->size(); i++) {
452 struct PendingFutureValue pfv = (*futurevalues)[i];
453 if (pfv.act->get_node()->add_future_value(pfv.value, pfv.expiration) &&
454 (!priv->next_backtrack || *pfv.act > *priv->next_backtrack))
455 priv->next_backtrack = pfv.act;
457 futurevalues->resize(0);
460 mo_graph->commitChanges();
461 get_thread(curr)->set_return_value(VALUE_NONE);
462 return updated_mod_order || updated_promises;
466 * @brief Process the current action for thread-related activity
468 * Performs current-action processing for a THREAD_* ModelAction. Proccesses
469 * may include setting Thread status, completing THREAD_FINISH/THREAD_JOIN
470 * synchronization, etc. This function is a no-op for non-THREAD actions
471 * (e.g., ATOMIC_{READ,WRITE,RMW,LOCK}, etc.)
473 * @param curr The current action
474 * @return True if synchronization was updated
476 bool ModelChecker::process_thread_action(ModelAction *curr)
478 bool synchronized = false;
480 switch (curr->get_type()) {
481 case THREAD_CREATE: {
482 Thread *th = (Thread *)curr->get_location();
483 th->set_creation(curr);
487 Thread *waiting, *blocking;
488 waiting = get_thread(curr);
489 blocking = (Thread *)curr->get_location();
490 if (!blocking->is_complete()) {
491 blocking->push_wait_list(curr);
492 scheduler->sleep(waiting);
494 do_complete_join(curr);
499 case THREAD_FINISH: {
500 Thread *th = get_thread(curr);
501 while (!th->wait_list_empty()) {
502 ModelAction *act = th->pop_wait_list();
503 Thread *wake = get_thread(act);
504 scheduler->wake(wake);
505 do_complete_join(act);
512 check_promises(NULL, curr->get_cv());
523 * Initialize the current action by performing one or more of the following
524 * actions, as appropriate: merging RMWR and RMWC/RMW actions, stepping forward
525 * in the NodeStack, manipulating backtracking sets, allocating and
526 * initializing clock vectors, and computing the promises to fulfill.
528 * @param curr The current action, as passed from the user context; may be
529 * freed/invalidated after the execution of this function
530 * @return The current action, as processed by the ModelChecker. Is only the
531 * same as the parameter @a curr if this is a newly-explored action.
533 ModelAction * ModelChecker::initialize_curr_action(ModelAction *curr)
535 ModelAction *newcurr;
537 if (curr->is_rmwc() || curr->is_rmw()) {
538 newcurr = process_rmw(curr);
541 if (newcurr->is_rmw())
542 compute_promises(newcurr);
546 newcurr = node_stack->explore_action(curr, scheduler->get_enabled());
548 /* First restore type and order in case of RMW operation */
550 newcurr->copy_typeandorder(curr);
552 ASSERT(curr->get_location() == newcurr->get_location());
553 newcurr->copy_from_new(curr);
555 /* Discard duplicate ModelAction; use action from NodeStack */
558 /* Always compute new clock vector */
559 newcurr->create_cv(get_parent_action(newcurr->get_tid()));
563 /* Always compute new clock vector */
564 newcurr->create_cv(get_parent_action(newcurr->get_tid()));
566 * Perform one-time actions when pushing new ModelAction onto
569 if (newcurr->is_write())
570 compute_promises(newcurr);
576 * This method checks whether a model action is enabled at the given point.
577 * At this point, it checks whether a lock operation would be successful at this point.
578 * If not, it puts the thread in a waiter list.
579 * @param curr is the ModelAction to check whether it is enabled.
580 * @return a bool that indicates whether the action is enabled.
582 bool ModelChecker::check_action_enabled(ModelAction *curr) {
583 if (curr->is_lock()) {
584 std::mutex * lock = (std::mutex *)curr->get_location();
585 struct std::mutex_state * state = lock->get_state();
586 if (state->islocked) {
587 //Stick the action in the appropriate waiting queue
588 lock_waiters_map->get_safe_ptr(curr->get_location())->push_back(curr);
597 * This is the heart of the model checker routine. It performs model-checking
598 * actions corresponding to a given "current action." Among other processes, it
599 * calculates reads-from relationships, updates synchronization clock vectors,
600 * forms a memory_order constraints graph, and handles replay/backtrack
601 * execution when running permutations of previously-observed executions.
603 * @param curr The current action to process
604 * @return The next Thread that must be executed. May be NULL if ModelChecker
605 * makes no choice (e.g., according to replay execution, combining RMW actions,
608 Thread * ModelChecker::check_current_action(ModelAction *curr)
612 bool second_part_of_rmw = curr->is_rmwc() || curr->is_rmw();
614 if (!check_action_enabled(curr)) {
615 /* Make the execution look like we chose to run this action
616 * much later, when a lock is actually available to release */
617 get_current_thread()->set_pending(curr);
618 scheduler->sleep(get_current_thread());
619 return get_next_thread(NULL);
622 ModelAction *newcurr = initialize_curr_action(curr);
624 /* Add the action to lists before any other model-checking tasks */
625 if (!second_part_of_rmw)
626 add_action_to_lists(newcurr);
628 /* Build may_read_from set for newly-created actions */
629 if (curr == newcurr && curr->is_read())
630 build_reads_from_past(curr);
633 /* Initialize work_queue with the "current action" work */
634 work_queue_t work_queue(1, CheckCurrWorkEntry(curr));
636 while (!work_queue.empty()) {
637 WorkQueueEntry work = work_queue.front();
638 work_queue.pop_front();
641 case WORK_CHECK_CURR_ACTION: {
642 ModelAction *act = work.action;
643 bool update = false; /* update this location's release seq's */
644 bool update_all = false; /* update all release seq's */
646 if (process_thread_action(curr))
649 if (act->is_read() && process_read(act, second_part_of_rmw))
652 if (act->is_write() && process_write(act))
655 if (act->is_mutex_op() && process_mutex(act))
659 work_queue.push_back(CheckRelSeqWorkEntry(NULL));
661 work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
664 case WORK_CHECK_RELEASE_SEQ:
665 resolve_release_sequences(work.location, &work_queue);
667 case WORK_CHECK_MO_EDGES: {
668 /** @todo Complete verification of work_queue */
669 ModelAction *act = work.action;
670 bool updated = false;
672 if (act->is_read()) {
673 const ModelAction *rf = act->get_reads_from();
674 if (rf != NULL && r_modification_order(act, rf))
677 if (act->is_write()) {
678 if (w_modification_order(act))
681 mo_graph->commitChanges();
684 work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
693 check_curr_backtracking(curr);
695 set_backtracking(curr);
697 return get_next_thread(curr);
701 * Complete a THREAD_JOIN operation, by synchronizing with the THREAD_FINISH
702 * operation from the Thread it is joining with. Must be called after the
703 * completion of the Thread in question.
704 * @param join The THREAD_JOIN action
706 void ModelChecker::do_complete_join(ModelAction *join)
708 Thread *blocking = (Thread *)join->get_location();
709 ModelAction *act = get_last_action(blocking->get_id());
710 join->synchronize_with(act);
713 void ModelChecker::check_curr_backtracking(ModelAction * curr) {
714 Node *currnode = curr->get_node();
715 Node *parnode = currnode->get_parent();
717 if ((!parnode->backtrack_empty() ||
718 !currnode->read_from_empty() ||
719 !currnode->future_value_empty() ||
720 !currnode->promise_empty())
721 && (!priv->next_backtrack ||
722 *curr > *priv->next_backtrack)) {
723 priv->next_backtrack = curr;
727 bool ModelChecker::promises_expired() {
728 for (unsigned int promise_index = 0; promise_index < promises->size(); promise_index++) {
729 Promise *promise = (*promises)[promise_index];
730 if (promise->get_expiration()<priv->used_sequence_numbers) {
737 /** @return whether the current partial trace must be a prefix of a
739 bool ModelChecker::isfeasibleprefix() {
740 return promises->size() == 0 && pending_acq_rel_seq->size() == 0;
743 /** @return whether the current partial trace is feasible. */
744 bool ModelChecker::isfeasible() {
745 if (DBG_ENABLED() && mo_graph->checkForRMWViolation())
746 DEBUG("Infeasible: RMW violation\n");
748 return !mo_graph->checkForRMWViolation() && isfeasibleotherthanRMW();
751 /** @return whether the current partial trace is feasible other than
752 * multiple RMW reading from the same store. */
753 bool ModelChecker::isfeasibleotherthanRMW() {
755 if (mo_graph->checkForCycles())
756 DEBUG("Infeasible: modification order cycles\n");
758 DEBUG("Infeasible: failed promise\n");
760 DEBUG("Infeasible: too many reads\n");
761 if (bad_synchronization)
762 DEBUG("Infeasible: bad synchronization ordering\n");
763 if (promises_expired())
764 DEBUG("Infeasible: promises expired\n");
766 return !mo_graph->checkForCycles() && !failed_promise && !too_many_reads && !bad_synchronization && !promises_expired();
769 /** Returns whether the current completed trace is feasible. */
770 bool ModelChecker::isfinalfeasible() {
771 if (DBG_ENABLED() && promises->size() != 0)
772 DEBUG("Infeasible: unrevolved promises\n");
774 return isfeasible() && promises->size() == 0;
777 /** Close out a RMWR by converting previous RMWR into a RMW or READ. */
778 ModelAction * ModelChecker::process_rmw(ModelAction *act) {
779 int tid = id_to_int(act->get_tid());
780 ModelAction *lastread = get_last_action(tid);
781 lastread->process_rmw(act);
782 if (act->is_rmw() && lastread->get_reads_from()!=NULL) {
783 mo_graph->addRMWEdge(lastread->get_reads_from(), lastread);
784 mo_graph->commitChanges();
790 * Checks whether a thread has read from the same write for too many times
791 * without seeing the effects of a later write.
794 * 1) there must a different write that we could read from that would satisfy the modification order,
795 * 2) we must have read from the same value in excess of maxreads times, and
796 * 3) that other write must have been in the reads_from set for maxreads times.
798 * If so, we decide that the execution is no longer feasible.
800 void ModelChecker::check_recency(ModelAction *curr, const ModelAction *rf) {
801 if (params.maxreads != 0) {
803 if (curr->get_node()->get_read_from_size() <= 1)
805 //Must make sure that execution is currently feasible... We could
806 //accidentally clear by rolling back
809 std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
810 int tid = id_to_int(curr->get_tid());
813 if ((int)thrd_lists->size() <= tid)
815 action_list_t *list = &(*thrd_lists)[tid];
817 action_list_t::reverse_iterator rit = list->rbegin();
819 for (; (*rit) != curr; rit++)
821 /* go past curr now */
824 action_list_t::reverse_iterator ritcopy = rit;
825 //See if we have enough reads from the same value
827 for (; count < params.maxreads; rit++,count++) {
828 if (rit==list->rend())
830 ModelAction *act = *rit;
834 if (act->get_reads_from() != rf)
836 if (act->get_node()->get_read_from_size() <= 1)
839 for (int i = 0; i<curr->get_node()->get_read_from_size(); i++) {
841 const ModelAction * write = curr->get_node()->get_read_from_at(i);
843 //Need a different write
847 /* Test to see whether this is a feasible write to read from*/
848 mo_graph->startChanges();
849 r_modification_order(curr, write);
850 bool feasiblereadfrom = isfeasible();
851 mo_graph->rollbackChanges();
853 if (!feasiblereadfrom)
857 bool feasiblewrite = true;
858 //new we need to see if this write works for everyone
860 for (int loop = count; loop>0; loop--,rit++) {
861 ModelAction *act=*rit;
862 bool foundvalue = false;
863 for (int j = 0; j<act->get_node()->get_read_from_size(); j++) {
864 if (act->get_node()->get_read_from_at(i)==write) {
870 feasiblewrite = false;
875 too_many_reads = true;
883 * Updates the mo_graph with the constraints imposed from the current
886 * Basic idea is the following: Go through each other thread and find
887 * the lastest action that happened before our read. Two cases:
889 * (1) The action is a write => that write must either occur before
890 * the write we read from or be the write we read from.
892 * (2) The action is a read => the write that that action read from
893 * must occur before the write we read from or be the same write.
895 * @param curr The current action. Must be a read.
896 * @param rf The action that curr reads from. Must be a write.
897 * @return True if modification order edges were added; false otherwise
899 bool ModelChecker::r_modification_order(ModelAction *curr, const ModelAction *rf)
901 std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
904 ASSERT(curr->is_read());
906 /* Iterate over all threads */
907 for (i = 0; i < thrd_lists->size(); i++) {
908 /* Iterate over actions in thread, starting from most recent */
909 action_list_t *list = &(*thrd_lists)[i];
910 action_list_t::reverse_iterator rit;
911 for (rit = list->rbegin(); rit != list->rend(); rit++) {
912 ModelAction *act = *rit;
915 * Include at most one act per-thread that "happens
916 * before" curr. Don't consider reflexively.
918 if (act->happens_before(curr) && act != curr) {
919 if (act->is_write()) {
921 mo_graph->addEdge(act, rf);
925 const ModelAction *prevreadfrom = act->get_reads_from();
926 if (prevreadfrom != NULL && rf != prevreadfrom) {
927 mo_graph->addEdge(prevreadfrom, rf);
939 /** This method fixes up the modification order when we resolve a
940 * promises. The basic problem is that actions that occur after the
941 * read curr could not property add items to the modification order
944 * So for each thread, we find the earliest item that happens after
945 * the read curr. This is the item we have to fix up with additional
946 * constraints. If that action is write, we add a MO edge between
947 * the Action rf and that action. If the action is a read, we add a
948 * MO edge between the Action rf, and whatever the read accessed.
950 * @param curr is the read ModelAction that we are fixing up MO edges for.
951 * @param rf is the write ModelAction that curr reads from.
954 void ModelChecker::post_r_modification_order(ModelAction *curr, const ModelAction *rf)
956 std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
958 ASSERT(curr->is_read());
960 /* Iterate over all threads */
961 for (i = 0; i < thrd_lists->size(); i++) {
962 /* Iterate over actions in thread, starting from most recent */
963 action_list_t *list = &(*thrd_lists)[i];
964 action_list_t::reverse_iterator rit;
965 ModelAction *lastact = NULL;
967 /* Find last action that happens after curr that is either not curr or a rmw */
968 for (rit = list->rbegin(); rit != list->rend(); rit++) {
969 ModelAction *act = *rit;
970 if (curr->happens_before(act) && (curr != act || curr->is_rmw())) {
976 /* Include at most one act per-thread that "happens before" curr */
977 if (lastact != NULL) {
979 //Case 1: The resolved read is a RMW, and we need to make sure
980 //that the write portion of the RMW mod order after rf
982 mo_graph->addEdge(rf, lastact);
983 } else if (lastact->is_read()) {
984 //Case 2: The resolved read is a normal read and the next
985 //operation is a read, and we need to make sure the value read
986 //is mod ordered after rf
988 const ModelAction *postreadfrom = lastact->get_reads_from();
989 if (postreadfrom != NULL&&rf != postreadfrom)
990 mo_graph->addEdge(rf, postreadfrom);
992 //Case 3: The resolved read is a normal read and the next
993 //operation is a write, and we need to make sure that the
994 //write is mod ordered after rf
996 mo_graph->addEdge(rf, lastact);
1004 * Updates the mo_graph with the constraints imposed from the current write.
1006 * Basic idea is the following: Go through each other thread and find
1007 * the lastest action that happened before our write. Two cases:
1009 * (1) The action is a write => that write must occur before
1012 * (2) The action is a read => the write that that action read from
1013 * must occur before the current write.
1015 * This method also handles two other issues:
1017 * (I) Sequential Consistency: Making sure that if the current write is
1018 * seq_cst, that it occurs after the previous seq_cst write.
1020 * (II) Sending the write back to non-synchronizing reads.
1022 * @param curr The current action. Must be a write.
1023 * @return True if modification order edges were added; false otherwise
1025 bool ModelChecker::w_modification_order(ModelAction *curr)
1027 std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
1030 ASSERT(curr->is_write());
1032 if (curr->is_seqcst()) {
1033 /* We have to at least see the last sequentially consistent write,
1034 so we are initialized. */
1035 ModelAction *last_seq_cst = get_last_seq_cst(curr);
1036 if (last_seq_cst != NULL) {
1037 mo_graph->addEdge(last_seq_cst, curr);
1042 /* Iterate over all threads */
1043 for (i = 0; i < thrd_lists->size(); i++) {
1044 /* Iterate over actions in thread, starting from most recent */
1045 action_list_t *list = &(*thrd_lists)[i];
1046 action_list_t::reverse_iterator rit;
1047 for (rit = list->rbegin(); rit != list->rend(); rit++) {
1048 ModelAction *act = *rit;
1051 * If RMW, we already have all relevant edges,
1052 * so just skip to next thread.
1053 * If normal write, we need to look at earlier
1054 * actions, so continue processing list.
1063 * Include at most one act per-thread that "happens
1066 if (act->happens_before(curr)) {
1068 * Note: if act is RMW, just add edge:
1070 * The following edge should be handled elsewhere:
1071 * readfrom(act) --mo--> act
1073 if (act->is_write())
1074 mo_graph->addEdge(act, curr);
1075 else if (act->is_read() && act->get_reads_from() != NULL)
1076 mo_graph->addEdge(act->get_reads_from(), curr);
1079 } else if (act->is_read() && !act->is_synchronizing(curr) &&
1080 !act->same_thread(curr)) {
1081 /* We have an action that:
1082 (1) did not happen before us
1083 (2) is a read and we are a write
1084 (3) cannot synchronize with us
1085 (4) is in a different thread
1087 that read could potentially read from our write.
1089 if (thin_air_constraint_may_allow(curr, act)) {
1091 (curr->is_rmw() && act->is_rmw() && curr->get_reads_from() == act->get_reads_from() && isfeasibleotherthanRMW())) {
1092 struct PendingFutureValue pfv = {curr->get_value(),curr->get_seq_number()+params.maxfuturedelay,act};
1093 futurevalues->push_back(pfv);
1103 /** Arbitrary reads from the future are not allowed. Section 29.3
1104 * part 9 places some constraints. This method checks one result of constraint
1105 * constraint. Others require compiler support. */
1106 bool ModelChecker::thin_air_constraint_may_allow(const ModelAction * writer, const ModelAction *reader) {
1107 if (!writer->is_rmw())
1110 if (!reader->is_rmw())
1113 for (const ModelAction *search = writer->get_reads_from(); search != NULL; search = search->get_reads_from()) {
1114 if (search == reader)
1116 if (search->get_tid() == reader->get_tid() &&
1117 search->happens_before(reader))
1125 * Finds the head(s) of the release sequence(s) containing a given ModelAction.
1126 * The ModelAction under consideration is expected to be taking part in
1127 * release/acquire synchronization as an object of the "reads from" relation.
1128 * Note that this can only provide release sequence support for RMW chains
1129 * which do not read from the future, as those actions cannot be traced until
1130 * their "promise" is fulfilled. Similarly, we may not even establish the
1131 * presence of a release sequence with certainty, as some modification order
1132 * constraints may be decided further in the future. Thus, this function
1133 * "returns" two pieces of data: a pass-by-reference vector of @a release_heads
1134 * and a boolean representing certainty.
1136 * @todo Finish lazy updating, when promises are fulfilled in the future
1137 * @param rf The action that might be part of a release sequence. Must be a
1139 * @param release_heads A pass-by-reference style return parameter. After
1140 * execution of this function, release_heads will contain the heads of all the
1141 * relevant release sequences, if any exists
1142 * @return true, if the ModelChecker is certain that release_heads is complete;
1145 bool ModelChecker::release_seq_head(const ModelAction *rf, rel_heads_list_t *release_heads) const
1147 /* Only check for release sequences if there are no cycles */
1148 if (mo_graph->checkForCycles())
1152 ASSERT(rf->is_write());
1154 if (rf->is_release())
1155 release_heads->push_back(rf);
1157 break; /* End of RMW chain */
1159 /** @todo Need to be smarter here... In the linux lock
1160 * example, this will run to the beginning of the program for
1162 /** @todo The way to be smarter here is to keep going until 1
1163 * thread has a release preceded by an acquire and you've seen
1166 /* acq_rel RMW is a sufficient stopping condition */
1167 if (rf->is_acquire() && rf->is_release())
1168 return true; /* complete */
1170 rf = rf->get_reads_from();
1173 /* read from future: need to settle this later */
1174 return false; /* incomplete */
1177 if (rf->is_release())
1178 return true; /* complete */
1180 /* else relaxed write; check modification order for contiguous subsequence
1181 * -> rf must be same thread as release */
1182 int tid = id_to_int(rf->get_tid());
1183 std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(rf->get_location());
1184 action_list_t *list = &(*thrd_lists)[tid];
1185 action_list_t::const_reverse_iterator rit;
1187 /* Find rf in the thread list */
1188 rit = std::find(list->rbegin(), list->rend(), rf);
1189 ASSERT(rit != list->rend());
1191 /* Find the last write/release */
1192 for (; rit != list->rend(); rit++)
1193 if ((*rit)->is_release())
1195 if (rit == list->rend()) {
1196 /* No write-release in this thread */
1197 return true; /* complete */
1199 ModelAction *release = *rit;
1201 ASSERT(rf->same_thread(release));
1203 bool certain = true;
1204 for (unsigned int i = 0; i < thrd_lists->size(); i++) {
1205 if (id_to_int(rf->get_tid()) == (int)i)
1207 list = &(*thrd_lists)[i];
1209 /* Can we ensure no future writes from this thread may break
1210 * the release seq? */
1211 bool future_ordered = false;
1213 ModelAction *last = get_last_action(int_to_id(i));
1214 if (last && (rf->happens_before(last) ||
1215 last->get_type() == THREAD_FINISH))
1216 future_ordered = true;
1218 for (rit = list->rbegin(); rit != list->rend(); rit++) {
1219 const ModelAction *act = *rit;
1220 /* Reach synchronization -> this thread is complete */
1221 if (act->happens_before(release))
1223 if (rf->happens_before(act)) {
1224 future_ordered = true;
1228 /* Only writes can break release sequences */
1229 if (!act->is_write())
1232 /* Check modification order */
1233 if (mo_graph->checkReachable(rf, act)) {
1234 /* rf --mo--> act */
1235 future_ordered = true;
1238 if (mo_graph->checkReachable(act, release))
1239 /* act --mo--> release */
1241 if (mo_graph->checkReachable(release, act) &&
1242 mo_graph->checkReachable(act, rf)) {
1243 /* release --mo-> act --mo--> rf */
1244 return true; /* complete */
1248 if (!future_ordered)
1249 return false; /* This thread is uncertain */
1253 release_heads->push_back(release);
1258 * A public interface for getting the release sequence head(s) with which a
1259 * given ModelAction must synchronize. This function only returns a non-empty
1260 * result when it can locate a release sequence head with certainty. Otherwise,
1261 * it may mark the internal state of the ModelChecker so that it will handle
1262 * the release sequence at a later time, causing @a act to update its
1263 * synchronization at some later point in execution.
1264 * @param act The 'acquire' action that may read from a release sequence
1265 * @param release_heads A pass-by-reference return parameter. Will be filled
1266 * with the head(s) of the release sequence(s), if they exists with certainty.
1267 * @see ModelChecker::release_seq_head
1269 void ModelChecker::get_release_seq_heads(ModelAction *act, rel_heads_list_t *release_heads)
1271 const ModelAction *rf = act->get_reads_from();
1273 complete = release_seq_head(rf, release_heads);
1275 /* add act to 'lazy checking' list */
1276 pending_acq_rel_seq->push_back(act);
1281 * Attempt to resolve all stashed operations that might synchronize with a
1282 * release sequence for a given location. This implements the "lazy" portion of
1283 * determining whether or not a release sequence was contiguous, since not all
1284 * modification order information is present at the time an action occurs.
1286 * @param location The location/object that should be checked for release
1287 * sequence resolutions. A NULL value means to check all locations.
1288 * @param work_queue The work queue to which to add work items as they are
1290 * @return True if any updates occurred (new synchronization, new mo_graph
1293 bool ModelChecker::resolve_release_sequences(void *location, work_queue_t *work_queue)
1295 bool updated = false;
1296 std::vector<ModelAction *>::iterator it = pending_acq_rel_seq->begin();
1297 while (it != pending_acq_rel_seq->end()) {
1298 ModelAction *act = *it;
1300 /* Only resolve sequences on the given location, if provided */
1301 if (location && act->get_location() != location) {
1306 const ModelAction *rf = act->get_reads_from();
1307 rel_heads_list_t release_heads;
1309 complete = release_seq_head(rf, &release_heads);
1310 for (unsigned int i = 0; i < release_heads.size(); i++) {
1311 if (!act->has_synchronized_with(release_heads[i])) {
1312 if (act->synchronize_with(release_heads[i]))
1315 set_bad_synchronization();
1320 /* Re-check act for mo_graph edges */
1321 work_queue->push_back(MOEdgeWorkEntry(act));
1323 /* propagate synchronization to later actions */
1324 action_list_t::reverse_iterator rit = action_trace->rbegin();
1325 for (; (*rit) != act; rit++) {
1326 ModelAction *propagate = *rit;
1327 if (act->happens_before(propagate)) {
1328 propagate->synchronize_with(act);
1329 /* Re-check 'propagate' for mo_graph edges */
1330 work_queue->push_back(MOEdgeWorkEntry(propagate));
1335 it = pending_acq_rel_seq->erase(it);
1340 // If we resolved promises or data races, see if we have realized a data race.
1341 if (checkDataRaces()) {
1349 * Performs various bookkeeping operations for the current ModelAction. For
1350 * instance, adds action to the per-object, per-thread action vector and to the
1351 * action trace list of all thread actions.
1353 * @param act is the ModelAction to add.
1355 void ModelChecker::add_action_to_lists(ModelAction *act)
1357 int tid = id_to_int(act->get_tid());
1358 action_trace->push_back(act);
1360 obj_map->get_safe_ptr(act->get_location())->push_back(act);
1362 std::vector<action_list_t> *vec = obj_thrd_map->get_safe_ptr(act->get_location());
1363 if (tid >= (int)vec->size())
1364 vec->resize(priv->next_thread_id);
1365 (*vec)[tid].push_back(act);
1367 if ((int)thrd_last_action->size() <= tid)
1368 thrd_last_action->resize(get_num_threads());
1369 (*thrd_last_action)[tid] = act;
1373 * @brief Get the last action performed by a particular Thread
1374 * @param tid The thread ID of the Thread in question
1375 * @return The last action in the thread
1377 ModelAction * ModelChecker::get_last_action(thread_id_t tid) const
1379 int threadid = id_to_int(tid);
1380 if (threadid < (int)thrd_last_action->size())
1381 return (*thrd_last_action)[id_to_int(tid)];
1387 * Gets the last memory_order_seq_cst write (in the total global sequence)
1388 * performed on a particular object (i.e., memory location), not including the
1390 * @param curr The current ModelAction; also denotes the object location to
1392 * @return The last seq_cst write
1394 ModelAction * ModelChecker::get_last_seq_cst(ModelAction *curr) const
1396 void *location = curr->get_location();
1397 action_list_t *list = obj_map->get_safe_ptr(location);
1398 /* Find: max({i in dom(S) | seq_cst(t_i) && isWrite(t_i) && samevar(t_i, t)}) */
1399 action_list_t::reverse_iterator rit;
1400 for (rit = list->rbegin(); rit != list->rend(); rit++)
1401 if ((*rit)->is_write() && (*rit)->is_seqcst() && (*rit) != curr)
1407 * Gets the last unlock operation performed on a particular mutex (i.e., memory
1408 * location). This function identifies the mutex according to the current
1409 * action, which is presumed to perform on the same mutex.
1410 * @param curr The current ModelAction; also denotes the object location to
1412 * @return The last unlock operation
1414 ModelAction * ModelChecker::get_last_unlock(ModelAction *curr) const
1416 void *location = curr->get_location();
1417 action_list_t *list = obj_map->get_safe_ptr(location);
1418 /* Find: max({i in dom(S) | isUnlock(t_i) && samevar(t_i, t)}) */
1419 action_list_t::reverse_iterator rit;
1420 for (rit = list->rbegin(); rit != list->rend(); rit++)
1421 if ((*rit)->is_unlock())
1426 ModelAction * ModelChecker::get_parent_action(thread_id_t tid)
1428 ModelAction *parent = get_last_action(tid);
1430 parent = get_thread(tid)->get_creation();
1435 * Returns the clock vector for a given thread.
1436 * @param tid The thread whose clock vector we want
1437 * @return Desired clock vector
1439 ClockVector * ModelChecker::get_cv(thread_id_t tid)
1441 return get_parent_action(tid)->get_cv();
1445 * Resolve a set of Promises with a current write. The set is provided in the
1446 * Node corresponding to @a write.
1447 * @param write The ModelAction that is fulfilling Promises
1448 * @return True if promises were resolved; false otherwise
1450 bool ModelChecker::resolve_promises(ModelAction *write)
1452 bool resolved = false;
1454 for (unsigned int i = 0, promise_index = 0; promise_index < promises->size(); i++) {
1455 Promise *promise = (*promises)[promise_index];
1456 if (write->get_node()->get_promise(i)) {
1457 ModelAction *read = promise->get_action();
1458 if (read->is_rmw()) {
1459 mo_graph->addRMWEdge(write, read);
1461 read->read_from(write);
1462 //First fix up the modification order for actions that happened
1464 r_modification_order(read, write);
1465 //Next fix up the modification order for actions that happened
1467 post_r_modification_order(read, write);
1468 //Make sure the promise's value matches the write's value
1469 ASSERT(promise->get_value() == write->get_value());
1471 promises->erase(promises->begin() + promise_index);
1480 * Compute the set of promises that could potentially be satisfied by this
1481 * action. Note that the set computation actually appears in the Node, not in
1483 * @param curr The ModelAction that may satisfy promises
1485 void ModelChecker::compute_promises(ModelAction *curr)
1487 for (unsigned int i = 0; i < promises->size(); i++) {
1488 Promise *promise = (*promises)[i];
1489 const ModelAction *act = promise->get_action();
1490 if (!act->happens_before(curr) &&
1492 !act->is_synchronizing(curr) &&
1493 !act->same_thread(curr) &&
1494 promise->get_value() == curr->get_value()) {
1495 curr->get_node()->set_promise(i);
1500 /** Checks promises in response to change in ClockVector Threads. */
1501 void ModelChecker::check_promises(ClockVector *old_cv, ClockVector *merge_cv)
1503 for (unsigned int i = 0; i < promises->size(); i++) {
1504 Promise *promise = (*promises)[i];
1505 const ModelAction *act = promise->get_action();
1506 if ((old_cv == NULL || !old_cv->synchronized_since(act)) &&
1507 merge_cv->synchronized_since(act)) {
1508 //This thread is no longer able to send values back to satisfy the promise
1509 int num_synchronized_threads = promise->increment_threads();
1510 if (num_synchronized_threads == get_num_threads()) {
1511 //Promise has failed
1512 failed_promise = true;
1520 * Build up an initial set of all past writes that this 'read' action may read
1521 * from. This set is determined by the clock vector's "happens before"
1523 * @param curr is the current ModelAction that we are exploring; it must be a
1526 void ModelChecker::build_reads_from_past(ModelAction *curr)
1528 std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
1530 ASSERT(curr->is_read());
1532 ModelAction *last_seq_cst = NULL;
1534 /* Track whether this object has been initialized */
1535 bool initialized = false;
1537 if (curr->is_seqcst()) {
1538 last_seq_cst = get_last_seq_cst(curr);
1539 /* We have to at least see the last sequentially consistent write,
1540 so we are initialized. */
1541 if (last_seq_cst != NULL)
1545 /* Iterate over all threads */
1546 for (i = 0; i < thrd_lists->size(); i++) {
1547 /* Iterate over actions in thread, starting from most recent */
1548 action_list_t *list = &(*thrd_lists)[i];
1549 action_list_t::reverse_iterator rit;
1550 for (rit = list->rbegin(); rit != list->rend(); rit++) {
1551 ModelAction *act = *rit;
1553 /* Only consider 'write' actions */
1554 if (!act->is_write() || act == curr)
1557 /* Don't consider more than one seq_cst write if we are a seq_cst read. */
1558 if (!curr->is_seqcst() || (!act->is_seqcst() && (last_seq_cst == NULL || !act->happens_before(last_seq_cst))) || act == last_seq_cst) {
1559 DEBUG("Adding action to may_read_from:\n");
1560 if (DBG_ENABLED()) {
1564 curr->get_node()->add_read_from(act);
1567 /* Include at most one act per-thread that "happens before" curr */
1568 if (act->happens_before(curr)) {
1576 /** @todo Need a more informative way of reporting errors. */
1577 printf("ERROR: may read from uninitialized atomic\n");
1580 if (DBG_ENABLED() || !initialized) {
1581 printf("Reached read action:\n");
1583 printf("Printing may_read_from\n");
1584 curr->get_node()->print_may_read_from();
1585 printf("End printing may_read_from\n");
1588 ASSERT(initialized);
1591 static void print_list(action_list_t *list)
1593 action_list_t::iterator it;
1595 printf("---------------------------------------------------------------------\n");
1598 for (it = list->begin(); it != list->end(); it++) {
1601 printf("---------------------------------------------------------------------\n");
1604 #if SUPPORT_MOD_ORDER_DUMP
1605 void ModelChecker::dumpGraph(char *filename) {
1607 sprintf(buffer, "%s.dot",filename);
1608 FILE *file=fopen(buffer, "w");
1609 fprintf(file, "digraph %s {\n",filename);
1610 mo_graph->dumpNodes(file);
1611 ModelAction ** thread_array=(ModelAction **)model_calloc(1, sizeof(ModelAction *)*get_num_threads());
1613 for (action_list_t::iterator it = action_trace->begin(); it != action_trace->end(); it++) {
1614 ModelAction *action=*it;
1615 if (action->is_read()) {
1616 fprintf(file, "N%u [label=\"%u, T%u\"];\n", action->get_seq_number(),action->get_seq_number(), action->get_tid());
1617 fprintf(file, "N%u -> N%u[label=\"rf\", color=red];\n", action->get_seq_number(), action->get_reads_from()->get_seq_number());
1619 if (thread_array[action->get_tid()] != NULL) {
1620 fprintf(file, "N%u -> N%u[label=\"sb\", color=blue];\n", thread_array[action->get_tid()]->get_seq_number(), action->get_seq_number());
1623 thread_array[action->get_tid()]=action;
1625 fprintf(file,"}\n");
1626 model_free(thread_array);
1631 void ModelChecker::print_summary()
1634 printf("Number of executions: %d\n", num_executions);
1635 printf("Number of feasible executions: %d\n", num_feasible_executions);
1636 printf("Total nodes created: %d\n", node_stack->get_total_nodes());
1638 #if SUPPORT_MOD_ORDER_DUMP
1640 char buffername[100];
1641 sprintf(buffername, "exec%04u", num_executions);
1642 mo_graph->dumpGraphToFile(buffername);
1643 sprintf(buffername, "graph%04u", num_executions);
1644 dumpGraph(buffername);
1647 if (!isfinalfeasible())
1648 printf("INFEASIBLE EXECUTION!\n");
1649 print_list(action_trace);
1654 * Add a Thread to the system for the first time. Should only be called once
1656 * @param t The Thread to add
1658 void ModelChecker::add_thread(Thread *t)
1660 thread_map->put(id_to_int(t->get_id()), t);
1661 scheduler->add_thread(t);
1665 * Removes a thread from the scheduler.
1666 * @param the thread to remove.
1668 void ModelChecker::remove_thread(Thread *t)
1670 scheduler->remove_thread(t);
1674 * Switch from a user-context to the "master thread" context (a.k.a. system
1675 * context). This switch is made with the intention of exploring a particular
1676 * model-checking action (described by a ModelAction object). Must be called
1677 * from a user-thread context.
1678 * @param act The current action that will be explored. Must not be NULL.
1679 * @return Return status from the 'swap' call (i.e., success/fail, 0/-1)
1681 int ModelChecker::switch_to_master(ModelAction *act)
1684 Thread *old = thread_current();
1685 set_current_action(act);
1686 old->set_state(THREAD_READY);
1687 return Thread::swap(old, &system_context);
1691 * Takes the next step in the execution, if possible.
1692 * @return Returns true (success) if a step was taken and false otherwise.
1694 bool ModelChecker::take_step() {
1698 Thread * curr = thread_current();
1700 if (curr->get_state() == THREAD_READY) {
1701 ASSERT(priv->current_action);
1703 priv->nextThread = check_current_action(priv->current_action);
1704 priv->current_action = NULL;
1706 if (curr->is_blocked() || curr->is_complete())
1707 scheduler->remove_thread(curr);
1712 Thread * next = scheduler->next_thread(priv->nextThread);
1714 /* Infeasible -> don't take any more steps */
1719 next->set_state(THREAD_RUNNING);
1720 DEBUG("(%d, %d)\n", curr ? curr->get_id() : -1, next ? next->get_id() : -1);
1722 /* next == NULL -> don't take any more steps */
1726 if ( next->get_pending() != NULL ) {
1727 //restart a pending action
1728 set_current_action(next->get_pending());
1729 next->set_pending(NULL);
1730 next->set_state(THREAD_READY);
1734 /* Return false only if swap fails with an error */
1735 return (Thread::swap(&system_context, next) == 0);
1738 /** Runs the current execution until threre are no more steps to take. */
1739 void ModelChecker::finish_execution() {
1742 while (take_step());