model.cc

   1 #include <stdio.h>
   2 #include <algorithm>
   3
   4 #include "model.h"
   5 #include "action.h"
   6 #include "nodestack.h"
   7 #include "schedule.h"
   8 #include "snapshot-interface.h"
   9 #include "common.h"
  10 #include "clockvector.h"
  11 #include "cyclegraph.h"
  12 #include "promise.h"
  13 #include "datarace.h"
  14
  15 #define INITIAL_THREAD_ID       0
  16
  17 ModelChecker *model;
  18
  19 /** @brief Constructor */
  20 ModelChecker::ModelChecker(struct model_params params) :
  21         /* Initialize default scheduler */
  22         scheduler(new Scheduler()),
  23         num_executions(0),
  24         num_feasible_executions(0),
  25         params(params),
  26         diverge(NULL),
  27         action_trace(new action_list_t()),
  28         thread_map(new HashTable<int, Thread *, int>()),
  29         obj_map(new HashTable<const void *, action_list_t, uintptr_t, 4>()),
  30         obj_thrd_map(new HashTable<void *, std::vector<action_list_t>, uintptr_t, 4 >()),
  31         promises(new std::vector<Promise *>()),
  32         futurevalues(new std::vector<struct PendingFutureValue>()),
  33         lazy_sync_with_release(new HashTable<void *, std::list<ModelAction *>, uintptr_t, 4>()),
  34         thrd_last_action(new std::vector<ModelAction *>(1)),
  35         node_stack(new NodeStack()),
  36         mo_graph(new CycleGraph()),
  37         failed_promise(false),
  38         too_many_reads(false),
  39         asserted(false)
  40 {
  41         /* Allocate this "size" on the snapshotting heap */
  42         priv = (struct model_snapshot_members *)calloc(1, sizeof(*priv));
  43         /* First thread created will have id INITIAL_THREAD_ID */
  44         priv->next_thread_id = INITIAL_THREAD_ID;
  45
  46         lazy_sync_size = &priv->lazy_sync_size;
  47 }
  48
  49 /** @brief Destructor */
  50 ModelChecker::~ModelChecker()
  51 {
  52         for (int i = 0; i < get_num_threads(); i++)
  53                 delete thread_map->get(i);
  54         delete thread_map;
  55
  56         delete obj_thrd_map;
  57         delete obj_map;
  58         delete action_trace;
  59
  60         for (unsigned int i = 0; i < promises->size(); i++)
  61                 delete (*promises)[i];
  62         delete promises;
  63
  64         delete lazy_sync_with_release;
  65
  66         delete thrd_last_action;
  67         delete node_stack;
  68         delete scheduler;
  69         delete mo_graph;
  70 }
  71
  72 /**
  73  * Restores user program to initial state and resets all model-checker data
  74  * structures.
  75  */
  76 void ModelChecker::reset_to_initial_state()
  77 {
  78         DEBUG("+++ Resetting to initial state +++\n");
  79         node_stack->reset_execution();
  80         failed_promise = false;
  81         too_many_reads = false;
  82         reset_asserted();
  83         snapshotObject->backTrackBeforeStep(0);
  84 }
  85
  86 /** @returns a thread ID for a new Thread */
  87 thread_id_t ModelChecker::get_next_id()
  88 {
  89         return priv->next_thread_id++;
  90 }
  91
  92 /** @returns the number of user threads created during this execution */
  93 int ModelChecker::get_num_threads()
  94 {
  95         return priv->next_thread_id;
  96 }
  97
  98 /** @returns a sequence number for a new ModelAction */
  99 modelclock_t ModelChecker::get_next_seq_num()
 100 {
 101         return ++priv->used_sequence_numbers;
 102 }
 103
 104 /**
 105  * @brief Choose the next thread to execute.
 106  *
 107  * This function chooses the next thread that should execute. It can force the
 108  * adjacency of read/write portions of a RMW action, force THREAD_CREATE to be
 109  * followed by a THREAD_START, or it can enforce execution replay/backtracking.
 110  * The model-checker may have no preference regarding the next thread (i.e.,
 111  * when exploring a new execution ordering), in which case this will return
 112  * NULL.
 113  * @param curr The current ModelAction. This action might guide the choice of
 114  * next thread.
 115  * @return The next thread to run. If the model-checker has no preference, NULL.
 116  */
 117 Thread * ModelChecker::get_next_thread(ModelAction *curr)
 118 {
 119         thread_id_t tid;
 120
 121         /* Do not split atomic actions. */
 122         if (curr->is_rmwr())
 123                 return thread_current();
 124         /* The THREAD_CREATE action points to the created Thread */
 125         else if (curr->get_type() == THREAD_CREATE)
 126                 return (Thread *)curr->get_location();
 127
 128         /* Have we completed exploring the preselected path? */
 129         if (diverge == NULL)
 130                 return NULL;
 131
 132         /* Else, we are trying to replay an execution */
 133         ModelAction *next = node_stack->get_next()->get_action();
 134
 135         if (next == diverge) {
 136                 Node *nextnode = next->get_node();
 137                 /* Reached divergence point */
 138                 if (nextnode->increment_promise()) {
 139                         /* The next node will try to satisfy a different set of promises. */
 140                         tid = next->get_tid();
 141                         node_stack->pop_restofstack(2);
 142                 } else if (nextnode->increment_read_from()) {
 143                         /* The next node will read from a different value. */
 144                         tid = next->get_tid();
 145                         node_stack->pop_restofstack(2);
 146                 } else if (nextnode->increment_future_value()) {
 147                         /* The next node will try to read from a different future value. */
 148                         tid = next->get_tid();
 149                         node_stack->pop_restofstack(2);
 150                 } else {
 151                         /* Make a different thread execute for next step */
 152                         Node *node = nextnode->get_parent();
 153                         tid = node->get_next_backtrack();
 154                         node_stack->pop_restofstack(1);
 155                 }
 156                 DEBUG("*** Divergence point ***\n");
 157                 diverge = NULL;
 158         } else {
 159                 tid = next->get_tid();
 160         }
 161         DEBUG("*** ModelChecker chose next thread = %d ***\n", tid);
 162         ASSERT(tid != THREAD_ID_T_NONE);
 163         return thread_map->get(id_to_int(tid));
 164 }
 165
 166 /**
 167  * Queries the model-checker for more executions to explore and, if one
 168  * exists, resets the model-checker state to execute a new execution.
 169  *
 170  * @return If there are more executions to explore, return true. Otherwise,
 171  * return false.
 172  */
 173 bool ModelChecker::next_execution()
 174 {
 175         DBG();
 176
 177         num_executions++;
 178         if (isfinalfeasible())
 179                 num_feasible_executions++;
 180
 181         if (isfinalfeasible() || DBG_ENABLED())
 182                 print_summary();
 183
 184         if ((diverge = get_next_backtrack()) == NULL)
 185                 return false;
 186
 187         if (DBG_ENABLED()) {
 188                 printf("Next execution will diverge at:\n");
 189                 diverge->print();
 190         }
 191
 192         reset_to_initial_state();
 193         return true;
 194 }
 195
 196 ModelAction * ModelChecker::get_last_conflict(ModelAction *act)
 197 {
 198         action_type type = act->get_type();
 199
 200         switch (type) {
 201                 case ATOMIC_READ:
 202                 case ATOMIC_WRITE:
 203                 case ATOMIC_RMW:
 204                         break;
 205                 default:
 206                         return NULL;
 207         }
 208         /* linear search: from most recent to oldest */
 209         action_list_t *list = obj_map->get_safe_ptr(act->get_location());
 210         action_list_t::reverse_iterator rit;
 211         for (rit = list->rbegin(); rit != list->rend(); rit++) {
 212                 ModelAction *prev = *rit;
 213                 if (act->is_synchronizing(prev))
 214                         return prev;
 215         }
 216         return NULL;
 217 }
 218
 219 void ModelChecker::set_backtracking(ModelAction *act)
 220 {
 221         ModelAction *prev;
 222         Node *node;
 223         Thread *t = get_thread(act);
 224
 225         prev = get_last_conflict(act);
 226         if (prev == NULL)
 227                 return;
 228
 229         node = prev->get_node()->get_parent();
 230
 231         while (!node->is_enabled(t))
 232                 t = t->get_parent();
 233
 234         /* Check if this has been explored already */
 235         if (node->has_been_explored(t->get_id()))
 236                 return;
 237
 238         /* Cache the latest backtracking point */
 239         if (!priv->next_backtrack || *prev > *priv->next_backtrack)
 240                 priv->next_backtrack = prev;
 241
 242         /* If this is a new backtracking point, mark the tree */
 243         if (!node->set_backtrack(t->get_id()))
 244                 return;
 245         DEBUG("Setting backtrack: conflict = %d, instead tid = %d\n",
 246                         prev->get_tid(), t->get_id());
 247         if (DBG_ENABLED()) {
 248                 prev->print();
 249                 act->print();
 250         }
 251 }
 252
 253 /**
 254  * Returns last backtracking point. The model checker will explore a different
 255  * path for this point in the next execution.
 256  * @return The ModelAction at which the next execution should diverge.
 257  */
 258 ModelAction * ModelChecker::get_next_backtrack()
 259 {
 260         ModelAction *next = priv->next_backtrack;
 261         priv->next_backtrack = NULL;
 262         return next;
 263 }
 264
 265 /**
 266  * Processes a read or rmw model action.
 267  * @param curr is the read model action to process.
 268  * @param second_part_of_rmw is boolean that is true is this is the second action of a rmw.
 269  * @return True if processing this read updates the mo_graph.
 270  */
 271 bool ModelChecker::process_read(ModelAction *curr, bool second_part_of_rmw)
 272 {
 273         uint64_t value;
 274         bool updated = false;
 275         while (true) {
 276                 const ModelAction *reads_from = curr->get_node()->get_read_from();
 277                 if (reads_from != NULL) {
 278                         mo_graph->startChanges();
 279
 280                         value = reads_from->get_value();
 281                         bool r_status = false;
 282
 283                         if (!second_part_of_rmw) {
 284                                 check_recency(curr);
 285                                 r_status = r_modification_order(curr, reads_from);
 286                         }
 287
 288
 289                         if (!second_part_of_rmw&&!isfeasible()&&(curr->get_node()->increment_read_from()||curr->get_node()->increment_future_value())) {
 290                                 mo_graph->rollbackChanges();
 291                                 too_many_reads = false;
 292                                 continue;
 293                         }
 294
 295                         curr->read_from(reads_from);
 296                         mo_graph->commitChanges();
 297                         updated |= r_status;
 298                 } else if (!second_part_of_rmw) {
 299                         /* Read from future value */
 300                         value = curr->get_node()->get_future_value();
 301                         modelclock_t expiration = curr->get_node()->get_future_value_expiration();
 302                         curr->read_from(NULL);
 303                         Promise *valuepromise = new Promise(curr, value, expiration);
 304                         promises->push_back(valuepromise);
 305                 }
 306                 get_thread(curr)->set_return_value(value);
 307                 return updated;
 308         }
 309 }
 310
 311 /**
 312  * Process a write ModelAction
 313  * @param curr The ModelAction to process
 314  * @return True if the mo_graph was updated or promises were resolved
 315  */
 316 bool ModelChecker::process_write(ModelAction *curr)
 317 {
 318         bool updated_mod_order = w_modification_order(curr);
 319         bool updated_promises = resolve_promises(curr);
 320
 321         if (promises->size() == 0) {
 322                 for (unsigned int i = 0; i<futurevalues->size(); i++) {
 323                         struct PendingFutureValue pfv = (*futurevalues)[i];
 324                         if (pfv.act->get_node()->add_future_value(pfv.value, pfv.expiration) &&
 325                                         (!priv->next_backtrack || *pfv.act > *priv->next_backtrack))
 326                                 priv->next_backtrack = pfv.act;
 327                 }
 328                 futurevalues->resize(0);
 329         }
 330
 331         mo_graph->commitChanges();
 332         get_thread(curr)->set_return_value(VALUE_NONE);
 333         return updated_mod_order || updated_promises;
 334 }
 335
 336 /**
 337  * This is the heart of the model checker routine. It performs model-checking
 338  * actions corresponding to a given "current action." Among other processes, it
 339  * calculates reads-from relationships, updates synchronization clock vectors,
 340  * forms a memory_order constraints graph, and handles replay/backtrack
 341  * execution when running permutations of previously-observed executions.
 342  *
 343  * @param curr The current action to process
 344  * @return The next Thread that must be executed. May be NULL if ModelChecker
 345  * makes no choice (e.g., according to replay execution, combining RMW actions,
 346  * etc.)
 347  */
 348 Thread * ModelChecker::check_current_action(ModelAction *curr)
 349 {
 350         bool second_part_of_rmw = false;
 351
 352         ASSERT(curr);
 353
 354         if (curr->is_rmwc() || curr->is_rmw()) {
 355                 ModelAction *tmp = process_rmw(curr);
 356                 second_part_of_rmw = true;
 357                 delete curr;
 358                 curr = tmp;
 359                 compute_promises(curr);
 360         } else {
 361                 ModelAction *tmp = node_stack->explore_action(curr);
 362                 if (tmp) {
 363                         /* Discard duplicate ModelAction; use action from NodeStack */
 364                         /* First restore type and order in case of RMW operation */
 365                         if (curr->is_rmwr())
 366                                 tmp->copy_typeandorder(curr);
 367
 368                         /* If we have diverged, we need to reset the clock vector. */
 369                         if (diverge == NULL)
 370                                 tmp->create_cv(get_parent_action(tmp->get_tid()));
 371
 372                         delete curr;
 373                         curr = tmp;
 374                 } else {
 375                         /*
 376                          * Perform one-time actions when pushing new ModelAction onto
 377                          * NodeStack
 378                          */
 379                         curr->create_cv(get_parent_action(curr->get_tid()));
 380                         /* Build may_read_from set */
 381                         if (curr->is_read())
 382                                 build_reads_from_past(curr);
 383                         if (curr->is_write())
 384                                 compute_promises(curr);
 385                 }
 386         }
 387
 388         /* Thread specific actions */
 389         switch (curr->get_type()) {
 390         case THREAD_CREATE: {
 391                 Thread *th = (Thread *)curr->get_location();
 392                 th->set_creation(curr);
 393                 break;
 394         }
 395         case THREAD_JOIN: {
 396                 Thread *waiting, *blocking;
 397                 waiting = get_thread(curr);
 398                 blocking = (Thread *)curr->get_location();
 399                 if (!blocking->is_complete()) {
 400                         blocking->push_wait_list(curr);
 401                         scheduler->sleep(waiting);
 402                 }
 403                 break;
 404         }
 405         case THREAD_FINISH: {
 406                 Thread *th = get_thread(curr);
 407                 while (!th->wait_list_empty()) {
 408                         ModelAction *act = th->pop_wait_list();
 409                         Thread *wake = get_thread(act);
 410                         scheduler->wake(wake);
 411                 }
 412                 th->complete();
 413                 break;
 414         }
 415         case THREAD_START: {
 416                 check_promises(NULL, curr->get_cv());
 417                 break;
 418         }
 419         default:
 420                 break;
 421         }
 422
 423         /* Add current action to lists before work_queue loop */
 424         if (!second_part_of_rmw)
 425                 add_action_to_lists(curr);
 426
 427         work_queue_t work_queue(1, CheckCurrWorkEntry(curr));
 428
 429         while (!work_queue.empty()) {
 430                 WorkQueueEntry work = work_queue.front();
 431                 work_queue.pop_front();
 432
 433                 switch (work.type) {
 434                 case WORK_CHECK_CURR_ACTION: {
 435                         ModelAction *act = work.action;
 436                         bool updated = false;
 437                         if (act->is_read() && process_read(act, second_part_of_rmw))
 438                                 updated = true;
 439
 440                         if (act->is_write() && process_write(act))
 441                                 updated = true;
 442
 443                         if (updated)
 444                                 work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
 445                         break;
 446                 }
 447                 case WORK_CHECK_RELEASE_SEQ:
 448                         resolve_release_sequences(work.location, &work_queue);
 449                         break;
 450                 case WORK_CHECK_MO_EDGES:
 451                         /** @todo Perform follow-up mo_graph checks */
 452                 default:
 453                         ASSERT(false);
 454                         break;
 455                 }
 456         }
 457
 458         check_curr_backtracking(curr);
 459
 460         set_backtracking(curr);
 461
 462         return get_next_thread(curr);
 463 }
 464
 465 void ModelChecker::check_curr_backtracking(ModelAction * curr) {
 466         Node *currnode = curr->get_node();
 467         Node *parnode = currnode->get_parent();
 468
 469         if ((!parnode->backtrack_empty() ||
 470                          !currnode->read_from_empty() ||
 471                          !currnode->future_value_empty() ||
 472                          !currnode->promise_empty())
 473                         && (!priv->next_backtrack ||
 474                                         *curr > *priv->next_backtrack)) {
 475                 priv->next_backtrack = curr;
 476         }
 477 }
 478
 479 bool ModelChecker::promises_expired() {
 480         for (unsigned int promise_index = 0; promise_index < promises->size(); promise_index++) {
 481                 Promise *promise = (*promises)[promise_index];
 482                 if (promise->get_expiration()<priv->used_sequence_numbers) {
 483                         return true;
 484                 }
 485         }
 486         return false;
 487 }
 488
 489 /** @returns whether the current partial trace must be a prefix of a
 490  * feasible trace. */
 491 bool ModelChecker::isfeasibleprefix() {
 492         return promises->size() == 0 && *lazy_sync_size == 0;
 493 }
 494
 495 /** @returns whether the current partial trace is feasible. */
 496 bool ModelChecker::isfeasible() {
 497         return !mo_graph->checkForRMWViolation() && isfeasibleotherthanRMW();
 498 }
 499
 500 /** @returns whether the current partial trace is feasible other than
 501  * multiple RMW reading from the same store. */
 502 bool ModelChecker::isfeasibleotherthanRMW() {
 503         return !mo_graph->checkForCycles() && !failed_promise && !too_many_reads && !promises_expired();
 504 }
 505
 506 /** Returns whether the current completed trace is feasible. */
 507 bool ModelChecker::isfinalfeasible() {
 508         return isfeasible() && promises->size() == 0;
 509 }
 510
 511 /** Close out a RMWR by converting previous RMWR into a RMW or READ. */
 512 ModelAction * ModelChecker::process_rmw(ModelAction *act) {
 513         int tid = id_to_int(act->get_tid());
 514         ModelAction *lastread = get_last_action(tid);
 515         lastread->process_rmw(act);
 516         if (act->is_rmw() && lastread->get_reads_from()!=NULL) {
 517                 mo_graph->addRMWEdge(lastread->get_reads_from(), lastread);
 518                 mo_graph->commitChanges();
 519         }
 520         return lastread;
 521 }
 522
 523 /**
 524  * Checks whether a thread has read from the same write for too many times
 525  * without seeing the effects of a later write.
 526  *
 527  * Basic idea:
 528  * 1) there must a different write that we could read from that would satisfy the modification order,
 529  * 2) we must have read from the same value in excess of maxreads times, and
 530  * 3) that other write must have been in the reads_from set for maxreads times.
 531  *
 532  * If so, we decide that the execution is no longer feasible.
 533  */
 534 void ModelChecker::check_recency(ModelAction *curr) {
 535         if (params.maxreads != 0) {
 536                 if (curr->get_node()->get_read_from_size() <= 1)
 537                         return;
 538
 539                 //Must make sure that execution is currently feasible...  We could
 540                 //accidentally clear by rolling back
 541                 if (!isfeasible())
 542                         return;
 543
 544                 std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
 545                 int tid = id_to_int(curr->get_tid());
 546
 547                 /* Skip checks */
 548                 if ((int)thrd_lists->size() <= tid)
 549                         return;
 550
 551                 action_list_t *list = &(*thrd_lists)[tid];
 552
 553                 action_list_t::reverse_iterator rit = list->rbegin();
 554                 /* Skip past curr */
 555                 for (; (*rit) != curr; rit++)
 556                         ;
 557                 /* go past curr now */
 558                 rit++;
 559
 560                 action_list_t::reverse_iterator ritcopy = rit;
 561                 //See if we have enough reads from the same value
 562                 int count = 0;
 563                 for (; count < params.maxreads; rit++,count++) {
 564                         if (rit==list->rend())
 565                                 return;
 566                         ModelAction *act = *rit;
 567                         if (!act->is_read())
 568                                 return;
 569                         if (act->get_reads_from() != curr->get_reads_from())
 570                                 return;
 571                         if (act->get_node()->get_read_from_size() <= 1)
 572                                 return;
 573                 }
 574
 575                 for (int i = 0; i<curr->get_node()->get_read_from_size(); i++) {
 576                         //Get write
 577                         const ModelAction * write = curr->get_node()->get_read_from_at(i);
 578                         //Need a different write
 579                         if (write==curr->get_reads_from())
 580                                 continue;
 581
 582                         /* Test to see whether this is a feasible write to read from*/
 583                         mo_graph->startChanges();
 584                         r_modification_order(curr, write);
 585                         bool feasiblereadfrom = isfeasible();
 586                         mo_graph->rollbackChanges();
 587
 588                         if (!feasiblereadfrom)
 589                                 continue;
 590                         rit = ritcopy;
 591
 592                         bool feasiblewrite = true;
 593                         //new we need to see if this write works for everyone
 594
 595                         for (int loop = count; loop>0; loop--,rit++) {
 596                                 ModelAction *act=*rit;
 597                                 bool foundvalue = false;
 598                                 for (int j = 0; j<act->get_node()->get_read_from_size(); j++) {
 599                                         if (act->get_node()->get_read_from_at(i)==write) {
 600                                                 foundvalue = true;
 601                                                 break;
 602                                         }
 603                                 }
 604                                 if (!foundvalue) {
 605                                         feasiblewrite = false;
 606                                         break;
 607                                 }
 608                         }
 609                         if (feasiblewrite) {
 610                                 too_many_reads = true;
 611                                 return;
 612                         }
 613                 }
 614         }
 615 }
 616
 617 /**
 618  * Updates the mo_graph with the constraints imposed from the current
 619  * read.
 620  *
 621  * Basic idea is the following: Go through each other thread and find
 622  * the lastest action that happened before our read.  Two cases:
 623  *
 624  * (1) The action is a write => that write must either occur before
 625  * the write we read from or be the write we read from.
 626  *
 627  * (2) The action is a read => the write that that action read from
 628  * must occur before the write we read from or be the same write.
 629  *
 630  * @param curr The current action. Must be a read.
 631  * @param rf The action that curr reads from. Must be a write.
 632  * @return True if modification order edges were added; false otherwise
 633  */
 634 bool ModelChecker::r_modification_order(ModelAction *curr, const ModelAction *rf)
 635 {
 636         std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
 637         unsigned int i;
 638         bool added = false;
 639         ASSERT(curr->is_read());
 640
 641         /* Iterate over all threads */
 642         for (i = 0; i < thrd_lists->size(); i++) {
 643                 /* Iterate over actions in thread, starting from most recent */
 644                 action_list_t *list = &(*thrd_lists)[i];
 645                 action_list_t::reverse_iterator rit;
 646                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 647                         ModelAction *act = *rit;
 648
 649                         /*
 650                          * Include at most one act per-thread that "happens
 651                          * before" curr. Don't consider reflexively.
 652                          */
 653                         if (act->happens_before(curr) && act != curr) {
 654                                 if (act->is_write()) {
 655                                         if (rf != act && act != curr) {
 656                                                 mo_graph->addEdge(act, rf);
 657                                                 added = true;
 658                                         }
 659                                 } else {
 660                                         const ModelAction *prevreadfrom = act->get_reads_from();
 661                                         if (prevreadfrom != NULL && rf != prevreadfrom) {
 662                                                 mo_graph->addEdge(prevreadfrom, rf);
 663                                                 added = true;
 664                                         }
 665                                 }
 666
 667                                 break;
 668                         }
 669                 }
 670         }
 671
 672         return added;
 673 }
 674
 675 /** This method fixes up the modification order when we resolve a
 676  *  promises.  The basic problem is that actions that occur after the
 677  *  read curr could not property add items to the modification order
 678  *  for our read.
 679  *
 680  *  So for each thread, we find the earliest item that happens after
 681  *  the read curr.  This is the item we have to fix up with additional
 682  *  constraints.  If that action is write, we add a MO edge between
 683  *  the Action rf and that action.  If the action is a read, we add a
 684  *  MO edge between the Action rf, and whatever the read accessed.
 685  *
 686  * @param curr is the read ModelAction that we are fixing up MO edges for.
 687  * @param rf is the write ModelAction that curr reads from.
 688  *
 689  */
 690
 691 void ModelChecker::post_r_modification_order(ModelAction *curr, const ModelAction *rf)
 692 {
 693         std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
 694         unsigned int i;
 695         ASSERT(curr->is_read());
 696
 697         /* Iterate over all threads */
 698         for (i = 0; i < thrd_lists->size(); i++) {
 699                 /* Iterate over actions in thread, starting from most recent */
 700                 action_list_t *list = &(*thrd_lists)[i];
 701                 action_list_t::reverse_iterator rit;
 702                 ModelAction *lastact = NULL;
 703
 704                 /* Find last action that happens after curr */
 705                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 706                         ModelAction *act = *rit;
 707                         if (curr->happens_before(act)) {
 708                                 lastact = act;
 709                         } else
 710                                 break;
 711                 }
 712
 713                         /* Include at most one act per-thread that "happens before" curr */
 714                 if (lastact != NULL) {
 715                         if (lastact->is_read()) {
 716                                 const ModelAction *postreadfrom = lastact->get_reads_from();
 717                                 if (postreadfrom != NULL&&rf != postreadfrom)
 718                                         mo_graph->addEdge(rf, postreadfrom);
 719                         } else if (rf != lastact) {
 720                                 mo_graph->addEdge(rf, lastact);
 721                         }
 722                         break;
 723                 }
 724         }
 725 }
 726
 727 /**
 728  * Updates the mo_graph with the constraints imposed from the current write.
 729  *
 730  * Basic idea is the following: Go through each other thread and find
 731  * the lastest action that happened before our write.  Two cases:
 732  *
 733  * (1) The action is a write => that write must occur before
 734  * the current write
 735  *
 736  * (2) The action is a read => the write that that action read from
 737  * must occur before the current write.
 738  *
 739  * This method also handles two other issues:
 740  *
 741  * (I) Sequential Consistency: Making sure that if the current write is
 742  * seq_cst, that it occurs after the previous seq_cst write.
 743  *
 744  * (II) Sending the write back to non-synchronizing reads.
 745  *
 746  * @param curr The current action. Must be a write.
 747  * @return True if modification order edges were added; false otherwise
 748  */
 749 bool ModelChecker::w_modification_order(ModelAction *curr)
 750 {
 751         std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
 752         unsigned int i;
 753         bool added = false;
 754         ASSERT(curr->is_write());
 755
 756         if (curr->is_seqcst()) {
 757                 /* We have to at least see the last sequentially consistent write,
 758                          so we are initialized. */
 759                 ModelAction *last_seq_cst = get_last_seq_cst(curr);
 760                 if (last_seq_cst != NULL) {
 761                         mo_graph->addEdge(last_seq_cst, curr);
 762                         added = true;
 763                 }
 764         }
 765
 766         /* Iterate over all threads */
 767         for (i = 0; i < thrd_lists->size(); i++) {
 768                 /* Iterate over actions in thread, starting from most recent */
 769                 action_list_t *list = &(*thrd_lists)[i];
 770                 action_list_t::reverse_iterator rit;
 771                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 772                         ModelAction *act = *rit;
 773
 774                         /*
 775                          * Include at most one act per-thread that "happens
 776                          * before" curr. Only consider reflexively if curr is
 777                          * RMW.
 778                          */
 779                         if (act->happens_before(curr) && (act != curr || curr->is_rmw())) {
 780                                 /*
 781                                  * Note: if act is RMW, just add edge:
 782                                  *   act --mo--> curr
 783                                  * The following edge should be handled elsewhere:
 784                                  *   readfrom(act) --mo--> act
 785                                  */
 786                                 if (act->is_write()) {
 787                                         //RMW shouldn't have an edge to themselves
 788                                         if (act!=curr)
 789                                                 mo_graph->addEdge(act, curr);
 790                                 } else if (act->is_read() && act->get_reads_from() != NULL)
 791                                         mo_graph->addEdge(act->get_reads_from(), curr);
 792                                 added = true;
 793                                 break;
 794                         } else if (act->is_read() && !act->is_synchronizing(curr) &&
 795                                                      !act->same_thread(curr)) {
 796                                 /* We have an action that:
 797                                    (1) did not happen before us
 798                                    (2) is a read and we are a write
 799                                    (3) cannot synchronize with us
 800                                    (4) is in a different thread
 801                                    =>
 802                                    that read could potentially read from our write.
 803                                  */
 804                                 if (thin_air_constraint_may_allow(curr, act)) {
 805                                         if (isfeasible() ||
 806                                                         (curr->is_rmw() && act->is_rmw() && curr->get_reads_from()==act->get_reads_from() && isfeasibleotherthanRMW())) {
 807                                                 struct PendingFutureValue pfv = {curr->get_value(),curr->get_seq_number()+params.maxfuturedelay,act};
 808                                                 futurevalues->push_back(pfv);
 809                                         }
 810                                 }
 811                         }
 812                 }
 813         }
 814
 815         return added;
 816 }
 817
 818 /** Arbitrary reads from the future are not allowed.  Section 29.3
 819  * part 9 places some constraints.  This method checks one result of constraint
 820  * constraint.  Others require compiler support. */
 821
 822 bool ModelChecker::thin_air_constraint_may_allow(const ModelAction * writer, const ModelAction *reader) {
 823         if (!writer->is_rmw())
 824                 return true;
 825
 826         if (!reader->is_rmw())
 827                 return true;
 828
 829         for (const ModelAction *search = writer->get_reads_from(); search != NULL; search = search->get_reads_from()) {
 830                 if (search==reader)
 831                         return false;
 832                 if (search->get_tid() == reader->get_tid() &&
 833                                 search->happens_before(reader))
 834                         break;
 835         }
 836
 837         return true;
 838 }
 839
 840 /**
 841  * Finds the head(s) of the release sequence(s) containing a given ModelAction.
 842  * The ModelAction under consideration is expected to be taking part in
 843  * release/acquire synchronization as an object of the "reads from" relation.
 844  * Note that this can only provide release sequence support for RMW chains
 845  * which do not read from the future, as those actions cannot be traced until
 846  * their "promise" is fulfilled. Similarly, we may not even establish the
 847  * presence of a release sequence with certainty, as some modification order
 848  * constraints may be decided further in the future. Thus, this function
 849  * "returns" two pieces of data: a pass-by-reference vector of @a release_heads
 850  * and a boolean representing certainty.
 851  *
 852  * @todo Finish lazy updating, when promises are fulfilled in the future
 853  * @param rf The action that might be part of a release sequence. Must be a
 854  * write.
 855  * @param release_heads A pass-by-reference style return parameter.  After
 856  * execution of this function, release_heads will contain the heads of all the
 857  * relevant release sequences, if any exists
 858  * @return true, if the ModelChecker is certain that release_heads is complete;
 859  * false otherwise
 860  */
 861 bool ModelChecker::release_seq_head(const ModelAction *rf,
 862                 std::vector< const ModelAction *, MyAlloc<const ModelAction *> > *release_heads) const
 863 {
 864         if (!rf) {
 865                 /* read from future: need to settle this later */
 866                 return false; /* incomplete */
 867         }
 868
 869         ASSERT(rf->is_write());
 870
 871         if (rf->is_release())
 872                 release_heads->push_back(rf);
 873         if (rf->is_rmw()) {
 874                 /* We need a RMW action that is both an acquire and release to stop */
 875                 /** @todo Need to be smarter here...  In the linux lock
 876                  * example, this will run to the beginning of the program for
 877                  * every acquire. */
 878                 if (rf->is_acquire() && rf->is_release())
 879                         return true; /* complete */
 880                 return release_seq_head(rf->get_reads_from(), release_heads);
 881         }
 882         if (rf->is_release())
 883                 return true; /* complete */
 884
 885         /* else relaxed write; check modification order for contiguous subsequence
 886          * -> rf must be same thread as release */
 887         int tid = id_to_int(rf->get_tid());
 888         std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(rf->get_location());
 889         action_list_t *list = &(*thrd_lists)[tid];
 890         action_list_t::const_reverse_iterator rit;
 891
 892         /* Find rf in the thread list */
 893         rit = std::find(list->rbegin(), list->rend(), rf);
 894         ASSERT(rit != list->rend());
 895
 896         /* Find the last write/release */
 897         for (; rit != list->rend(); rit++)
 898                 if ((*rit)->is_release())
 899                         break;
 900         if (rit == list->rend()) {
 901                 /* No write-release in this thread */
 902                 return true; /* complete */
 903         }
 904         ModelAction *release = *rit;
 905
 906         ASSERT(rf->same_thread(release));
 907
 908         bool certain = true;
 909         for (unsigned int i = 0; i < thrd_lists->size(); i++) {
 910                 if (id_to_int(rf->get_tid()) == (int)i)
 911                         continue;
 912                 list = &(*thrd_lists)[i];
 913
 914                 /* Can we ensure no future writes from this thread may break
 915                  * the release seq? */
 916                 bool future_ordered = false;
 917
 918                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 919                         const ModelAction *act = *rit;
 920                         if (!act->is_write())
 921                                 continue;
 922                         /* Reach synchronization -> this thread is complete */
 923                         if (act->happens_before(release))
 924                                 break;
 925                         if (rf->happens_before(act)) {
 926                                 future_ordered = true;
 927                                 continue;
 928                         }
 929
 930                         /* Check modification order */
 931                         if (mo_graph->checkReachable(rf, act)) {
 932                                 /* rf --mo--> act */
 933                                 future_ordered = true;
 934                                 continue;
 935                         }
 936                         if (mo_graph->checkReachable(act, release))
 937                                 /* act --mo--> release */
 938                                 break;
 939                         if (mo_graph->checkReachable(release, act) &&
 940                                       mo_graph->checkReachable(act, rf)) {
 941                                 /* release --mo-> act --mo--> rf */
 942                                 return true; /* complete */
 943                         }
 944                         certain = false;
 945                 }
 946                 if (!future_ordered)
 947                         return false; /* This thread is uncertain */
 948         }
 949
 950         if (certain)
 951                 release_heads->push_back(release);
 952         return certain;
 953 }
 954
 955 /**
 956  * A public interface for getting the release sequence head(s) with which a
 957  * given ModelAction must synchronize. This function only returns a non-empty
 958  * result when it can locate a release sequence head with certainty. Otherwise,
 959  * it may mark the internal state of the ModelChecker so that it will handle
 960  * the release sequence at a later time, causing @a act to update its
 961  * synchronization at some later point in execution.
 962  * @param act The 'acquire' action that may read from a release sequence
 963  * @param release_heads A pass-by-reference return parameter. Will be filled
 964  * with the head(s) of the release sequence(s), if they exists with certainty.
 965  * @see ModelChecker::release_seq_head
 966  */
 967 void ModelChecker::get_release_seq_heads(ModelAction *act,
 968                 std::vector< const ModelAction *, MyAlloc<const ModelAction *> > *release_heads)
 969 {
 970         const ModelAction *rf = act->get_reads_from();
 971         bool complete;
 972         complete = release_seq_head(rf, release_heads);
 973         if (!complete) {
 974                 /* add act to 'lazy checking' list */
 975                 std::list<ModelAction *> *list;
 976                 list = lazy_sync_with_release->get_safe_ptr(act->get_location());
 977                 list->push_back(act);
 978                 (*lazy_sync_size)++;
 979         }
 980 }
 981
 982 /**
 983  * Attempt to resolve all stashed operations that might synchronize with a
 984  * release sequence for a given location. This implements the "lazy" portion of
 985  * determining whether or not a release sequence was contiguous, since not all
 986  * modification order information is present at the time an action occurs.
 987  *
 988  * @param location The location/object that should be checked for release
 989  * sequence resolutions
 990  * @param work_queue The work queue to which to add work items as they are
 991  * generated
 992  * @return True if any updates occurred (new synchronization, new mo_graph
 993  * edges)
 994  */
 995 bool ModelChecker::resolve_release_sequences(void *location, work_queue_t *work_queue)
 996 {
 997         std::list<ModelAction *> *list;
 998         list = lazy_sync_with_release->getptr(location);
 999         if (!list)
1000                 return false;
1001
1002         bool updated = false;
1003         std::list<ModelAction *>::iterator it = list->begin();
1004         while (it != list->end()) {
1005                 ModelAction *act = *it;
1006                 const ModelAction *rf = act->get_reads_from();
1007                 std::vector< const ModelAction *, MyAlloc<const ModelAction *> > release_heads;
1008                 bool complete;
1009                 complete = release_seq_head(rf, &release_heads);
1010                 for (unsigned int i = 0; i < release_heads.size(); i++) {
1011                         if (!act->has_synchronized_with(release_heads[i])) {
1012                                 updated = true;
1013                                 act->synchronize_with(release_heads[i]);
1014                         }
1015                 }
1016
1017                 if (updated) {
1018                         /* Re-check act for mo_graph edges */
1019                         work_queue->push_back(MOEdgeWorkEntry(act));
1020
1021                         /* propagate synchronization to later actions */
1022                         action_list_t::reverse_iterator it = action_trace->rbegin();
1023                         while ((*it) != act) {
1024                                 ModelAction *propagate = *it;
1025                                 if (act->happens_before(propagate)) {
1026                                         propagate->synchronize_with(act);
1027                                         /* Re-check 'propagate' for mo_graph edges */
1028                                         work_queue->push_back(MOEdgeWorkEntry(propagate));
1029                                 }
1030                         }
1031                 }
1032                 if (complete) {
1033                         it = list->erase(it);
1034                         (*lazy_sync_size)--;
1035                 } else
1036                         it++;
1037         }
1038
1039         // If we resolved promises or data races, see if we have realized a data race.
1040         if (checkDataRaces()) {
1041                 set_assert();
1042         }
1043
1044         return updated;
1045 }
1046
1047 /**
1048  * Performs various bookkeeping operations for the current ModelAction. For
1049  * instance, adds action to the per-object, per-thread action vector and to the
1050  * action trace list of all thread actions.
1051  *
1052  * @param act is the ModelAction to add.
1053  */
1054 void ModelChecker::add_action_to_lists(ModelAction *act)
1055 {
1056         int tid = id_to_int(act->get_tid());
1057         action_trace->push_back(act);
1058
1059         obj_map->get_safe_ptr(act->get_location())->push_back(act);
1060
1061         std::vector<action_list_t> *vec = obj_thrd_map->get_safe_ptr(act->get_location());
1062         if (tid >= (int)vec->size())
1063                 vec->resize(priv->next_thread_id);
1064         (*vec)[tid].push_back(act);
1065
1066         if ((int)thrd_last_action->size() <= tid)
1067                 thrd_last_action->resize(get_num_threads());
1068         (*thrd_last_action)[tid] = act;
1069 }
1070
1071 ModelAction * ModelChecker::get_last_action(thread_id_t tid)
1072 {
1073         int nthreads = get_num_threads();
1074         if ((int)thrd_last_action->size() < nthreads)
1075                 thrd_last_action->resize(nthreads);
1076         return (*thrd_last_action)[id_to_int(tid)];
1077 }
1078
1079 /**
1080  * Gets the last memory_order_seq_cst write (in the total global sequence)
1081  * performed on a particular object (i.e., memory location), not including the
1082  * current action.
1083  * @param curr The current ModelAction; also denotes the object location to
1084  * check
1085  * @return The last seq_cst write
1086  */
1087 ModelAction * ModelChecker::get_last_seq_cst(ModelAction *curr)
1088 {
1089         void *location = curr->get_location();
1090         action_list_t *list = obj_map->get_safe_ptr(location);
1091         /* Find: max({i in dom(S) | seq_cst(t_i) && isWrite(t_i) && samevar(t_i, t)}) */
1092         action_list_t::reverse_iterator rit;
1093         for (rit = list->rbegin(); rit != list->rend(); rit++)
1094                 if ((*rit)->is_write() && (*rit)->is_seqcst() && (*rit) != curr)
1095                         return *rit;
1096         return NULL;
1097 }
1098
1099 ModelAction * ModelChecker::get_parent_action(thread_id_t tid)
1100 {
1101         ModelAction *parent = get_last_action(tid);
1102         if (!parent)
1103                 parent = get_thread(tid)->get_creation();
1104         return parent;
1105 }
1106
1107 /**
1108  * Returns the clock vector for a given thread.
1109  * @param tid The thread whose clock vector we want
1110  * @return Desired clock vector
1111  */
1112 ClockVector * ModelChecker::get_cv(thread_id_t tid)
1113 {
1114         return get_parent_action(tid)->get_cv();
1115 }
1116
1117 /**
1118  * Resolve a set of Promises with a current write. The set is provided in the
1119  * Node corresponding to @a write.
1120  * @param write The ModelAction that is fulfilling Promises
1121  * @return True if promises were resolved; false otherwise
1122  */
1123 bool ModelChecker::resolve_promises(ModelAction *write)
1124 {
1125         bool resolved = false;
1126
1127         for (unsigned int i = 0, promise_index = 0; promise_index < promises->size(); i++) {
1128                 Promise *promise = (*promises)[promise_index];
1129                 if (write->get_node()->get_promise(i)) {
1130                         ModelAction *read = promise->get_action();
1131                         read->read_from(write);
1132                         if (read->is_rmw()) {
1133                                 mo_graph->addRMWEdge(write, read);
1134                         }
1135                         //First fix up the modification order for actions that happened
1136                         //before the read
1137                         r_modification_order(read, write);
1138                         //Next fix up the modification order for actions that happened
1139                         //after the read.
1140                         post_r_modification_order(read, write);
1141                         promises->erase(promises->begin() + promise_index);
1142                         resolved = true;
1143                 } else
1144                         promise_index++;
1145         }
1146         return resolved;
1147 }
1148
1149 /**
1150  * Compute the set of promises that could potentially be satisfied by this
1151  * action. Note that the set computation actually appears in the Node, not in
1152  * ModelChecker.
1153  * @param curr The ModelAction that may satisfy promises
1154  */
1155 void ModelChecker::compute_promises(ModelAction *curr)
1156 {
1157         for (unsigned int i = 0; i < promises->size(); i++) {
1158                 Promise *promise = (*promises)[i];
1159                 const ModelAction *act = promise->get_action();
1160                 if (!act->happens_before(curr) &&
1161                                 act->is_read() &&
1162                                 !act->is_synchronizing(curr) &&
1163                                 !act->same_thread(curr) &&
1164                                 promise->get_value() == curr->get_value()) {
1165                         curr->get_node()->set_promise(i);
1166                 }
1167         }
1168 }
1169
1170 /** Checks promises in response to change in ClockVector Threads. */
1171 void ModelChecker::check_promises(ClockVector *old_cv, ClockVector *merge_cv)
1172 {
1173         for (unsigned int i = 0; i < promises->size(); i++) {
1174                 Promise *promise = (*promises)[i];
1175                 const ModelAction *act = promise->get_action();
1176                 if ((old_cv == NULL || !old_cv->synchronized_since(act)) &&
1177                                 merge_cv->synchronized_since(act)) {
1178                         //This thread is no longer able to send values back to satisfy the promise
1179                         int num_synchronized_threads = promise->increment_threads();
1180                         if (num_synchronized_threads == get_num_threads()) {
1181                                 //Promise has failed
1182                                 failed_promise = true;
1183                                 return;
1184                         }
1185                 }
1186         }
1187 }
1188
1189 /**
1190  * Build up an initial set of all past writes that this 'read' action may read
1191  * from. This set is determined by the clock vector's "happens before"
1192  * relationship.
1193  * @param curr is the current ModelAction that we are exploring; it must be a
1194  * 'read' operation.
1195  */
1196 void ModelChecker::build_reads_from_past(ModelAction *curr)
1197 {
1198         std::vector<action_list_t> *thrd_lists = obj_thrd_map->get_safe_ptr(curr->get_location());
1199         unsigned int i;
1200         ASSERT(curr->is_read());
1201
1202         ModelAction *last_seq_cst = NULL;
1203
1204         /* Track whether this object has been initialized */
1205         bool initialized = false;
1206
1207         if (curr->is_seqcst()) {
1208                 last_seq_cst = get_last_seq_cst(curr);
1209                 /* We have to at least see the last sequentially consistent write,
1210                          so we are initialized. */
1211                 if (last_seq_cst != NULL)
1212                         initialized = true;
1213         }
1214
1215         /* Iterate over all threads */
1216         for (i = 0; i < thrd_lists->size(); i++) {
1217                 /* Iterate over actions in thread, starting from most recent */
1218                 action_list_t *list = &(*thrd_lists)[i];
1219                 action_list_t::reverse_iterator rit;
1220                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
1221                         ModelAction *act = *rit;
1222
1223                         /* Only consider 'write' actions */
1224                         if (!act->is_write())
1225                                 continue;
1226
1227                         /* Don't consider more than one seq_cst write if we are a seq_cst read. */
1228                         if (!curr->is_seqcst()|| (!act->is_seqcst() && (last_seq_cst==NULL||!act->happens_before(last_seq_cst))) || act == last_seq_cst) {
1229                                 DEBUG("Adding action to may_read_from:\n");
1230                                 if (DBG_ENABLED()) {
1231                                         act->print();
1232                                         curr->print();
1233                                 }
1234                                 curr->get_node()->add_read_from(act);
1235                         }
1236
1237                         /* Include at most one act per-thread that "happens before" curr */
1238                         if (act->happens_before(curr)) {
1239                                 initialized = true;
1240                                 break;
1241                         }
1242                 }
1243         }
1244
1245         if (!initialized) {
1246                 /** @todo Need a more informative way of reporting errors. */
1247                 printf("ERROR: may read from uninitialized atomic\n");
1248         }
1249
1250         if (DBG_ENABLED() || !initialized) {
1251                 printf("Reached read action:\n");
1252                 curr->print();
1253                 printf("Printing may_read_from\n");
1254                 curr->get_node()->print_may_read_from();
1255                 printf("End printing may_read_from\n");
1256         }
1257
1258         ASSERT(initialized);
1259 }
1260
1261 static void print_list(action_list_t *list)
1262 {
1263         action_list_t::iterator it;
1264
1265         printf("---------------------------------------------------------------------\n");
1266         printf("Trace:\n");
1267
1268         for (it = list->begin(); it != list->end(); it++) {
1269                 (*it)->print();
1270         }
1271         printf("---------------------------------------------------------------------\n");
1272 }
1273
1274 void ModelChecker::print_summary()
1275 {
1276         printf("\n");
1277         printf("Number of executions: %d\n", num_executions);
1278         printf("Number of feasible executions: %d\n", num_feasible_executions);
1279         printf("Total nodes created: %d\n", node_stack->get_total_nodes());
1280
1281 #if SUPPORT_MOD_ORDER_DUMP
1282         scheduler->print();
1283         char buffername[100];
1284         sprintf(buffername, "exec%u",num_executions);
1285         mo_graph->dumpGraphToFile(buffername);
1286 #endif
1287
1288         if (!isfinalfeasible())
1289                 printf("INFEASIBLE EXECUTION!\n");
1290         print_list(action_trace);
1291         printf("\n");
1292 }
1293
1294 /**
1295  * Add a Thread to the system for the first time. Should only be called once
1296  * per thread.
1297  * @param t The Thread to add
1298  */
1299 void ModelChecker::add_thread(Thread *t)
1300 {
1301         thread_map->put(id_to_int(t->get_id()), t);
1302         scheduler->add_thread(t);
1303 }
1304
1305 void ModelChecker::remove_thread(Thread *t)
1306 {
1307         scheduler->remove_thread(t);
1308 }
1309
1310 /**
1311  * Switch from a user-context to the "master thread" context (a.k.a. system
1312  * context). This switch is made with the intention of exploring a particular
1313  * model-checking action (described by a ModelAction object). Must be called
1314  * from a user-thread context.
1315  * @param act The current action that will be explored. Must not be NULL.
1316  * @return Return status from the 'swap' call (i.e., success/fail, 0/-1)
1317  */
1318 int ModelChecker::switch_to_master(ModelAction *act)
1319 {
1320         DBG();
1321         Thread *old = thread_current();
1322         set_current_action(act);
1323         old->set_state(THREAD_READY);
1324         return Thread::swap(old, &system_context);
1325 }
1326
1327 /**
1328  * Takes the next step in the execution, if possible.
1329  * @return Returns true (success) if a step was taken and false otherwise.
1330  */
1331 bool ModelChecker::take_step() {
1332         Thread *curr, *next;
1333
1334         if (has_asserted())
1335                 return false;
1336
1337         curr = thread_current();
1338         if (curr) {
1339                 if (curr->get_state() == THREAD_READY) {
1340                         ASSERT(priv->current_action);
1341
1342                         priv->nextThread = check_current_action(priv->current_action);
1343                         priv->current_action = NULL;
1344                         if (!curr->is_blocked() && !curr->is_complete())
1345                                 scheduler->add_thread(curr);
1346                 } else {
1347                         ASSERT(false);
1348                 }
1349         }
1350         next = scheduler->next_thread(priv->nextThread);
1351
1352         /* Infeasible -> don't take any more steps */
1353         if (!isfeasible())
1354                 return false;
1355
1356         if (next)
1357                 next->set_state(THREAD_RUNNING);
1358         DEBUG("(%d, %d)\n", curr ? curr->get_id() : -1, next ? next->get_id() : -1);
1359
1360         /* next == NULL -> don't take any more steps */
1361         if (!next)
1362                 return false;
1363         /* Return false only if swap fails with an error */
1364         return (Thread::swap(&system_context, next) == 0);
1365 }
1366
1367 /** Runs the current execution until threre are no more steps to take. */
1368 void ModelChecker::finish_execution() {
1369         DBG();
1370
1371         while (take_step());
1372 }