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_promises()) {
 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_values()) {
 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 (isfeasible() || 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                         delete curr;
 283                         curr = tmp;
 284                 } else {
 285                         /*
 286                          * Perform one-time actions when pushing new ModelAction onto
 287                          * NodeStack
 288                          */
 289                         curr->create_cv(get_parent_action(curr->get_tid()));
 290                         /* Build may_read_from set */
 291                         if (curr->is_read())
 292                                 build_reads_from_past(curr);
 293                         if (curr->is_write())
 294                                 compute_promises(curr);
 295                 }
 296         }
 297
 298         /* Assign 'creation' parent */
 299         if (curr->get_type() == THREAD_CREATE) {
 300                 Thread *th = (Thread *)curr->get_location();
 301                 th->set_creation(curr);
 302         }
 303
 304         /* Deal with new thread */
 305         if (curr->get_type() == THREAD_START) {
 306                 check_promises(NULL, curr->get_cv());
 307         }
 308
 309         /* Assign reads_from values */
 310         Thread *th = get_thread(curr->get_tid());
 311         uint64_t value = VALUE_NONE;
 312         if (curr->is_read()) {
 313                 const ModelAction *reads_from = curr->get_node()->get_read_from();
 314                 if (reads_from!=NULL) {
 315                         value = reads_from->get_value();
 316                         /* Assign reads_from, perform release/acquire synchronization */
 317                         curr->read_from(reads_from);
 318                         r_modification_order(curr,reads_from);
 319                 } else {
 320                         /* Read from future value */
 321                         value = curr->get_node()->get_future_value();
 322                         curr->read_from(NULL);
 323                         Promise * valuepromise=new Promise(curr, value);
 324                         promises->push_back(valuepromise);
 325                 }
 326         } else if (curr->is_write()) {
 327                 w_modification_order(curr);
 328                 resolve_promises(curr);
 329         }
 330
 331         th->set_return_value(value);
 332
 333         /* Add action to list.  */
 334         if (!already_added)
 335                 add_action_to_lists(curr);
 336
 337         /* Is there a better interface for setting the next thread rather
 338                  than this field/convoluted approach?  Perhaps like just returning
 339                  it or something? */
 340
 341         /* Do not split atomic actions. */
 342         if (curr->is_rmwr()) {
 343                 nextThread = thread_current()->get_id();
 344         } else {
 345                 nextThread = get_next_replay_thread();
 346         }
 347
 348         Node *currnode = curr->get_node();
 349         Node *parnode = currnode->get_parent();
 350
 351         if (!parnode->backtrack_empty()||!currnode->readsfrom_empty()||!currnode->futurevalues_empty()||!currnode->promises_empty())
 352                 if (!next_backtrack || *curr > *next_backtrack)
 353                         next_backtrack = curr;
 354
 355         set_backtracking(curr);
 356 }
 357
 358 /** @returns whether the current trace is feasible. */
 359 bool ModelChecker::isfeasible() {
 360         return !cyclegraph->checkForCycles() && !failed_promise;
 361 }
 362
 363 /** Close out a RMWR by converting previous RMWR into a RMW or READ. */
 364 ModelAction * ModelChecker::process_rmw(ModelAction * act) {
 365         int tid = id_to_int(act->get_tid());
 366         ModelAction *lastread=get_last_action(tid);
 367         lastread->process_rmw(act);
 368         if (act->is_rmw())
 369                 cyclegraph->addRMWEdge(lastread, lastread->get_reads_from());
 370         return lastread;
 371 }
 372
 373 /**
 374  * Updates the cyclegraph with the constraints imposed from the current read.
 375  * @param curr The current action. Must be a read.
 376  * @param rf The action that curr reads from. Must be a write.
 377  */
 378 void ModelChecker::r_modification_order(ModelAction * curr, const ModelAction *rf) {
 379         std::vector<action_list_t> *thrd_lists = obj_thrd_map->ensureptr(curr->get_location());
 380         unsigned int i;
 381         ASSERT(curr->is_read());
 382
 383         /* Iterate over all threads */
 384         for (i = 0; i < thrd_lists->size(); i++) {
 385                 /* Iterate over actions in thread, starting from most recent */
 386                 action_list_t *list = &(*thrd_lists)[i];
 387                 action_list_t::reverse_iterator rit;
 388                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 389                         ModelAction *act = *rit;
 390
 391                         /* Include at most one act per-thread that "happens before" curr */
 392                         if (act->happens_before(curr)) {
 393                                 if (act->is_read()) {
 394                                         const ModelAction * prevreadfrom=act->get_reads_from();
 395                                         if (rf!=prevreadfrom)
 396                                                 cyclegraph->addEdge(rf, prevreadfrom);
 397                                 } else if (rf!=act) {
 398                                         cyclegraph->addEdge(rf, act);
 399                                 }
 400                                 break;
 401                         }
 402                 }
 403         }
 404 }
 405
 406 /**
 407  * Updates the cyclegraph with the constraints imposed from the current write.
 408  * @param curr The current action. Must be a write.
 409  */
 410 void ModelChecker::w_modification_order(ModelAction * curr) {
 411         std::vector<action_list_t> *thrd_lists = obj_thrd_map->ensureptr(curr->get_location());
 412         unsigned int i;
 413         ASSERT(curr->is_write());
 414
 415         if (curr->is_seqcst()) {
 416                 /* We have to at least see the last sequentially consistent write,
 417                          so we are initialized. */
 418                 ModelAction * last_seq_cst=get_last_seq_cst(curr->get_location());
 419                 if (last_seq_cst!=NULL)
 420                         cyclegraph->addEdge(curr, last_seq_cst);
 421         }
 422
 423         /* Iterate over all threads */
 424         for (i = 0; i < thrd_lists->size(); i++) {
 425                 /* Iterate over actions in thread, starting from most recent */
 426                 action_list_t *list = &(*thrd_lists)[i];
 427                 action_list_t::reverse_iterator rit;
 428                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 429                         ModelAction *act = *rit;
 430
 431                         /* Include at most one act per-thread that "happens before" curr */
 432                         if (act->happens_before(curr)) {
 433                                 if (act->is_read()) {
 434                                         cyclegraph->addEdge(curr, act->get_reads_from());
 435                                 } else
 436                                         cyclegraph->addEdge(curr, act);
 437                                 break;
 438                         } else {
 439                                 if (act->is_read()&&!act->is_synchronizing(curr)&&!act->same_thread(curr)) {
 440                                         /* We have an action that:
 441                                                  (1) did not happen before us
 442                                                  (2) is a read and we are a write
 443                                                  (3) cannot synchronize with us
 444                                                  (4) is in a different thread
 445                                                  =>
 446                                                  that read could potentially read from our write.
 447                                         */
 448
 449                                         if (act->get_node()->add_future_value(curr->get_value())&&
 450                                                         (!next_backtrack || *act > * next_backtrack))
 451                                                 next_backtrack = act;
 452                                 }
 453                         }
 454                 }
 455         }
 456 }
 457
 458 /**
 459  * Performs various bookkeeping operations for the current ModelAction. For
 460  * instance, adds action to the per-object, per-thread action vector and to the
 461  * action trace list of all thread actions.
 462  *
 463  * @param act is the ModelAction to add.
 464  */
 465 void ModelChecker::add_action_to_lists(ModelAction *act)
 466 {
 467         int tid = id_to_int(act->get_tid());
 468         action_trace->push_back(act);
 469
 470         obj_map->ensureptr(act->get_location())->push_back(act);
 471
 472         std::vector<action_list_t> *vec = obj_thrd_map->ensureptr(act->get_location());
 473         if (tid >= (int)vec->size())
 474                 vec->resize(next_thread_id);
 475         (*vec)[tid].push_back(act);
 476
 477         if ((int)thrd_last_action->size() <= tid)
 478                 thrd_last_action->resize(get_num_threads());
 479         (*thrd_last_action)[tid] = act;
 480 }
 481
 482 ModelAction * ModelChecker::get_last_action(thread_id_t tid)
 483 {
 484         int nthreads = get_num_threads();
 485         if ((int)thrd_last_action->size() < nthreads)
 486                 thrd_last_action->resize(nthreads);
 487         return (*thrd_last_action)[id_to_int(tid)];
 488 }
 489
 490 /**
 491  * Gets the last memory_order_seq_cst action (in the total global sequence)
 492  * performed on a particular object (i.e., memory location).
 493  * @param location The object location to check
 494  * @return The last seq_cst action performed
 495  */
 496 ModelAction * ModelChecker::get_last_seq_cst(const void *location)
 497 {
 498         action_list_t *list = obj_map->ensureptr(location);
 499         /* Find: max({i in dom(S) | seq_cst(t_i) && isWrite(t_i) && samevar(t_i, t)}) */
 500         action_list_t::reverse_iterator rit;
 501         for (rit = list->rbegin(); rit != list->rend(); rit++)
 502                 if ((*rit)->is_write() && (*rit)->is_seqcst())
 503                         return *rit;
 504         return NULL;
 505 }
 506
 507 ModelAction * ModelChecker::get_parent_action(thread_id_t tid)
 508 {
 509         ModelAction *parent = get_last_action(tid);
 510         if (!parent)
 511                 parent = get_thread(tid)->get_creation();
 512         return parent;
 513 }
 514
 515 /**
 516  * Returns the clock vector for a given thread.
 517  * @param tid The thread whose clock vector we want
 518  * @return Desired clock vector
 519  */
 520 ClockVector * ModelChecker::get_cv(thread_id_t tid) {
 521         return get_parent_action(tid)->get_cv();
 522 }
 523
 524
 525 /** Resolve promises. */
 526
 527 void ModelChecker::resolve_promises(ModelAction *write) {
 528         for(unsigned int i=0;i<promises->size();i++) {
 529                 Promise * promise=(*promises)[i];
 530                 if (write->get_node()->get_promise(i)) {
 531                         ModelAction * read=promise->get_action();
 532                         read->read_from(write);
 533                         r_modification_order(read, write);
 534                 }
 535         }
 536 }
 537
 538 void ModelChecker::compute_promises(ModelAction *curr) {
 539         for(unsigned int i=0;i<promises->size();i++) {
 540                 Promise * promise=(*promises)[i];
 541                 const ModelAction * act=promise->get_action();
 542                 if (!act->happens_before(curr)&&
 543                                 act->is_read()&&
 544                                 !act->is_synchronizing(curr)&&
 545                                 !act->same_thread(curr)&&
 546                                 promise->get_value()==curr->get_value()) {
 547                         curr->get_node()->set_promise(i);
 548                 }
 549         }
 550 }
 551
 552 /** Checks promises in response to change in ClockVector Threads. */
 553
 554 void ModelChecker::check_promises(ClockVector *old_cv, ClockVector * merge_cv) {
 555         for(unsigned int i=0;i<promises->size();i++) {
 556                 Promise * promise=(*promises)[i];
 557                 const ModelAction * act=promise->get_action();
 558                 if ((old_cv==NULL||!old_cv->synchronized_since(act))&&
 559                                 merge_cv->synchronized_since(act)) {
 560                         //This thread is no longer able to send values back to satisfy the promise
 561                         int num_synchronized_threads=promise->increment_threads();
 562                         if (num_synchronized_threads==model->get_num_threads()) {
 563                                 //Promise has failed
 564                                 failed_promise = true;
 565                                 return;
 566                         }
 567                 }
 568         }
 569 }
 570
 571 /**
 572  * Build up an initial set of all past writes that this 'read' action may read
 573  * from. This set is determined by the clock vector's "happens before"
 574  * relationship.
 575  * @param curr is the current ModelAction that we are exploring; it must be a
 576  * 'read' operation.
 577  */
 578 void ModelChecker::build_reads_from_past(ModelAction *curr)
 579 {
 580         std::vector<action_list_t> *thrd_lists = obj_thrd_map->ensureptr(curr->get_location());
 581         unsigned int i;
 582         ASSERT(curr->is_read());
 583
 584         ModelAction *last_seq_cst = NULL;
 585
 586         /* Track whether this object has been initialized */
 587         bool initialized = false;
 588
 589         if (curr->is_seqcst()) {
 590                 last_seq_cst = get_last_seq_cst(curr->get_location());
 591                 /* We have to at least see the last sequentially consistent write,
 592                          so we are initialized. */
 593                 if (last_seq_cst != NULL)
 594                         initialized = true;
 595         }
 596
 597         /* Iterate over all threads */
 598         for (i = 0; i < thrd_lists->size(); i++) {
 599                 /* Iterate over actions in thread, starting from most recent */
 600                 action_list_t *list = &(*thrd_lists)[i];
 601                 action_list_t::reverse_iterator rit;
 602                 for (rit = list->rbegin(); rit != list->rend(); rit++) {
 603                         ModelAction *act = *rit;
 604
 605                         /* Only consider 'write' actions */
 606                         if (!act->is_write())
 607                                 continue;
 608
 609                         /* Don't consider more than one seq_cst write if we are a seq_cst read. */
 610                         if (!act->is_seqcst() || !curr->is_seqcst() || act == last_seq_cst) {
 611                                 DEBUG("Adding action to may_read_from:\n");
 612                                 if (DBG_ENABLED()) {
 613                                         act->print();
 614                                         curr->print();
 615                                 }
 616                                 curr->get_node()->add_read_from(act);
 617                         }
 618
 619                         /* Include at most one act per-thread that "happens before" curr */
 620                         if (act->happens_before(curr)) {
 621                                 initialized = true;
 622                                 break;
 623                         }
 624                 }
 625         }
 626
 627         if (!initialized) {
 628                 /** @todo Need a more informative way of reporting errors. */
 629                 printf("ERROR: may read from uninitialized atomic\n");
 630         }
 631
 632         if (DBG_ENABLED() || !initialized) {
 633                 printf("Reached read action:\n");
 634                 curr->print();
 635                 printf("Printing may_read_from\n");
 636                 curr->get_node()->print_may_read_from();
 637                 printf("End printing may_read_from\n");
 638         }
 639
 640         ASSERT(initialized);
 641 }
 642
 643 static void print_list(action_list_t *list)
 644 {
 645         action_list_t::iterator it;
 646
 647         printf("---------------------------------------------------------------------\n");
 648         printf("Trace:\n");
 649
 650         for (it = list->begin(); it != list->end(); it++) {
 651                 (*it)->print();
 652         }
 653         printf("---------------------------------------------------------------------\n");
 654 }
 655
 656 void ModelChecker::print_summary(void)
 657 {
 658         printf("\n");
 659         printf("Number of executions: %d\n", num_executions);
 660         printf("Total nodes created: %d\n", node_stack->get_total_nodes());
 661
 662         scheduler->print();
 663
 664         if (!isfeasible())
 665                 printf("INFEASIBLE EXECUTION!\n");
 666         print_list(action_trace);
 667         printf("\n");
 668 }
 669
 670 int ModelChecker::add_thread(Thread *t)
 671 {
 672         thread_map->put(id_to_int(t->get_id()), t);
 673         scheduler->add_thread(t);
 674         return 0;
 675 }
 676
 677 void ModelChecker::remove_thread(Thread *t)
 678 {
 679         scheduler->remove_thread(t);
 680 }
 681
 682 /**
 683  * Switch from a user-context to the "master thread" context (a.k.a. system
 684  * context). This switch is made with the intention of exploring a particular
 685  * model-checking action (described by a ModelAction object). Must be called
 686  * from a user-thread context.
 687  * @param act The current action that will be explored. May be NULL, although
 688  * there is little reason to switch to the model-checker without an action to
 689  * explore (note: act == NULL is sometimes used as a hack to allow a thread to
 690  * yield control without performing any progress; see thrd_join()).
 691  * @return Return status from the 'swap' call (i.e., success/fail, 0/-1)
 692  */
 693 int ModelChecker::switch_to_master(ModelAction *act)
 694 {
 695         DBG();
 696         Thread * old = thread_current();
 697         set_current_action(act);
 698         old->set_state(THREAD_READY);
 699         return Thread::swap(old, get_system_context());
 700 }