model.cc

   1 #include <stdio.h>
   2
   3 #include "model.h"
   4 #include "action.h"
   5 #include "nodestack.h"
   6 #include "schedule.h"
   7 #include "snapshot-interface.h"
   8 #include "common.h"
   9 #include "clockvector.h"
  10 #include "cyclegraph.h"
  11 #include "promise.h"
  12
  13 #define INITIAL_THREAD_ID       0
  14
  15 ModelChecker *model;
  16
  17 /** @brief Constructor */
  18 ModelChecker::ModelChecker()
  19         :
  20         /* Initialize default scheduler */
  21         scheduler(new Scheduler()),
  22         /* First thread created will have id INITIAL_THREAD_ID */
  23         next_thread_id(INITIAL_THREAD_ID),
  24         used_sequence_numbers(0),
  25         num_executions(0),
  26         current_action(NULL),
  27         diverge(NULL),
  28         nextThread(THREAD_ID_T_NONE),
  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         obj_thrd_map(new HashTable<void *, std::vector<action_list_t>, uintptr_t, 4 >()),
  33         promises(new std::vector<Promise *>()),
  34         thrd_last_action(new std::vector<ModelAction *>(1)),
  35         node_stack(new NodeStack()),
  36         next_backtrack(NULL),
  37         cyclegraph(new CycleGraph()),
  38         failed_promise(false)
  39 {
  40 }
  41
  42 /** @brief Destructor */
  43 ModelChecker::~ModelChecker()
  44 {
  45         /*      std::map<int, Thread *>::iterator it;
  46         for (it = thread_map->begin(); it != thread_map->end(); it++)
  47         delete (*it).second;*/
  48         delete thread_map;
  49
  50         delete obj_thrd_map;
  51         delete obj_map;
  52         delete action_trace;
  53         delete thrd_last_action;
  54         delete node_stack;
  55         delete scheduler;
  56         delete cyclegraph;
  57 }
  58
  59 /**
  60  * Restores user program to initial state and resets all model-checker data
  61  * structures.
  62  */
  63 void ModelChecker::reset_to_initial_state()
  64 {
  65         DEBUG("+++ Resetting to initial state +++\n");
  66         node_stack->reset_execution();
  67         current_action = NULL;
  68         next_thread_id = INITIAL_THREAD_ID;
  69         used_sequence_numbers = 0;
  70         nextThread = 0;
  71         next_backtrack = NULL;
  72         failed_promise = false;
  73         snapshotObject->backTrackBeforeStep(0);
  74 }
  75
  76 /** @returns a thread ID for a new Thread */
  77 thread_id_t ModelChecker::get_next_id()
  78 {
  79         return next_thread_id++;
  80 }
  81
  82 /** @returns the number of user threads created during this execution */
  83 int ModelChecker::get_num_threads()
  84 {
  85         return next_thread_id;
  86 }
  87
  88 /** @returns a sequence number for a new ModelAction */
  89 modelclock_t ModelChecker::get_next_seq_num()
  90 {
  91         return ++used_sequence_numbers;
  92 }
  93
  94 /**
  95  * Performs the "scheduling" for the model-checker. That is, it checks if the
  96  * model-checker has selected a "next thread to run" and returns it, if
  97  * available. This function should be called from the Scheduler routine, where
  98  * the Scheduler falls back to a default scheduling routine if needed.
  99  *
 100  * @return The next thread chosen by the model-checker. If the model-checker
 101  * makes no selection, retuns NULL.
 102  */
 103 Thread * ModelChecker::schedule_next_thread()
 104 {
 105         Thread *t;
 106         if (nextThread == THREAD_ID_T_NONE)
 107                 return NULL;
 108         t = thread_map->get(id_to_int(nextThread));
 109
 110         ASSERT(t != NULL);
 111
 112         return t;
 113 }
 114
 115 /**
 116  * Choose the next thread in the replay sequence.
 117  *
 118  * If the replay sequence has reached the 'diverge' point, returns a thread
 119  * from the backtracking set. Otherwise, simply returns the next thread in the
 120  * sequence that is being replayed.
 121  */
 122 thread_id_t ModelChecker::get_next_replay_thread()
 123 {
 124         thread_id_t tid;
 125
 126         /* Have we completed exploring the preselected path? */
 127         if (diverge == NULL)
 128                 return THREAD_ID_T_NONE;
 129
 130         /* Else, we are trying to replay an execution */
 131         ModelAction * next = node_stack->get_next()->get_action();
 132
 133         if (next == diverge) {
 134                 Node *nextnode = next->get_node();
 135                 /* Reached divergence point */
 136                 if (nextnode->increment_promise()) {
 137                         /* The next node will try to satisfy a different set of promises. */
 138                         tid = next->get_tid();
 139                         node_stack->pop_restofstack(2);
 140                 } else if (nextnode->increment_read_from()) {
 141                         /* The next node will read from a different value. */
 142                         tid = next->get_tid();
 143                         node_stack->pop_restofstack(2);
 144                 } else if (nextnode->increment_future_value()) {
 145                         /* The next node will try to read from a different future value. */
 146                         tid = next->get_tid();
 147                         node_stack->pop_restofstack(2);
 148                 } else {
 149                         /* Make a different thread execute for next step */
 150                         Node *node = nextnode->get_parent();
 151                         tid = node->get_next_backtrack();
 152                         node_stack->pop_restofstack(1);
 153                 }
 154                 DEBUG("*** Divergence point ***\n");
 155                 diverge = NULL;
 156         } else {
 157                 tid = next->get_tid();
 158         }
 159         DEBUG("*** ModelChecker chose next thread = %d ***\n", tid);
 160         return tid;
 161 }
 162
 163 /**
 164  * Queries the model-checker for more executions to explore and, if one
 165  * exists, resets the model-checker state to execute a new execution.
 166  *
 167  * @return If there are more executions to explore, return true. Otherwise,
 168  * return false.
 169  */
 170 bool ModelChecker::next_execution()
 171 {
 172         DBG();
 173
 174         num_executions++;
 175
 176         if (isfinalfeasible() || DBG_ENABLED())
 177                 print_summary();
 178
 179         if ((diverge = model->get_next_backtrack()) == NULL)
 180                 return false;
 181
 182         if (DBG_ENABLED()) {
 183                 printf("Next execution will diverge at:\n");
 184                 diverge->print();
 185         }
 186
 187         model->reset_to_initial_state();
 188         return true;
 189 }
 190
 191 ModelAction * ModelChecker::get_last_conflict(ModelAction *act)
 192 {
 193         action_type type = act->get_type();
 194
 195         switch (type) {
 196                 case ATOMIC_READ:
 197                 case ATOMIC_WRITE:
 198                 case ATOMIC_RMW:
 199                         break;
 200                 default:
 201                         return NULL;
 202         }
 203         /* linear search: from most recent to oldest */
 204         action_list_t *list = obj_map->ensureptr(act->get_location());
 205         action_list_t::reverse_iterator rit;
 206         for (rit = list->rbegin(); rit != list->rend(); rit++) {
 207                 ModelAction *prev = *rit;
 208                 if (act->is_synchronizing(prev))
 209                         return prev;
 210         }
 211         return NULL;
 212 }
 213
 214 void ModelChecker::set_backtracking(ModelAction *act)
 215 {
 216         ModelAction *prev;
 217         Node *node;
 218         Thread *t = get_thread(act->get_tid());
 219
 220         prev = get_last_conflict(act);
 221         if (prev == NULL)
 222                 return;
 223
 224         node = prev->get_node()->get_parent();
 225
 226         while (!node->is_enabled(t))
 227                 t = t->get_parent();
 228
 229         /* Check if this has been explored already */
 230         if (node->has_been_explored(t->get_id()))
 231                 return;
 232
 233         /* Cache the latest backtracking point */
 234         if (!next_backtrack || *prev > *next_backtrack)
 235                 next_backtrack = prev;
 236
 237         /* If this is a new backtracking point, mark the tree */
 238         if (!node->set_backtrack(t->get_id()))
 239                 return;
 240         DEBUG("Setting backtrack: conflict = %d, instead tid = %d\n",
 241                         prev->get_tid(), t->get_id());
 242         if (DBG_ENABLED()) {
 243                 prev->print();
 244                 act->print();
 245         }
 246 }
 247
 248 /**
 249  * Returns last backtracking point. The model checker will explore a different
 250  * path for this point in the next execution.
 251  * @return The ModelAction at which the next execution should diverge.
 252  */
 253 ModelAction * ModelChecker::get_next_backtrack()
 254 {
 255         ModelAction *next = next_backtrack;
 256         next_backtrack = NULL;
 257         return next;
 258 }
 259
 260 void ModelChecker::check_current_action(void)
 261 {
 262         ModelAction *curr = this->current_action;
 263         bool already_added = false;
 264         this->current_action = NULL;
 265         if (!curr) {
 266                 DEBUG("trying to push NULL action...\n");
 267                 return;
 268         }
 269
 270         if (curr->is_rmwc()||curr->is_rmw()) {
 271                 ModelAction *tmp=process_rmw(curr);
 272                 already_added = true;
 273                 delete curr;
 274                 curr=tmp;
 275         } else {
 276                 ModelAction * tmp = node_stack->explore_action(curr);
 277                 if (tmp) {
 278                         /* Discard duplicate ModelAction; use action from NodeStack */
 279                         /* First restore type and order in case of RMW operation */
 280                         if (curr->is_rmwr())
 281                                 tmp->copy_typeandorder(curr);
 282
 283                         /* If we have diverged, we need to reset the clock vector. */
 284                         if (diverge==NULL) {
 285                                 tmp->create_cv(get_parent_action(tmp->get_tid()));
 286                         }
 287
 288                         delete curr;
 289                         curr = tmp;
 290                 } else {
 291                         /*
 292                          * Perform one-time actions when pushing new ModelAction onto
 293                          * NodeStack
 294                          */
 295                         curr->create_cv(get_parent_action(curr->get_tid()));
 296                         /* Build may_read_from set */
 297                         if (curr->is_read())
 298                                 build_reads_from_past(curr);
 299                         if (curr->is_write())
 300                                 compute_promises(curr);
 301                 }
 302         }
 303
 304         /* Assign 'creation' parent */
 305         if (curr->get_type() == THREAD_CREATE) {
 306                 Thread *th = (Thread *)curr->get_location();
 307                 th->set_creation(curr);
 308         }
 309
 310         /* Deal with new thread */
 311         if (curr->get_type() == THREAD_START) {
 312                 check_promises(NULL, curr->get_cv());
 313         }
 314
 315         /* Assign reads_from values */
 316         Thread *th = get_thread(curr->get_tid());
 317         uint64_t value = VALUE_NONE;
 318         if (curr->is_read()) {
 319                 const ModelAction *reads_from = curr->get_node()->get_read_from();
 320                 if (reads_from!=NULL) {
 321                         value = reads_from->get_value();
 322                         /* Assign reads_from, perform release/acquire synchronization */
 323                         curr->read_from(reads_from);
 324                         r_modification_order(curr,reads_from);
 325                 } else {
 326                         /* Read from future value */
 327                         value = curr->get_node()->get_future_value();
 328                         curr->read_from(NULL);
 329                         Promise * valuepromise=new Promise(curr, value);
 330                         promises->push_back(valuepromise);
 331                 }
 332         } else if (curr->is_write()) {
 333                 w_modification_order(curr);
 334                 resolve_promises(curr);
 335         }
 336
 337         th->set_return_value(value);
 338
 339         /* Add action to list.  */
 340         if (!already_added)
 341                 add_action_to_lists(curr);
 342
 343         /* Is there a better interface for setting the next thread rather
 344                  than this field/convoluted approach?  Perhaps like just returning
 345                  it or something? */
 346
 347         /* Do not split atomic actions. */
 348         if (curr->is_rmwr()) {
 349                 nextThread = thread_current()->get_id();
 350         } else {
 351                 nextThread = get_next_replay_thread();
 352         }
 353
 354         Node *currnode = curr->get_node();
 355         Node *parnode = currnode->get_parent();
 356
 357         if (!parnode->backtrack_empty()||!currnode->read_from_empty()||!currnode->future_value_empty()||!currnode->promise_empty())
 358                 if (!next_backtrack || *curr > *next_backtrack)
 359                         next_backtrack = curr;
 360
 361         set_backtracking(curr);
 362 }
 363
 364 /** @returns whether the current trace is feasible. */
 365 bool ModelChecker::isfeasible() {
 366         return !cyclegraph->checkForCycles() && !failed_promise;
 367 }
 368
 369 /** Returns whether the current trace is feasible. */
 370 bool ModelChecker::isfinalfeasible() {
 371         return isfeasible() && promises->size()==0;
 372 }
 373
 374 /** Close out a RMWR by converting previous RMWR into a RMW or READ. */
 375 ModelAction * ModelChecker::process_rmw(ModelAction * act) {
 376         int tid = id_to_int(act->get_tid());
 377         ModelAction *lastread=get_last_action(tid);
 378         lastread->process_rmw(act);
 379         if (act->is_rmw())
 380                 cyclegraph->addRMWEdge(lastread, lastread->get_reads_from());
 381         return lastread;
 382 }
 383
 384 /**
 385  * Updates the cyclegraph with the constraints imposed from the current read.
 386  * @param curr The current action. Must be a read.
 387  * @param rf The action that curr reads from. Must be a write.
 388  */
 389 void ModelChecker::r_modification_order(ModelAction * curr, const ModelAction *rf) {
 390         std::vector<action_list_t> *thrd_lists = obj_thrd_map->ensureptr(curr->get_location());
 391         unsigned int i;
 392         ASSERT(curr->is_read());
 393
 394         /* Iterate over all threads */
 395         for (i = 0; i < thrd_lists->size(); i++) {
 396                 /* Iterate over actions in thread, starting from most recent */
 397                 action_list_t *list = &(*thrd_lists)[i];
 398                 action_list_t::reverse_iterator rit;
 399                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 400                         ModelAction *act = *rit;
 401
 402                         /* Include at most one act per-thread that "happens before" curr */
 403                         if (act->happens_before(curr)) {
 404                                 if (act->is_read()) {
 405                                         const ModelAction * prevreadfrom=act->get_reads_from();
 406                                         if (rf!=prevreadfrom)
 407                                                 cyclegraph->addEdge(rf, prevreadfrom);
 408                                 } else if (rf!=act) {
 409                                         cyclegraph->addEdge(rf, act);
 410                                 }
 411                                 break;
 412                         }
 413                 }
 414         }
 415 }
 416
 417 /**
 418  * Updates the cyclegraph with the constraints imposed from the current write.
 419  * @param curr The current action. Must be a write.
 420  */
 421 void ModelChecker::w_modification_order(ModelAction * curr) {
 422         std::vector<action_list_t> *thrd_lists = obj_thrd_map->ensureptr(curr->get_location());
 423         unsigned int i;
 424         ASSERT(curr->is_write());
 425
 426         if (curr->is_seqcst()) {
 427                 /* We have to at least see the last sequentially consistent write,
 428                          so we are initialized. */
 429                 ModelAction * last_seq_cst=get_last_seq_cst(curr->get_location());
 430                 if (last_seq_cst!=NULL)
 431                         cyclegraph->addEdge(curr, last_seq_cst);
 432         }
 433
 434         /* Iterate over all threads */
 435         for (i = 0; i < thrd_lists->size(); i++) {
 436                 /* Iterate over actions in thread, starting from most recent */
 437                 action_list_t *list = &(*thrd_lists)[i];
 438                 action_list_t::reverse_iterator rit;
 439                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 440                         ModelAction *act = *rit;
 441
 442                         /* Include at most one act per-thread that "happens before" curr */
 443                         if (act->happens_before(curr)) {
 444                                 if (act->is_read()) {
 445                                         cyclegraph->addEdge(curr, act->get_reads_from());
 446                                 } else
 447                                         cyclegraph->addEdge(curr, act);
 448                                 break;
 449                         } else {
 450                                 if (act->is_read()&&!act->is_synchronizing(curr)&&!act->same_thread(curr)) {
 451                                         /* We have an action that:
 452                                                  (1) did not happen before us
 453                                                  (2) is a read and we are a write
 454                                                  (3) cannot synchronize with us
 455                                                  (4) is in a different thread
 456                                                  =>
 457                                                  that read could potentially read from our write.
 458                                         */
 459                                         if (act->get_node()->add_future_value(curr->get_value())&&
 460                                                         (!next_backtrack || *act > * next_backtrack))
 461                                                 next_backtrack = act;
 462                                 }
 463                         }
 464                 }
 465         }
 466 }
 467
 468 /**
 469  * Performs various bookkeeping operations for the current ModelAction. For
 470  * instance, adds action to the per-object, per-thread action vector and to the
 471  * action trace list of all thread actions.
 472  *
 473  * @param act is the ModelAction to add.
 474  */
 475 void ModelChecker::add_action_to_lists(ModelAction *act)
 476 {
 477         int tid = id_to_int(act->get_tid());
 478         action_trace->push_back(act);
 479
 480         obj_map->ensureptr(act->get_location())->push_back(act);
 481
 482         std::vector<action_list_t> *vec = obj_thrd_map->ensureptr(act->get_location());
 483         if (tid >= (int)vec->size())
 484                 vec->resize(next_thread_id);
 485         (*vec)[tid].push_back(act);
 486
 487         if ((int)thrd_last_action->size() <= tid)
 488                 thrd_last_action->resize(get_num_threads());
 489         (*thrd_last_action)[tid] = act;
 490 }
 491
 492 ModelAction * ModelChecker::get_last_action(thread_id_t tid)
 493 {
 494         int nthreads = get_num_threads();
 495         if ((int)thrd_last_action->size() < nthreads)
 496                 thrd_last_action->resize(nthreads);
 497         return (*thrd_last_action)[id_to_int(tid)];
 498 }
 499
 500 /**
 501  * Gets the last memory_order_seq_cst action (in the total global sequence)
 502  * performed on a particular object (i.e., memory location).
 503  * @param location The object location to check
 504  * @return The last seq_cst action performed
 505  */
 506 ModelAction * ModelChecker::get_last_seq_cst(const void *location)
 507 {
 508         action_list_t *list = obj_map->ensureptr(location);
 509         /* Find: max({i in dom(S) | seq_cst(t_i) && isWrite(t_i) && samevar(t_i, t)}) */
 510         action_list_t::reverse_iterator rit;
 511         for (rit = list->rbegin(); rit != list->rend(); rit++)
 512                 if ((*rit)->is_write() && (*rit)->is_seqcst())
 513                         return *rit;
 514         return NULL;
 515 }
 516
 517 ModelAction * ModelChecker::get_parent_action(thread_id_t tid)
 518 {
 519         ModelAction *parent = get_last_action(tid);
 520         if (!parent)
 521                 parent = get_thread(tid)->get_creation();
 522         return parent;
 523 }
 524
 525 /**
 526  * Returns the clock vector for a given thread.
 527  * @param tid The thread whose clock vector we want
 528  * @return Desired clock vector
 529  */
 530 ClockVector * ModelChecker::get_cv(thread_id_t tid) {
 531         return get_parent_action(tid)->get_cv();
 532 }
 533
 534
 535 /** Resolve promises. */
 536
 537 void ModelChecker::resolve_promises(ModelAction *write) {
 538         for(unsigned int i=0, promise_index=0;promise_index<promises->size(); i++) {
 539                 Promise * promise=(*promises)[promise_index];
 540                 if (write->get_node()->get_promise(i)) {
 541                         ModelAction * read=promise->get_action();
 542                         read->read_from(write);
 543                         r_modification_order(read, write);
 544                         promises->erase(promises->begin()+promise_index);
 545                 } else
 546                         promise_index++;
 547         }
 548 }
 549
 550 void ModelChecker::compute_promises(ModelAction *curr) {
 551         for(unsigned int i=0;i<promises->size();i++) {
 552                 Promise * promise=(*promises)[i];
 553                 const ModelAction * act=promise->get_action();
 554                 if (!act->happens_before(curr)&&
 555                                 act->is_read()&&
 556                                 !act->is_synchronizing(curr)&&
 557                                 !act->same_thread(curr)&&
 558                                 promise->get_value()==curr->get_value()) {
 559                         curr->get_node()->set_promise(i);
 560                 }
 561         }
 562 }
 563
 564 /** Checks promises in response to change in ClockVector Threads. */
 565
 566 void ModelChecker::check_promises(ClockVector *old_cv, ClockVector * merge_cv) {
 567         for(unsigned int i=0;i<promises->size();i++) {
 568                 Promise * promise=(*promises)[i];
 569                 const ModelAction * act=promise->get_action();
 570                 if ((old_cv==NULL||!old_cv->synchronized_since(act))&&
 571                                 merge_cv->synchronized_since(act)) {
 572                         //This thread is no longer able to send values back to satisfy the promise
 573                         int num_synchronized_threads=promise->increment_threads();
 574                         if (num_synchronized_threads==model->get_num_threads()) {
 575                                 //Promise has failed
 576                                 failed_promise = true;
 577                                 return;
 578                         }
 579                 }
 580         }
 581 }
 582
 583 /**
 584  * Build up an initial set of all past writes that this 'read' action may read
 585  * from. This set is determined by the clock vector's "happens before"
 586  * relationship.
 587  * @param curr is the current ModelAction that we are exploring; it must be a
 588  * 'read' operation.
 589  */
 590 void ModelChecker::build_reads_from_past(ModelAction *curr)
 591 {
 592         std::vector<action_list_t> *thrd_lists = obj_thrd_map->ensureptr(curr->get_location());
 593         unsigned int i;
 594         ASSERT(curr->is_read());
 595
 596         ModelAction *last_seq_cst = NULL;
 597
 598         /* Track whether this object has been initialized */
 599         bool initialized = false;
 600
 601         if (curr->is_seqcst()) {
 602                 last_seq_cst = get_last_seq_cst(curr->get_location());
 603                 /* We have to at least see the last sequentially consistent write,
 604                          so we are initialized. */
 605                 if (last_seq_cst != NULL)
 606                         initialized = true;
 607         }
 608
 609         /* Iterate over all threads */
 610         for (i = 0; i < thrd_lists->size(); i++) {
 611                 /* Iterate over actions in thread, starting from most recent */
 612                 action_list_t *list = &(*thrd_lists)[i];
 613                 action_list_t::reverse_iterator rit;
 614                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 615                         ModelAction *act = *rit;
 616
 617                         /* Only consider 'write' actions */
 618                         if (!act->is_write())
 619                                 continue;
 620
 621                         /* Don't consider more than one seq_cst write if we are a seq_cst read. */
 622                         if (!act->is_seqcst() || !curr->is_seqcst() || act == last_seq_cst) {
 623                                 DEBUG("Adding action to may_read_from:\n");
 624                                 if (DBG_ENABLED()) {
 625                                         act->print();
 626                                         curr->print();
 627                                 }
 628                                 curr->get_node()->add_read_from(act);
 629                         }
 630
 631                         /* Include at most one act per-thread that "happens before" curr */
 632                         if (act->happens_before(curr)) {
 633                                 initialized = true;
 634                                 break;
 635                         }
 636                 }
 637         }
 638
 639         if (!initialized) {
 640                 /** @todo Need a more informative way of reporting errors. */
 641                 printf("ERROR: may read from uninitialized atomic\n");
 642         }
 643
 644         if (DBG_ENABLED() || !initialized) {
 645                 printf("Reached read action:\n");
 646                 curr->print();
 647                 printf("Printing may_read_from\n");
 648                 curr->get_node()->print_may_read_from();
 649                 printf("End printing may_read_from\n");
 650         }
 651
 652         ASSERT(initialized);
 653 }
 654
 655 static void print_list(action_list_t *list)
 656 {
 657         action_list_t::iterator it;
 658
 659         printf("---------------------------------------------------------------------\n");
 660         printf("Trace:\n");
 661
 662         for (it = list->begin(); it != list->end(); it++) {
 663                 (*it)->print();
 664         }
 665         printf("---------------------------------------------------------------------\n");
 666 }
 667
 668 void ModelChecker::print_summary(void)
 669 {
 670         printf("\n");
 671         printf("Number of executions: %d\n", num_executions);
 672         printf("Total nodes created: %d\n", node_stack->get_total_nodes());
 673
 674         scheduler->print();
 675
 676         if (!isfinalfeasible())
 677                 printf("INFEASIBLE EXECUTION!\n");
 678         print_list(action_trace);
 679         printf("\n");
 680 }
 681
 682 int ModelChecker::add_thread(Thread *t)
 683 {
 684         thread_map->put(id_to_int(t->get_id()), t);
 685         scheduler->add_thread(t);
 686         return 0;
 687 }
 688
 689 void ModelChecker::remove_thread(Thread *t)
 690 {
 691         scheduler->remove_thread(t);
 692 }
 693
 694 /**
 695  * Switch from a user-context to the "master thread" context (a.k.a. system
 696  * context). This switch is made with the intention of exploring a particular
 697  * model-checking action (described by a ModelAction object). Must be called
 698  * from a user-thread context.
 699  * @param act The current action that will be explored. May be NULL, although
 700  * there is little reason to switch to the model-checker without an action to
 701  * explore (note: act == NULL is sometimes used as a hack to allow a thread to
 702  * yield control without performing any progress; see thrd_join()).
 703  * @return Return status from the 'swap' call (i.e., success/fail, 0/-1)
 704  */
 705 int ModelChecker::switch_to_master(ModelAction *act)
 706 {
 707         DBG();
 708         Thread * old = thread_current();
 709         set_current_action(act);
 710         old->set_state(THREAD_READY);
 711         return Thread::swap(old, get_system_context());
 712 }