make hashtables only contain primitive types or pointers

[cdsspec-compiler.git] / model.cc

#include <stdio.h>
#include <algorithm>
#include <mutex>

#include "model.h"
#include "action.h"
#include "nodestack.h"
#include "schedule.h"
#include "snapshot-interface.h"
#include "common.h"
#include "clockvector.h"
#include "cyclegraph.h"
#include "promise.h"
#include "datarace.h"
#include "threads-model.h"

#define INITIAL_THREAD_ID       0

ModelChecker *model;

/**
 * Structure for holding small ModelChecker members that should be snapshotted
 */
struct model_snapshot_members {
        ModelAction *current_action;
        unsigned int next_thread_id;
        modelclock_t used_sequence_numbers;
        Thread *nextThread;
        ModelAction *next_backtrack;
};

/** @brief Constructor */
ModelChecker::ModelChecker(struct model_params params) :
        /* Initialize default scheduler */
        params(params),
        scheduler(new Scheduler()),
        num_executions(0),
        num_feasible_executions(0),
        diverge(NULL),
        earliest_diverge(NULL),
        action_trace(new action_list_t()),
        thread_map(new HashTable<int, Thread *, int>()),
        obj_map(new HashTable<const void *, action_list_t *, uintptr_t, 4>()),
        lock_waiters_map(new HashTable<const void *, action_list_t *, uintptr_t, 4>()),
        condvar_waiters_map(new HashTable<const void *, action_list_t *, uintptr_t, 4>()),
        obj_thrd_map(new HashTable<void *, std::vector<action_list_t> *, uintptr_t, 4 >()),
        promises(new std::vector< Promise *, SnapshotAlloc<Promise *> >()),
        futurevalues(new std::vector< struct PendingFutureValue, SnapshotAlloc<struct PendingFutureValue> >()),
        pending_rel_seqs(new std::vector< struct release_seq *, SnapshotAlloc<struct release_seq *> >()),
        thrd_last_action(new std::vector< ModelAction *, SnapshotAlloc<ModelAction *> >(1)),
        node_stack(new NodeStack()),
        mo_graph(new CycleGraph()),
        failed_promise(false),
        too_many_reads(false),
        asserted(false),
        bad_synchronization(false)
{
        /* Allocate this "size" on the snapshotting heap */
        priv = (struct model_snapshot_members *)calloc(1, sizeof(*priv));
        /* First thread created will have id INITIAL_THREAD_ID */
        priv->next_thread_id = INITIAL_THREAD_ID;

        /* Initialize a model-checker thread, for special ModelActions */
        model_thread = new Thread(get_next_id());
        thread_map->put(id_to_int(model_thread->get_id()), model_thread);
}

/** @brief Destructor */
ModelChecker::~ModelChecker()
{
        for (unsigned int i = 0; i < get_num_threads(); i++)
                delete thread_map->get(i);
        delete thread_map;

        delete obj_thrd_map;
        delete obj_map;
        delete lock_waiters_map;
        delete condvar_waiters_map;
        delete action_trace;

        for (unsigned int i = 0; i < promises->size(); i++)
                delete (*promises)[i];
        delete promises;

        delete pending_rel_seqs;

        delete thrd_last_action;
        delete node_stack;
        delete scheduler;
        delete mo_graph;
}

static action_list_t * get_safe_ptr_action(HashTable<const void *, action_list_t *, uintptr_t, 4> * hash, void * ptr) {
        action_list_t * tmp=hash->get(ptr);
        if (tmp==NULL) {
                tmp=new action_list_t();
                hash->put(ptr, tmp);
        }
        return tmp;
}

static std::vector<action_list_t> * get_safe_ptr_vect_action(HashTable<void *, std::vector<action_list_t> *, uintptr_t, 4> * hash, void * ptr) {
        std::vector<action_list_t> * tmp=hash->get(ptr);
        if (tmp==NULL) {
                tmp=new std::vector<action_list_t>();
                hash->put(ptr, tmp);
        }
        return tmp;
}

/**
 * Restores user program to initial state and resets all model-checker data
 * structures.
 */
void ModelChecker::reset_to_initial_state()
{
        DEBUG("+++ Resetting to initial state +++\n");
        node_stack->reset_execution();
        failed_promise = false;
        too_many_reads = false;
        bad_synchronization = false;
        reset_asserted();
        snapshotObject->backTrackBeforeStep(0);
}

/** @return a thread ID for a new Thread */
thread_id_t ModelChecker::get_next_id()
{
        return priv->next_thread_id++;
}

/** @return the number of user threads created during this execution */
unsigned int ModelChecker::get_num_threads() const
{
        return priv->next_thread_id;
}

/** @return The currently executing Thread. */
Thread * ModelChecker::get_current_thread()
{
        return scheduler->get_current_thread();
}

/** @return a sequence number for a new ModelAction */
modelclock_t ModelChecker::get_next_seq_num()
{
        return ++priv->used_sequence_numbers;
}

Node * ModelChecker::get_curr_node() {
        return node_stack->get_head();
}

/**
 * @brief Choose the next thread to execute.
 *
 * This function chooses the next thread that should execute. It can force the
 * adjacency of read/write portions of a RMW action, force THREAD_CREATE to be
 * followed by a THREAD_START, or it can enforce execution replay/backtracking.
 * The model-checker may have no preference regarding the next thread (i.e.,
 * when exploring a new execution ordering), in which case this will return
 * NULL.
 * @param curr The current ModelAction. This action might guide the choice of
 * next thread.
 * @return The next thread to run. If the model-checker has no preference, NULL.
 */
Thread * ModelChecker::get_next_thread(ModelAction *curr)
{
        thread_id_t tid;

        if (curr!=NULL) {
                /* Do not split atomic actions. */
                if (curr->is_rmwr())
                        return thread_current();
                /* The THREAD_CREATE action points to the created Thread */
                else if (curr->get_type() == THREAD_CREATE)
                        return (Thread *)curr->get_location();
        }

        /* Have we completed exploring the preselected path? */
        if (diverge == NULL)
                return NULL;

        /* Else, we are trying to replay an execution */
        ModelAction *next = node_stack->get_next()->get_action();

        if (next == diverge) {
                if (earliest_diverge == NULL || *diverge < *earliest_diverge)
                        earliest_diverge=diverge;

                Node *nextnode = next->get_node();
                Node *prevnode = nextnode->get_parent();
                scheduler->update_sleep_set(prevnode);

                /* Reached divergence point */
                if (nextnode->increment_misc()) {
                        /* The next node will try to satisfy a different misc_index values. */
                        tid = next->get_tid();
                        node_stack->pop_restofstack(2);
                } else if (nextnode->increment_promise()) {
                        /* The next node will try to satisfy a different set of promises. */
                        tid = next->get_tid();
                        node_stack->pop_restofstack(2);
                } else if (nextnode->increment_read_from()) {
                        /* The next node will read from a different value. */
                        tid = next->get_tid();
                        node_stack->pop_restofstack(2);
                } else if (nextnode->increment_future_value()) {
                        /* The next node will try to read from a different future value. */
                        tid = next->get_tid();
                        node_stack->pop_restofstack(2);
                } else if (nextnode->increment_relseq_break()) {
                        /* The next node will try to resolve a release sequence differently */
                        tid = next->get_tid();
                        node_stack->pop_restofstack(2);
                } else {
                        /* Make a different thread execute for next step */
                        scheduler->add_sleep(thread_map->get(id_to_int(next->get_tid())));
                        tid = prevnode->get_next_backtrack();
                        /* Make sure the backtracked thread isn't sleeping. */
                        node_stack->pop_restofstack(1);
                        if (diverge==earliest_diverge) {
                                earliest_diverge=prevnode->get_action();
                        }
                }
                /* The correct sleep set is in the parent node. */
                execute_sleep_set();

                DEBUG("*** Divergence point ***\n");

                diverge = NULL;
        } else {
                tid = next->get_tid();
        }
        DEBUG("*** ModelChecker chose next thread = %d ***\n", id_to_int(tid));
        ASSERT(tid != THREAD_ID_T_NONE);
        return thread_map->get(id_to_int(tid));
}

/**
 * We need to know what the next actions of all threads in the sleep
 * set will be.  This method computes them and stores the actions at
 * the corresponding thread object's pending action.
 */

void ModelChecker::execute_sleep_set() {
        for(unsigned int i=0;i<get_num_threads();i++) {
                thread_id_t tid=int_to_id(i);
                Thread *thr=get_thread(tid);
                if ( scheduler->get_enabled(thr) == THREAD_SLEEP_SET &&
                                 thr->get_pending() == NULL ) {
                        thr->set_state(THREAD_RUNNING);
                        scheduler->next_thread(thr);
                        Thread::swap(&system_context, thr);
                        priv->current_action->set_sleep_flag();
                        thr->set_pending(priv->current_action);
                }
        }
        priv->current_action = NULL;
}

void ModelChecker::wake_up_sleeping_actions(ModelAction * curr) {
        for(unsigned int i=0;i<get_num_threads();i++) {
                thread_id_t tid=int_to_id(i);
                Thread *thr=get_thread(tid);
                if ( scheduler->get_enabled(thr) == THREAD_SLEEP_SET ) {
                        ModelAction *pending_act=thr->get_pending();
                        if ((!curr->is_rmwr())&&pending_act->could_synchronize_with(curr)) {
                                //Remove this thread from sleep set
                                scheduler->remove_sleep(thr);
                        }
                }
        }
}

/**
 * Check if we are in a deadlock. Should only be called at the end of an
 * execution, although it should not give false positives in the middle of an
 * execution (there should be some ENABLED thread).
 *
 * @return True if program is in a deadlock; false otherwise
 */
bool ModelChecker::is_deadlocked() const
{
        bool blocking_threads = false;
        for (unsigned int i = 0; i < get_num_threads(); i++) {
                thread_id_t tid = int_to_id(i);
                if (is_enabled(tid))
                        return false;
                Thread *t = get_thread(tid);
                if (!t->is_model_thread() && t->get_pending())
                        blocking_threads = true;
        }
        return blocking_threads;
}

/**
 * Queries the model-checker for more executions to explore and, if one
 * exists, resets the model-checker state to execute a new execution.
 *
 * @return If there are more executions to explore, return true. Otherwise,
 * return false.
 */
bool ModelChecker::next_execution()
{
        DBG();

        num_executions++;

        if (is_deadlocked())
                printf("ERROR: DEADLOCK\n");
        if (isfinalfeasible()) {
                printf("Earliest divergence point since last feasible execution:\n");
                if (earliest_diverge)
                        earliest_diverge->print();
                else
                        printf("(Not set)\n");

                earliest_diverge = NULL;
                num_feasible_executions++;
        }

        DEBUG("Number of acquires waiting on pending release sequences: %zu\n",
                        pending_rel_seqs->size());


        if (isfinalfeasible() || DBG_ENABLED()) {
                checkDataRaces();
                print_summary();
        }

        if ((diverge = get_next_backtrack()) == NULL)
                return false;

        if (DBG_ENABLED()) {
                printf("Next execution will diverge at:\n");
                diverge->print();
        }

        reset_to_initial_state();
        return true;
}

ModelAction * ModelChecker::get_last_conflict(ModelAction *act)
{
        switch (act->get_type()) {
        case ATOMIC_READ:
        case ATOMIC_WRITE:
        case ATOMIC_RMW: {
                /* linear search: from most recent to oldest */
                action_list_t *list = get_safe_ptr_action(obj_map, act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (prev->could_synchronize_with(act))
                                return prev;
                }
                break;
        }
        case ATOMIC_LOCK:
        case ATOMIC_TRYLOCK: {
                /* linear search: from most recent to oldest */
                action_list_t *list = get_safe_ptr_action(obj_map, act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (act->is_conflicting_lock(prev))
                                return prev;
                }
                break;
        }
        case ATOMIC_UNLOCK: {
                /* linear search: from most recent to oldest */
                action_list_t *list = get_safe_ptr_action(obj_map, act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (!act->same_thread(prev)&&prev->is_failed_trylock())
                                return prev;
                }
                break;
        }
        case ATOMIC_WAIT: {
                /* linear search: from most recent to oldest */
                action_list_t *list = get_safe_ptr_action(obj_map, act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (!act->same_thread(prev)&&prev->is_failed_trylock())
                                return prev;
                        if (!act->same_thread(prev)&&prev->is_notify())
                                return prev;
                }
                break;
        }

        case ATOMIC_NOTIFY_ALL:
        case ATOMIC_NOTIFY_ONE: {
                /* linear search: from most recent to oldest */
                action_list_t *list = get_safe_ptr_action(obj_map, act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (!act->same_thread(prev)&&prev->is_wait())
                                return prev;
                }
                break;
        }
        default:
                break;
        }
        return NULL;
}

/** This method finds backtracking points where we should try to
 * reorder the parameter ModelAction against.
 *
 * @param the ModelAction to find backtracking points for.
 */
void ModelChecker::set_backtracking(ModelAction *act)
{
        Thread *t = get_thread(act);
        ModelAction * prev = get_last_conflict(act);
        if (prev == NULL)
                return;

        Node * node = prev->get_node()->get_parent();

        int low_tid, high_tid;
        if (node->is_enabled(t)) {
                low_tid = id_to_int(act->get_tid());
                high_tid = low_tid+1;
        } else {
                low_tid = 0;
                high_tid = get_num_threads();
        }

        for(int i = low_tid; i < high_tid; i++) {
                thread_id_t tid = int_to_id(i);

                /* Make sure this thread can be enabled here. */
                if (i >= node->get_num_threads())
                        break;

                /* Don't backtrack into a point where the thread is disabled or sleeping. */
                if (node->enabled_status(tid)!=THREAD_ENABLED)
                        continue;

                /* Check if this has been explored already */
                if (node->has_been_explored(tid))
                        continue;

                /* See if fairness allows */
                if (model->params.fairwindow != 0 && !node->has_priority(tid)) {
                        bool unfair=false;
                        for(int t=0;t<node->get_num_threads();t++) {
                                thread_id_t tother=int_to_id(t);
                                if (node->is_enabled(tother) && node->has_priority(tother)) {
                                        unfair=true;
                                        break;
                                }
                        }
                        if (unfair)
                                continue;
                }
                /* Cache the latest backtracking point */
                if (!priv->next_backtrack || *prev > *priv->next_backtrack)
                        priv->next_backtrack = prev;

                /* If this is a new backtracking point, mark the tree */
                if (!node->set_backtrack(tid))
                        continue;
                DEBUG("Setting backtrack: conflict = %d, instead tid = %d\n",
                                        id_to_int(prev->get_tid()),
                                        id_to_int(t->get_id()));
                if (DBG_ENABLED()) {
                        prev->print();
                        act->print();
                }
        }
}

/**
 * Returns last backtracking point. The model checker will explore a different
 * path for this point in the next execution.
 * @return The ModelAction at which the next execution should diverge.
 */
ModelAction * ModelChecker::get_next_backtrack()
{
        ModelAction *next = priv->next_backtrack;
        priv->next_backtrack = NULL;
        return next;
}

/**
 * Processes a read or rmw model action.
 * @param curr is the read model action to process.
 * @param second_part_of_rmw is boolean that is true is this is the second action of a rmw.
 * @return True if processing this read updates the mo_graph.
 */
bool ModelChecker::process_read(ModelAction *curr, bool second_part_of_rmw)
{
        uint64_t value = VALUE_NONE;
        bool updated = false;
        while (true) {
                const ModelAction *reads_from = curr->get_node()->get_read_from();
                if (reads_from != NULL) {
                        mo_graph->startChanges();

                        value = reads_from->get_value();
                        bool r_status = false;

                        if (!second_part_of_rmw) {
                                check_recency(curr, reads_from);
                                r_status = r_modification_order(curr, reads_from);
                        }


                        if (!second_part_of_rmw&&!isfeasible()&&(curr->get_node()->increment_read_from()||curr->get_node()->increment_future_value())) {
                                mo_graph->rollbackChanges();
                                too_many_reads = false;
                                continue;
                        }

                        curr->read_from(reads_from);
                        mo_graph->commitChanges();
                        mo_check_promises(curr->get_tid(), reads_from);

                        updated |= r_status;
                } else if (!second_part_of_rmw) {
                        /* Read from future value */
                        value = curr->get_node()->get_future_value();
                        modelclock_t expiration = curr->get_node()->get_future_value_expiration();
                        curr->read_from(NULL);
                        Promise *valuepromise = new Promise(curr, value, expiration);
                        promises->push_back(valuepromise);
                }
                get_thread(curr)->set_return_value(value);
                return updated;
        }
}

/**
 * Processes a lock, trylock, or unlock model action.  @param curr is
 * the read model action to process.
 *
 * The try lock operation checks whether the lock is taken.  If not,
 * it falls to the normal lock operation case.  If so, it returns
 * fail.
 *
 * The lock operation has already been checked that it is enabled, so
 * it just grabs the lock and synchronizes with the previous unlock.
 *
 * The unlock operation has to re-enable all of the threads that are
 * waiting on the lock.
 *
 * @return True if synchronization was updated; false otherwise
 */
bool ModelChecker::process_mutex(ModelAction *curr) {
        std::mutex *mutex=NULL;
        struct std::mutex_state *state=NULL;

        if (curr->is_trylock() || curr->is_lock() || curr->is_unlock()) {
                mutex = (std::mutex *)curr->get_location();
                state = mutex->get_state();
        } else if(curr->is_wait()) {
                mutex = (std::mutex *)curr->get_value();
                state = mutex->get_state();
        }

        switch (curr->get_type()) {
        case ATOMIC_TRYLOCK: {
                bool success = !state->islocked;
                curr->set_try_lock(success);
                if (!success) {
                        get_thread(curr)->set_return_value(0);
                        break;
                }
                get_thread(curr)->set_return_value(1);
        }
                //otherwise fall into the lock case
        case ATOMIC_LOCK: {
                if (curr->get_cv()->getClock(state->alloc_tid) <= state->alloc_clock) {
                        printf("Lock access before initialization\n");
                        set_assert();
                }
                state->islocked = true;
                ModelAction *unlock = get_last_unlock(curr);
                //synchronize with the previous unlock statement
                if (unlock != NULL) {
                        curr->synchronize_with(unlock);
                        return true;
                }
                break;
        }
        case ATOMIC_UNLOCK: {
                //unlock the lock
                state->islocked = false;
                //wake up the other threads
                action_list_t *waiters = get_safe_ptr_action(lock_waiters_map, curr->get_location());
                //activate all the waiting threads
                for (action_list_t::iterator rit = waiters->begin(); rit != waiters->end(); rit++) {
                        scheduler->wake(get_thread(*rit));
                }
                waiters->clear();
                break;
        }
        case ATOMIC_WAIT: {
                //unlock the lock
                state->islocked = false;
                //wake up the other threads
                action_list_t *waiters = get_safe_ptr_action(lock_waiters_map, (void *) curr->get_value());
                //activate all the waiting threads
                for (action_list_t::iterator rit = waiters->begin(); rit != waiters->end(); rit++) {
                        scheduler->wake(get_thread(*rit));
                }
                waiters->clear();
                //check whether we should go to sleep or not...simulate spurious failures
                if (curr->get_node()->get_misc()==0) {
                        get_safe_ptr_action(condvar_waiters_map, curr->get_location())->push_back(curr);
                        //disable us
                        scheduler->sleep(get_current_thread());
                }
                break;
        }
        case ATOMIC_NOTIFY_ALL: {
                action_list_t *waiters = get_safe_ptr_action(condvar_waiters_map, curr->get_location());
                //activate all the waiting threads
                for (action_list_t::iterator rit = waiters->begin(); rit != waiters->end(); rit++) {
                        scheduler->wake(get_thread(*rit));
                }
                waiters->clear();
                break;
        }
        case ATOMIC_NOTIFY_ONE: {
                action_list_t *waiters = get_safe_ptr_action(condvar_waiters_map, curr->get_location());
                int wakeupthread=curr->get_node()->get_misc();
                action_list_t::iterator it = waiters->begin();
                advance(it, wakeupthread);
                scheduler->wake(get_thread(*it));
                waiters->erase(it);
                break;
        }

        default:
                ASSERT(0);
        }
        return false;
}

/**
 * Process a write ModelAction
 * @param curr The ModelAction to process
 * @return True if the mo_graph was updated or promises were resolved
 */
bool ModelChecker::process_write(ModelAction *curr)
{
        bool updated_mod_order = w_modification_order(curr);
        bool updated_promises = resolve_promises(curr);

        if (promises->size() == 0) {
                for (unsigned int i = 0; i < futurevalues->size(); i++) {
                        struct PendingFutureValue pfv = (*futurevalues)[i];
                        //Do more ambitious checks now that mo is more complete
                        if (mo_may_allow(pfv.writer, pfv.act)&&
                                        pfv.act->get_node()->add_future_value(pfv.writer->get_value(), pfv.writer->get_seq_number()+params.maxfuturedelay) &&
                                        (!priv->next_backtrack || *pfv.act > *priv->next_backtrack))
                                priv->next_backtrack = pfv.act;
                }
                futurevalues->resize(0);
        }

        mo_graph->commitChanges();
        mo_check_promises(curr->get_tid(), curr);

        get_thread(curr)->set_return_value(VALUE_NONE);
        return updated_mod_order || updated_promises;
}

/**
 * @brief Process the current action for thread-related activity
 *
 * Performs current-action processing for a THREAD_* ModelAction. Proccesses
 * may include setting Thread status, completing THREAD_FINISH/THREAD_JOIN
 * synchronization, etc.  This function is a no-op for non-THREAD actions
 * (e.g., ATOMIC_{READ,WRITE,RMW,LOCK}, etc.)
 *
 * @param curr The current action
 * @return True if synchronization was updated or a thread completed
 */
bool ModelChecker::process_thread_action(ModelAction *curr)
{
        bool updated = false;

        switch (curr->get_type()) {
        case THREAD_CREATE: {
                Thread *th = (Thread *)curr->get_location();
                th->set_creation(curr);
                break;
        }
        case THREAD_JOIN: {
                Thread *blocking = (Thread *)curr->get_location();
                ModelAction *act = get_last_action(blocking->get_id());
                curr->synchronize_with(act);
                updated = true; /* trigger rel-seq checks */
                break;
        }
        case THREAD_FINISH: {
                Thread *th = get_thread(curr);
                while (!th->wait_list_empty()) {
                        ModelAction *act = th->pop_wait_list();
                        scheduler->wake(get_thread(act));
                }
                th->complete();
                updated = true; /* trigger rel-seq checks */
                break;
        }
        case THREAD_START: {
                check_promises(curr->get_tid(), NULL, curr->get_cv());
                break;
        }
        default:
                break;
        }

        return updated;
}

/**
 * @brief Process the current action for release sequence fixup activity
 *
 * Performs model-checker release sequence fixups for the current action,
 * forcing a single pending release sequence to break (with a given, potential
 * "loose" write) or to complete (i.e., synchronize). If a pending release
 * sequence forms a complete release sequence, then we must perform the fixup
 * synchronization, mo_graph additions, etc.
 *
 * @param curr The current action; must be a release sequence fixup action
 * @param work_queue The work queue to which to add work items as they are
 * generated
 */
void ModelChecker::process_relseq_fixup(ModelAction *curr, work_queue_t *work_queue)
{
        const ModelAction *write = curr->get_node()->get_relseq_break();
        struct release_seq *sequence = pending_rel_seqs->back();
        pending_rel_seqs->pop_back();
        ASSERT(sequence);
        ModelAction *acquire = sequence->acquire;
        const ModelAction *rf = sequence->rf;
        const ModelAction *release = sequence->release;
        ASSERT(acquire);
        ASSERT(release);
        ASSERT(rf);
        ASSERT(release->same_thread(rf));

        if (write == NULL) {
                /**
                 * @todo Forcing a synchronization requires that we set
                 * modification order constraints. For instance, we can't allow
                 * a fixup sequence in which two separate read-acquire
                 * operations read from the same sequence, where the first one
                 * synchronizes and the other doesn't. Essentially, we can't
                 * allow any writes to insert themselves between 'release' and
                 * 'rf'
                 */

                /* Must synchronize */
                if (!acquire->synchronize_with(release)) {
                        set_bad_synchronization();
                        return;
                }
                /* Re-check all pending release sequences */
                work_queue->push_back(CheckRelSeqWorkEntry(NULL));
                /* Re-check act for mo_graph edges */
                work_queue->push_back(MOEdgeWorkEntry(acquire));

                /* propagate synchronization to later actions */
                action_list_t::reverse_iterator rit = action_trace->rbegin();
                for (; (*rit) != acquire; rit++) {
                        ModelAction *propagate = *rit;
                        if (acquire->happens_before(propagate)) {
                                propagate->synchronize_with(acquire);
                                /* Re-check 'propagate' for mo_graph edges */
                                work_queue->push_back(MOEdgeWorkEntry(propagate));
                        }
                }
        } else {
                /* Break release sequence with new edges:
                 *   release --mo--> write --mo--> rf */
                mo_graph->addEdge(release, write);
                mo_graph->addEdge(write, rf);
        }

        /* See if we have realized a data race */
        if (checkDataRaces())
                set_assert();
}

/**
 * Initialize the current action by performing one or more of the following
 * actions, as appropriate: merging RMWR and RMWC/RMW actions, stepping forward
 * in the NodeStack, manipulating backtracking sets, allocating and
 * initializing clock vectors, and computing the promises to fulfill.
 *
 * @param curr The current action, as passed from the user context; may be
 * freed/invalidated after the execution of this function, with a different
 * action "returned" its place (pass-by-reference)
 * @return True if curr is a newly-explored action; false otherwise
 */
bool ModelChecker::initialize_curr_action(ModelAction **curr)
{
        ModelAction *newcurr;

        if ((*curr)->is_rmwc() || (*curr)->is_rmw()) {
                newcurr = process_rmw(*curr);
                delete *curr;

                if (newcurr->is_rmw())
                        compute_promises(newcurr);

                *curr = newcurr;
                return false;
        }

        (*curr)->set_seq_number(get_next_seq_num());

        newcurr = node_stack->explore_action(*curr, scheduler->get_enabled());
        if (newcurr) {
                /* First restore type and order in case of RMW operation */
                if ((*curr)->is_rmwr())
                        newcurr->copy_typeandorder(*curr);

                ASSERT((*curr)->get_location() == newcurr->get_location());
                newcurr->copy_from_new(*curr);

                /* Discard duplicate ModelAction; use action from NodeStack */
                delete *curr;

                /* Always compute new clock vector */
                newcurr->create_cv(get_parent_action(newcurr->get_tid()));

                *curr = newcurr;
                return false; /* Action was explored previously */
        } else {
                newcurr = *curr;

                /* Always compute new clock vector */
                newcurr->create_cv(get_parent_action(newcurr->get_tid()));
                /*
                 * Perform one-time actions when pushing new ModelAction onto
                 * NodeStack
                 */
                if (newcurr->is_write())
                        compute_promises(newcurr);
                else if (newcurr->is_relseq_fixup())
                        compute_relseq_breakwrites(newcurr);
                else if (newcurr->is_wait())
                        newcurr->get_node()->set_misc_max(2);
                else if (newcurr->is_notify_one()) {
                        newcurr->get_node()->set_misc_max(get_safe_ptr_action(condvar_waiters_map, newcurr->get_location())->size());
                }
                return true; /* This was a new ModelAction */
        }
}

/**
 * @brief Check whether a model action is enabled.
 *
 * Checks whether a lock or join operation would be successful (i.e., is the
 * lock already locked, or is the joined thread already complete). If not, put
 * the action in a waiter list.
 *
 * @param curr is the ModelAction to check whether it is enabled.
 * @return a bool that indicates whether the action is enabled.
 */
bool ModelChecker::check_action_enabled(ModelAction *curr) {
        if (curr->is_lock()) {
                std::mutex * lock = (std::mutex *)curr->get_location();
                struct std::mutex_state * state = lock->get_state();
                if (state->islocked) {
                        //Stick the action in the appropriate waiting queue
                        get_safe_ptr_action(lock_waiters_map, curr->get_location())->push_back(curr);
                        return false;
                }
        } else if (curr->get_type() == THREAD_JOIN) {
                Thread *blocking = (Thread *)curr->get_location();
                if (!blocking->is_complete()) {
                        blocking->push_wait_list(curr);
                        return false;
                }
        }

        return true;
}

/**
 * Stores the ModelAction for the current thread action.  Call this
 * immediately before switching from user- to system-context to pass
 * data between them.
 * @param act The ModelAction created by the user-thread action
 */
void ModelChecker::set_current_action(ModelAction *act) {
        priv->current_action = act;
}

/**
 * This is the heart of the model checker routine. It performs model-checking
 * actions corresponding to a given "current action." Among other processes, it
 * calculates reads-from relationships, updates synchronization clock vectors,
 * forms a memory_order constraints graph, and handles replay/backtrack
 * execution when running permutations of previously-observed executions.
 *
 * @param curr The current action to process
 * @return The next Thread that must be executed. May be NULL if ModelChecker
 * makes no choice (e.g., according to replay execution, combining RMW actions,
 * etc.)
 */
Thread * ModelChecker::check_current_action(ModelAction *curr)
{
        ASSERT(curr);
        bool second_part_of_rmw = curr->is_rmwc() || curr->is_rmw();

        if (!check_action_enabled(curr)) {
                /* Make the execution look like we chose to run this action
                 * much later, when a lock/join can succeed */
                get_current_thread()->set_pending(curr);
                scheduler->sleep(get_current_thread());
                return get_next_thread(NULL);
        }

        bool newly_explored = initialize_curr_action(&curr);

        wake_up_sleeping_actions(curr);

        /* Add the action to lists before any other model-checking tasks */
        if (!second_part_of_rmw)
                add_action_to_lists(curr);

        /* Build may_read_from set for newly-created actions */
        if (newly_explored && curr->is_read())
                build_reads_from_past(curr);

        /* Initialize work_queue with the "current action" work */
        work_queue_t work_queue(1, CheckCurrWorkEntry(curr));
        while (!work_queue.empty()) {
                WorkQueueEntry work = work_queue.front();
                work_queue.pop_front();

                switch (work.type) {
                case WORK_CHECK_CURR_ACTION: {
                        ModelAction *act = work.action;
                        bool update = false; /* update this location's release seq's */
                        bool update_all = false; /* update all release seq's */

                        if (process_thread_action(curr))
                                update_all = true;

                        if (act->is_read() && process_read(act, second_part_of_rmw))
                                update = true;

                        if (act->is_write() && process_write(act))
                                update = true;

                        if (act->is_mutex_op() && process_mutex(act))
                                update_all = true;

                        if (act->is_relseq_fixup())
                                process_relseq_fixup(curr, &work_queue);

                        if (update_all)
                                work_queue.push_back(CheckRelSeqWorkEntry(NULL));
                        else if (update)
                                work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
                        break;
                }
                case WORK_CHECK_RELEASE_SEQ:
                        resolve_release_sequences(work.location, &work_queue);
                        break;
                case WORK_CHECK_MO_EDGES: {
                        /** @todo Complete verification of work_queue */
                        ModelAction *act = work.action;
                        bool updated = false;

                        if (act->is_read()) {
                                const ModelAction *rf = act->get_reads_from();
                                if (rf != NULL && r_modification_order(act, rf))
                                        updated = true;
                        }
                        if (act->is_write()) {
                                if (w_modification_order(act))
                                        updated = true;
                        }
                        mo_graph->commitChanges();

                        if (updated)
                                work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
                        break;
                }
                default:
                        ASSERT(false);
                        break;
                }
        }

        check_curr_backtracking(curr);
        set_backtracking(curr);
        return get_next_thread(curr);
}

void ModelChecker::check_curr_backtracking(ModelAction * curr) {
        Node *currnode = curr->get_node();
        Node *parnode = currnode->get_parent();

        if ((!parnode->backtrack_empty() ||
                         !currnode->misc_empty() ||
                         !currnode->read_from_empty() ||
                         !currnode->future_value_empty() ||
                         !currnode->promise_empty() ||
                         !currnode->relseq_break_empty())
                        && (!priv->next_backtrack ||
                                        *curr > *priv->next_backtrack)) {
                priv->next_backtrack = curr;
        }
}

bool ModelChecker::promises_expired() {
        for (unsigned int promise_index = 0; promise_index < promises->size(); promise_index++) {
                Promise *promise = (*promises)[promise_index];
                if (promise->get_expiration()<priv->used_sequence_numbers) {
                        return true;
                }
        }
        return false;
}

/** @return whether the current partial trace must be a prefix of a
 * feasible trace. */
bool ModelChecker::isfeasibleprefix() {
        return promises->size() == 0 && pending_rel_seqs->size() == 0 && isfeasible();
}

/** @return whether the current partial trace is feasible. */
bool ModelChecker::isfeasible() {
        if (DBG_ENABLED() && mo_graph->checkForRMWViolation())
                DEBUG("Infeasible: RMW violation\n");

        return !mo_graph->checkForRMWViolation() && isfeasibleotherthanRMW();
}

/** @return whether the current partial trace is feasible other than
 * multiple RMW reading from the same store. */
bool ModelChecker::isfeasibleotherthanRMW() {
        if (DBG_ENABLED()) {
                if (mo_graph->checkForCycles())
                        DEBUG("Infeasible: modification order cycles\n");
                if (failed_promise)
                        DEBUG("Infeasible: failed promise\n");
                if (too_many_reads)
                        DEBUG("Infeasible: too many reads\n");
                if (bad_synchronization)
                        DEBUG("Infeasible: bad synchronization ordering\n");
                if (promises_expired())
                        DEBUG("Infeasible: promises expired\n");
        }
        return !mo_graph->checkForCycles() && !failed_promise && !too_many_reads && !bad_synchronization && !promises_expired();
}

/** Returns whether the current completed trace is feasible. */
bool ModelChecker::isfinalfeasible() {
        if (DBG_ENABLED() && promises->size() != 0)
                DEBUG("Infeasible: unrevolved promises\n");

        return isfeasible() && promises->size() == 0;
}

/** Close out a RMWR by converting previous RMWR into a RMW or READ. */
ModelAction * ModelChecker::process_rmw(ModelAction *act) {
        ModelAction *lastread = get_last_action(act->get_tid());
        lastread->process_rmw(act);
        if (act->is_rmw() && lastread->get_reads_from()!=NULL) {
                mo_graph->addRMWEdge(lastread->get_reads_from(), lastread);
                mo_graph->commitChanges();
        }
        return lastread;
}

/**
 * Checks whether a thread has read from the same write for too many times
 * without seeing the effects of a later write.
 *
 * Basic idea:
 * 1) there must a different write that we could read from that would satisfy the modification order,
 * 2) we must have read from the same value in excess of maxreads times, and
 * 3) that other write must have been in the reads_from set for maxreads times.
 *
 * If so, we decide that the execution is no longer feasible.
 */
void ModelChecker::check_recency(ModelAction *curr, const ModelAction *rf) {
        if (params.maxreads != 0) {

                if (curr->get_node()->get_read_from_size() <= 1)
                        return;
                //Must make sure that execution is currently feasible...  We could
                //accidentally clear by rolling back
                if (!isfeasible())
                        return;
                std::vector<action_list_t> *thrd_lists = get_safe_ptr_vect_action(obj_thrd_map, curr->get_location());
                int tid = id_to_int(curr->get_tid());

                /* Skip checks */
                if ((int)thrd_lists->size() <= tid)
                        return;
                action_list_t *list = &(*thrd_lists)[tid];

                action_list_t::reverse_iterator rit = list->rbegin();
                /* Skip past curr */
                for (; (*rit) != curr; rit++)
                        ;
                /* go past curr now */
                rit++;

                action_list_t::reverse_iterator ritcopy = rit;
                //See if we have enough reads from the same value
                int count = 0;
                for (; count < params.maxreads; rit++,count++) {
                        if (rit==list->rend())
                                return;
                        ModelAction *act = *rit;
                        if (!act->is_read())
                                return;

                        if (act->get_reads_from() != rf)
                                return;
                        if (act->get_node()->get_read_from_size() <= 1)
                                return;
                }
                for (int i = 0; i<curr->get_node()->get_read_from_size(); i++) {
                        //Get write
                        const ModelAction * write = curr->get_node()->get_read_from_at(i);

                        //Need a different write
                        if (write==rf)
                                continue;

                        /* Test to see whether this is a feasible write to read from*/
                        mo_graph->startChanges();
                        r_modification_order(curr, write);
                        bool feasiblereadfrom = isfeasible();
                        mo_graph->rollbackChanges();

                        if (!feasiblereadfrom)
                                continue;
                        rit = ritcopy;

                        bool feasiblewrite = true;
                        //new we need to see if this write works for everyone

                        for (int loop = count; loop>0; loop--,rit++) {
                                ModelAction *act=*rit;
                                bool foundvalue = false;
                                for (int j = 0; j<act->get_node()->get_read_from_size(); j++) {
                                        if (act->get_node()->get_read_from_at(j)==write) {
                                                foundvalue = true;
                                                break;
                                        }
                                }
                                if (!foundvalue) {
                                        feasiblewrite = false;
                                        break;
                                }
                        }
                        if (feasiblewrite) {
                                too_many_reads = true;
                                return;
                        }
                }
        }
}

/**
 * Updates the mo_graph with the constraints imposed from the current
 * read.
 *
 * Basic idea is the following: Go through each other thread and find
 * the lastest action that happened before our read.  Two cases:
 *
 * (1) The action is a write => that write must either occur before
 * the write we read from or be the write we read from.
 *
 * (2) The action is a read => the write that that action read from
 * must occur before the write we read from or be the same write.
 *
 * @param curr The current action. Must be a read.
 * @param rf The action that curr reads from. Must be a write.
 * @return True if modification order edges were added; false otherwise
 */
bool ModelChecker::r_modification_order(ModelAction *curr, const ModelAction *rf)
{
        std::vector<action_list_t> *thrd_lists = get_safe_ptr_vect_action(obj_thrd_map, curr->get_location());
        unsigned int i;
        bool added = false;
        ASSERT(curr->is_read());

        /* Iterate over all threads */
        for (i = 0; i < thrd_lists->size(); i++) {
                /* Iterate over actions in thread, starting from most recent */
                action_list_t *list = &(*thrd_lists)[i];
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;

                        /*
                         * Include at most one act per-thread that "happens
                         * before" curr. Don't consider reflexively.
                         */
                        if (act->happens_before(curr) && act != curr) {
                                if (act->is_write()) {
                                        if (rf != act) {
                                                mo_graph->addEdge(act, rf);
                                                added = true;
                                        }
                                } else {
                                        const ModelAction *prevreadfrom = act->get_reads_from();
                                        //if the previous read is unresolved, keep going...
                                        if (prevreadfrom == NULL)
                                                continue;

                                        if (rf != prevreadfrom) {
                                                mo_graph->addEdge(prevreadfrom, rf);
                                                added = true;
                                        }
                                }
                                break;
                        }
                }
        }

        return added;
}

/** This method fixes up the modification order when we resolve a
 *  promises.  The basic problem is that actions that occur after the
 *  read curr could not property add items to the modification order
 *  for our read.
 *
 *  So for each thread, we find the earliest item that happens after
 *  the read curr.  This is the item we have to fix up with additional
 *  constraints.  If that action is write, we add a MO edge between
 *  the Action rf and that action.  If the action is a read, we add a
 *  MO edge between the Action rf, and whatever the read accessed.
 *
 * @param curr is the read ModelAction that we are fixing up MO edges for.
 * @param rf is the write ModelAction that curr reads from.
 *
 */
void ModelChecker::post_r_modification_order(ModelAction *curr, const ModelAction *rf)
{
        std::vector<action_list_t> *thrd_lists = get_safe_ptr_vect_action(obj_thrd_map, curr->get_location());
        unsigned int i;
        ASSERT(curr->is_read());

        /* Iterate over all threads */
        for (i = 0; i < thrd_lists->size(); i++) {
                /* Iterate over actions in thread, starting from most recent */
                action_list_t *list = &(*thrd_lists)[i];
                action_list_t::reverse_iterator rit;
                ModelAction *lastact = NULL;

                /* Find last action that happens after curr that is either not curr or a rmw */
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;
                        if (curr->happens_before(act) && (curr != act || curr->is_rmw())) {
                                lastact = act;
                        } else
                                break;
                }

                        /* Include at most one act per-thread that "happens before" curr */
                if (lastact != NULL) {
                        if (lastact==curr) {
                                //Case 1: The resolved read is a RMW, and we need to make sure
                                //that the write portion of the RMW mod order after rf

                                mo_graph->addEdge(rf, lastact);
                        } else if (lastact->is_read()) {
                                //Case 2: The resolved read is a normal read and the next
                                //operation is a read, and we need to make sure the value read
                                //is mod ordered after rf

                                const ModelAction *postreadfrom = lastact->get_reads_from();
                                if (postreadfrom != NULL&&rf != postreadfrom)
                                        mo_graph->addEdge(rf, postreadfrom);
                        } else {
                                //Case 3: The resolved read is a normal read and the next
                                //operation is a write, and we need to make sure that the
                                //write is mod ordered after rf
                                if (lastact!=rf)
                                        mo_graph->addEdge(rf, lastact);
                        }
                        break;
                }
        }
}

/**
 * Updates the mo_graph with the constraints imposed from the current write.
 *
 * Basic idea is the following: Go through each other thread and find
 * the lastest action that happened before our write.  Two cases:
 *
 * (1) The action is a write => that write must occur before
 * the current write
 *
 * (2) The action is a read => the write that that action read from
 * must occur before the current write.
 *
 * This method also handles two other issues:
 *
 * (I) Sequential Consistency: Making sure that if the current write is
 * seq_cst, that it occurs after the previous seq_cst write.
 *
 * (II) Sending the write back to non-synchronizing reads.
 *
 * @param curr The current action. Must be a write.
 * @return True if modification order edges were added; false otherwise
 */
bool ModelChecker::w_modification_order(ModelAction *curr)
{
        std::vector<action_list_t> *thrd_lists = get_safe_ptr_vect_action(obj_thrd_map, curr->get_location());
        unsigned int i;
        bool added = false;
        ASSERT(curr->is_write());

        if (curr->is_seqcst()) {
                /* We have to at least see the last sequentially consistent write,
                         so we are initialized. */
                ModelAction *last_seq_cst = get_last_seq_cst(curr);
                if (last_seq_cst != NULL) {
                        mo_graph->addEdge(last_seq_cst, curr);
                        added = true;
                }
        }

        /* Iterate over all threads */
        for (i = 0; i < thrd_lists->size(); i++) {
                /* Iterate over actions in thread, starting from most recent */
                action_list_t *list = &(*thrd_lists)[i];
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;
                        if (act == curr) {
                                /*
                                 * 1) If RMW and it actually read from something, then we
                                 * already have all relevant edges, so just skip to next
                                 * thread.
                                 *
                                 * 2) If RMW and it didn't read from anything, we should
                                 * whatever edge we can get to speed up convergence.
                                 *
                                 * 3) If normal write, we need to look at earlier actions, so
                                 * continue processing list.
                                 */
                                if (curr->is_rmw()) {
                                        if (curr->get_reads_from()!=NULL)
                                                break;
                                        else
                                                continue;
                                } else
                                        continue;
                        }

                        /*
                         * Include at most one act per-thread that "happens
                         * before" curr
                         */
                        if (act->happens_before(curr)) {
                                /*
                                 * Note: if act is RMW, just add edge:
                                 *   act --mo--> curr
                                 * The following edge should be handled elsewhere:
                                 *   readfrom(act) --mo--> act
                                 */
                                if (act->is_write())
                                        mo_graph->addEdge(act, curr);
                                else if (act->is_read()) {
                                        //if previous read accessed a null, just keep going
                                        if (act->get_reads_from() == NULL)
                                                continue;
                                        mo_graph->addEdge(act->get_reads_from(), curr);
                                }
                                added = true;
                                break;
                        } else if (act->is_read() && !act->could_synchronize_with(curr) &&
                                                     !act->same_thread(curr)) {
                                /* We have an action that:
                                   (1) did not happen before us
                                   (2) is a read and we are a write
                                   (3) cannot synchronize with us
                                   (4) is in a different thread
                                   =>
                                   that read could potentially read from our write.  Note that
                                   these checks are overly conservative at this point, we'll
                                   do more checks before actually removing the
                                   pendingfuturevalue.

                                 */
                                if (thin_air_constraint_may_allow(curr, act)) {
                                        if (isfeasible() ||
                                                        (curr->is_rmw() && act->is_rmw() && curr->get_reads_from() == act->get_reads_from() && isfeasibleotherthanRMW())) {
                                                struct PendingFutureValue pfv = {curr,act};
                                                futurevalues->push_back(pfv);
                                        }
                                }
                        }
                }
        }

        return added;
}

/** Arbitrary reads from the future are not allowed.  Section 29.3
 * part 9 places some constraints.  This method checks one result of constraint
 * constraint.  Others require compiler support. */
bool ModelChecker::thin_air_constraint_may_allow(const ModelAction * writer, const ModelAction *reader) {
        if (!writer->is_rmw())
                return true;

        if (!reader->is_rmw())
                return true;

        for (const ModelAction *search = writer->get_reads_from(); search != NULL; search = search->get_reads_from()) {
                if (search == reader)
                        return false;
                if (search->get_tid() == reader->get_tid() &&
                                search->happens_before(reader))
                        break;
        }

        return true;
}

/**
 * Arbitrary reads from the future are not allowed. Section 29.3 part 9 places
 * some constraints. This method checks one the following constraint (others
 * require compiler support):
 *
 *   If X --hb-> Y --mo-> Z, then X should not read from Z.
 */
bool ModelChecker::mo_may_allow(const ModelAction *writer, const ModelAction *reader)
{
        std::vector<action_list_t> *thrd_lists = get_safe_ptr_vect_action(obj_thrd_map, reader->get_location());
        unsigned int i;
        /* Iterate over all threads */
        for (i = 0; i < thrd_lists->size(); i++) {
                const ModelAction *write_after_read = NULL;

                /* Iterate over actions in thread, starting from most recent */
                action_list_t *list = &(*thrd_lists)[i];
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;

                        if (!reader->happens_before(act))
                                break;
                        else if (act->is_write())
                                write_after_read = act;
                        else if (act->is_read() && act->get_reads_from() != NULL && act != reader) {
                                write_after_read = act->get_reads_from();
                        }
                }

                if (write_after_read && write_after_read!=writer && mo_graph->checkReachable(write_after_read, writer))
                        return false;
        }
        return true;
}

/**
 * Finds the head(s) of the release sequence(s) containing a given ModelAction.
 * The ModelAction under consideration is expected to be taking part in
 * release/acquire synchronization as an object of the "reads from" relation.
 * Note that this can only provide release sequence support for RMW chains
 * which do not read from the future, as those actions cannot be traced until
 * their "promise" is fulfilled. Similarly, we may not even establish the
 * presence of a release sequence with certainty, as some modification order
 * constraints may be decided further in the future. Thus, this function
 * "returns" two pieces of data: a pass-by-reference vector of @a release_heads
 * and a boolean representing certainty.
 *
 * @param rf The action that might be part of a release sequence. Must be a
 * write.
 * @param release_heads A pass-by-reference style return parameter. After
 * execution of this function, release_heads will contain the heads of all the
 * relevant release sequences, if any exists with certainty
 * @param pending A pass-by-reference style return parameter which is only used
 * when returning false (i.e., uncertain). Returns most information regarding
 * an uncertain release sequence, including any write operations that might
 * break the sequence.
 * @return true, if the ModelChecker is certain that release_heads is complete;
 * false otherwise
 */
bool ModelChecker::release_seq_heads(const ModelAction *rf,
                rel_heads_list_t *release_heads,
                struct release_seq *pending) const
{
        /* Only check for release sequences if there are no cycles */
        if (mo_graph->checkForCycles())
                return false;

        while (rf) {
                ASSERT(rf->is_write());

                if (rf->is_release())
                        release_heads->push_back(rf);
                if (!rf->is_rmw())
                        break; /* End of RMW chain */

                /** @todo Need to be smarter here...  In the linux lock
                 * example, this will run to the beginning of the program for
                 * every acquire. */
                /** @todo The way to be smarter here is to keep going until 1
                 * thread has a release preceded by an acquire and you've seen
                 *       both. */

                /* acq_rel RMW is a sufficient stopping condition */
                if (rf->is_acquire() && rf->is_release())
                        return true; /* complete */

                rf = rf->get_reads_from();
        };
        if (!rf) {
                /* read from future: need to settle this later */
                pending->rf = NULL;
                return false; /* incomplete */
        }

        if (rf->is_release())
                return true; /* complete */

        /* else relaxed write; check modification order for contiguous subsequence
         * -> rf must be same thread as release */
        int tid = id_to_int(rf->get_tid());
        std::vector<action_list_t> *thrd_lists = get_safe_ptr_vect_action(obj_thrd_map, rf->get_location());
        action_list_t *list = &(*thrd_lists)[tid];
        action_list_t::const_reverse_iterator rit;

        /* Find rf in the thread list */
        rit = std::find(list->rbegin(), list->rend(), rf);
        ASSERT(rit != list->rend());

        /* Find the last write/release */
        for (; rit != list->rend(); rit++)
                if ((*rit)->is_release())
                        break;
        if (rit == list->rend()) {
                /* No write-release in this thread */
                return true; /* complete */
        }
        ModelAction *release = *rit;

        ASSERT(rf->same_thread(release));

        pending->writes.clear();

        bool certain = true;
        for (unsigned int i = 0; i < thrd_lists->size(); i++) {
                if (id_to_int(rf->get_tid()) == (int)i)
                        continue;
                list = &(*thrd_lists)[i];

                /* Can we ensure no future writes from this thread may break
                 * the release seq? */
                bool future_ordered = false;

                ModelAction *last = get_last_action(int_to_id(i));
                Thread *th = get_thread(int_to_id(i));
                if ((last && rf->happens_before(last)) ||
                                !is_enabled(th) ||
                                th->is_complete())
                        future_ordered = true;

                ASSERT(!th->is_model_thread() || future_ordered);

                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        const ModelAction *act = *rit;
                        /* Reach synchronization -> this thread is complete */
                        if (act->happens_before(release))
                                break;
                        if (rf->happens_before(act)) {
                                future_ordered = true;
                                continue;
                        }

                        /* Only non-RMW writes can break release sequences */
                        if (!act->is_write() || act->is_rmw())
                                continue;

                        /* Check modification order */
                        if (mo_graph->checkReachable(rf, act)) {
                                /* rf --mo--> act */
                                future_ordered = true;
                                continue;
                        }
                        if (mo_graph->checkReachable(act, release))
                                /* act --mo--> release */
                                break;
                        if (mo_graph->checkReachable(release, act) &&
                                      mo_graph->checkReachable(act, rf)) {
                                /* release --mo-> act --mo--> rf */
                                return true; /* complete */
                        }
                        /* act may break release sequence */
                        pending->writes.push_back(act);
                        certain = false;
                }
                if (!future_ordered)
                        certain = false; /* This thread is uncertain */
        }

        if (certain) {
                release_heads->push_back(release);
                pending->writes.clear();
        } else {
                pending->release = release;
                pending->rf = rf;
        }
        return certain;
}

/**
 * A public interface for getting the release sequence head(s) with which a
 * given ModelAction must synchronize. This function only returns a non-empty
 * result when it can locate a release sequence head with certainty. Otherwise,
 * it may mark the internal state of the ModelChecker so that it will handle
 * the release sequence at a later time, causing @a act to update its
 * synchronization at some later point in execution.
 * @param act The 'acquire' action that may read from a release sequence
 * @param release_heads A pass-by-reference return parameter. Will be filled
 * with the head(s) of the release sequence(s), if they exists with certainty.
 * @see ModelChecker::release_seq_heads
 */
void ModelChecker::get_release_seq_heads(ModelAction *act, rel_heads_list_t *release_heads)
{
        const ModelAction *rf = act->get_reads_from();
        struct release_seq *sequence = (struct release_seq *)snapshot_calloc(1, sizeof(struct release_seq));
        sequence->acquire = act;

        if (!release_seq_heads(rf, release_heads, sequence)) {
                /* add act to 'lazy checking' list */
                pending_rel_seqs->push_back(sequence);
        } else {
                snapshot_free(sequence);
        }
}

/**
 * Attempt to resolve all stashed operations that might synchronize with a
 * release sequence for a given location. This implements the "lazy" portion of
 * determining whether or not a release sequence was contiguous, since not all
 * modification order information is present at the time an action occurs.
 *
 * @param location The location/object that should be checked for release
 * sequence resolutions. A NULL value means to check all locations.
 * @param work_queue The work queue to which to add work items as they are
 * generated
 * @return True if any updates occurred (new synchronization, new mo_graph
 * edges)
 */
bool ModelChecker::resolve_release_sequences(void *location, work_queue_t *work_queue)
{
        bool updated = false;
        std::vector< struct release_seq *, SnapshotAlloc<struct release_seq *> >::iterator it = pending_rel_seqs->begin();
        while (it != pending_rel_seqs->end()) {
                struct release_seq *pending = *it;
                ModelAction *act = pending->acquire;

                /* Only resolve sequences on the given location, if provided */
                if (location && act->get_location() != location) {
                        it++;
                        continue;
                }

                const ModelAction *rf = act->get_reads_from();
                rel_heads_list_t release_heads;
                bool complete;
                complete = release_seq_heads(rf, &release_heads, pending);
                for (unsigned int i = 0; i < release_heads.size(); i++) {
                        if (!act->has_synchronized_with(release_heads[i])) {
                                if (act->synchronize_with(release_heads[i]))
                                        updated = true;
                                else
                                        set_bad_synchronization();
                        }
                }

                if (updated) {
                        /* Re-check all pending release sequences */
                        work_queue->push_back(CheckRelSeqWorkEntry(NULL));
                        /* Re-check act for mo_graph edges */
                        work_queue->push_back(MOEdgeWorkEntry(act));

                        /* propagate synchronization to later actions */
                        action_list_t::reverse_iterator rit = action_trace->rbegin();
                        for (; (*rit) != act; rit++) {
                                ModelAction *propagate = *rit;
                                if (act->happens_before(propagate)) {
                                        propagate->synchronize_with(act);
                                        /* Re-check 'propagate' for mo_graph edges */
                                        work_queue->push_back(MOEdgeWorkEntry(propagate));
                                }
                        }
                }
                if (complete) {
                        it = pending_rel_seqs->erase(it);
                        snapshot_free(pending);
                } else {
                        it++;
                }
        }

        // If we resolved promises or data races, see if we have realized a data race.
        if (checkDataRaces()) {
                set_assert();
        }

        return updated;
}

/**
 * Performs various bookkeeping operations for the current ModelAction. For
 * instance, adds action to the per-object, per-thread action vector and to the
 * action trace list of all thread actions.
 *
 * @param act is the ModelAction to add.
 */
void ModelChecker::add_action_to_lists(ModelAction *act)
{
        int tid = id_to_int(act->get_tid());
        action_trace->push_back(act);

        get_safe_ptr_action(obj_map, act->get_location())->push_back(act);

        std::vector<action_list_t> *vec = get_safe_ptr_vect_action(obj_thrd_map, act->get_location());
        if (tid >= (int)vec->size())
                vec->resize(priv->next_thread_id);
        (*vec)[tid].push_back(act);

        if ((int)thrd_last_action->size() <= tid)
                thrd_last_action->resize(get_num_threads());
        (*thrd_last_action)[tid] = act;

        if (act->is_wait()) {
                void *mutex_loc=(void *) act->get_value();
                get_safe_ptr_action(obj_map, mutex_loc)->push_back(act);

                std::vector<action_list_t> *vec = get_safe_ptr_vect_action(obj_thrd_map, mutex_loc);
                if (tid >= (int)vec->size())
                        vec->resize(priv->next_thread_id);
                (*vec)[tid].push_back(act);

                if ((int)thrd_last_action->size() <= tid)
                        thrd_last_action->resize(get_num_threads());
                (*thrd_last_action)[tid] = act;
        }
}

/**
 * @brief Get the last action performed by a particular Thread
 * @param tid The thread ID of the Thread in question
 * @return The last action in the thread
 */
ModelAction * ModelChecker::get_last_action(thread_id_t tid) const
{
        int threadid = id_to_int(tid);
        if (threadid < (int)thrd_last_action->size())
                return (*thrd_last_action)[id_to_int(tid)];
        else
                return NULL;
}

/**
 * Gets the last memory_order_seq_cst write (in the total global sequence)
 * performed on a particular object (i.e., memory location), not including the
 * current action.
 * @param curr The current ModelAction; also denotes the object location to
 * check
 * @return The last seq_cst write
 */
ModelAction * ModelChecker::get_last_seq_cst(ModelAction *curr) const
{
        void *location = curr->get_location();
        action_list_t *list = get_safe_ptr_action(obj_map, location);
        /* Find: max({i in dom(S) | seq_cst(t_i) && isWrite(t_i) && samevar(t_i, t)}) */
        action_list_t::reverse_iterator rit;
        for (rit = list->rbegin(); rit != list->rend(); rit++)
                if ((*rit)->is_write() && (*rit)->is_seqcst() && (*rit) != curr)
                        return *rit;
        return NULL;
}

/**
 * Gets the last unlock operation performed on a particular mutex (i.e., memory
 * location). This function identifies the mutex according to the current
 * action, which is presumed to perform on the same mutex.
 * @param curr The current ModelAction; also denotes the object location to
 * check
 * @return The last unlock operation
 */
ModelAction * ModelChecker::get_last_unlock(ModelAction *curr) const
{
        void *location = curr->get_location();
        action_list_t *list = get_safe_ptr_action(obj_map, location);
        /* Find: max({i in dom(S) | isUnlock(t_i) && samevar(t_i, t)}) */
        action_list_t::reverse_iterator rit;
        for (rit = list->rbegin(); rit != list->rend(); rit++)
                if ((*rit)->is_unlock() || (*rit)->is_wait())
                        return *rit;
        return NULL;
}

ModelAction * ModelChecker::get_parent_action(thread_id_t tid)
{
        ModelAction *parent = get_last_action(tid);
        if (!parent)
                parent = get_thread(tid)->get_creation();
        return parent;
}

/**
 * Returns the clock vector for a given thread.
 * @param tid The thread whose clock vector we want
 * @return Desired clock vector
 */
ClockVector * ModelChecker::get_cv(thread_id_t tid)
{
        return get_parent_action(tid)->get_cv();
}

/**
 * Resolve a set of Promises with a current write. The set is provided in the
 * Node corresponding to @a write.
 * @param write The ModelAction that is fulfilling Promises
 * @return True if promises were resolved; false otherwise
 */
bool ModelChecker::resolve_promises(ModelAction *write)
{
        bool resolved = false;
        std::vector< thread_id_t, ModelAlloc<thread_id_t> > threads_to_check;

        for (unsigned int i = 0, promise_index = 0; promise_index < promises->size(); i++) {
                Promise *promise = (*promises)[promise_index];
                if (write->get_node()->get_promise(i)) {
                        ModelAction *read = promise->get_action();
                        if (read->is_rmw()) {
                                mo_graph->addRMWEdge(write, read);
                        }
                        read->read_from(write);
                        //First fix up the modification order for actions that happened
                        //before the read
                        r_modification_order(read, write);
                        //Next fix up the modification order for actions that happened
                        //after the read.
                        post_r_modification_order(read, write);
                        //Make sure the promise's value matches the write's value
                        ASSERT(promise->get_value() == write->get_value());
                        delete(promise);

                        promises->erase(promises->begin() + promise_index);
                        threads_to_check.push_back(read->get_tid());

                        resolved = true;
                } else
                        promise_index++;
        }

        //Check whether reading these writes has made threads unable to
        //resolve promises

        for(unsigned int i=0;i<threads_to_check.size();i++)
                mo_check_promises(threads_to_check[i], write);

        return resolved;
}

/**
 * Compute the set of promises that could potentially be satisfied by this
 * action. Note that the set computation actually appears in the Node, not in
 * ModelChecker.
 * @param curr The ModelAction that may satisfy promises
 */
void ModelChecker::compute_promises(ModelAction *curr)
{
        for (unsigned int i = 0; i < promises->size(); i++) {
                Promise *promise = (*promises)[i];
                const ModelAction *act = promise->get_action();
                if (!act->happens_before(curr) &&
                                act->is_read() &&
                                !act->could_synchronize_with(curr) &&
                                !act->same_thread(curr) &&
                                act->get_location() == curr->get_location() &&
                                promise->get_value() == curr->get_value()) {
                        curr->get_node()->set_promise(i, act->is_rmw());
                }
        }
}

/** Checks promises in response to change in ClockVector Threads. */
void ModelChecker::check_promises(thread_id_t tid, ClockVector *old_cv, ClockVector *merge_cv)
{
        for (unsigned int i = 0; i < promises->size(); i++) {
                Promise *promise = (*promises)[i];
                const ModelAction *act = promise->get_action();
                if ((old_cv == NULL || !old_cv->synchronized_since(act)) &&
                                merge_cv->synchronized_since(act)) {
                        if (promise->increment_threads(tid)) {
                                //Promise has failed
                                failed_promise = true;
                                return;
                        }
                }
        }
}

void ModelChecker::check_promises_thread_disabled() {
        for (unsigned int i = 0; i < promises->size(); i++) {
                Promise *promise = (*promises)[i];
                if (promise->check_promise()) {
                        failed_promise = true;
                        return;
                }
        }
}

/** Checks promises in response to addition to modification order for threads.
 * Definitions:
 * pthread is the thread that performed the read that created the promise
 *
 * pread is the read that created the promise
 *
 * pwrite is either the first write to same location as pread by
 * pthread that is sequenced after pread or the value read by the
 * first read to the same lcoation as pread by pthread that is
 * sequenced after pread..
 *
 *      1. If tid=pthread, then we check what other threads are reachable
 * through the mode order starting with pwrite.  Those threads cannot
 * perform a write that will resolve the promise due to modification
 * order constraints.
 *
 * 2. If the tid is not pthread, we check whether pwrite can reach the
 * action write through the modification order.  If so, that thread
 * cannot perform a future write that will resolve the promise due to
 * modificatin order constraints.
 *
 *      @parem tid The thread that either read from the model action
 *      write, or actually did the model action write.
 *
 *      @parem write The ModelAction representing the relevant write.
 */

void ModelChecker::mo_check_promises(thread_id_t tid, const ModelAction *write) {
        void * location = write->get_location();
        for (unsigned int i = 0; i < promises->size(); i++) {
                Promise *promise = (*promises)[i];
                const ModelAction *act = promise->get_action();

                //Is this promise on the same location?
                if ( act->get_location() != location )
                        continue;

                //same thread as the promise
                if ( act->get_tid()==tid ) {

                        //do we have a pwrite for the promise, if not, set it
                        if (promise->get_write() == NULL ) {
                                promise->set_write(write);
                                //The pwrite cannot happen before the promise
                                if (write->happens_before(act) && (write != act)) {
                                        failed_promise = true;
                                        return;
                                }
                        }
                        if (mo_graph->checkPromise(write, promise)) {
                                failed_promise = true;
                                return;
                        }
                }

                //Don't do any lookups twice for the same thread
                if (promise->has_sync_thread(tid))
                        continue;

                if (promise->get_write()&&mo_graph->checkReachable(promise->get_write(), write)) {
                        if (promise->increment_threads(tid)) {
                                failed_promise = true;
                                return;
                        }
                }
        }
}

/**
 * Compute the set of writes that may break the current pending release
 * sequence. This information is extracted from previou release sequence
 * calculations.
 *
 * @param curr The current ModelAction. Must be a release sequence fixup
 * action.
 */
void ModelChecker::compute_relseq_breakwrites(ModelAction *curr)
{
        if (pending_rel_seqs->empty())
                return;

        struct release_seq *pending = pending_rel_seqs->back();
        for (unsigned int i = 0; i < pending->writes.size(); i++) {
                const ModelAction *write = pending->writes[i];
                curr->get_node()->add_relseq_break(write);
        }

        /* NULL means don't break the sequence; just synchronize */
        curr->get_node()->add_relseq_break(NULL);
}

/**
 * Build up an initial set of all past writes that this 'read' action may read
 * from. This set is determined by the clock vector's "happens before"
 * relationship.
 * @param curr is the current ModelAction that we are exploring; it must be a
 * 'read' operation.
 */
void ModelChecker::build_reads_from_past(ModelAction *curr)
{
        std::vector<action_list_t> *thrd_lists = get_safe_ptr_vect_action(obj_thrd_map, curr->get_location());
        unsigned int i;
        ASSERT(curr->is_read());

        ModelAction *last_seq_cst = NULL;

        /* Track whether this object has been initialized */
        bool initialized = false;

        if (curr->is_seqcst()) {
                last_seq_cst = get_last_seq_cst(curr);
                /* We have to at least see the last sequentially consistent write,
                         so we are initialized. */
                if (last_seq_cst != NULL)
                        initialized = true;
        }

        /* Iterate over all threads */
        for (i = 0; i < thrd_lists->size(); i++) {
                /* Iterate over actions in thread, starting from most recent */
                action_list_t *list = &(*thrd_lists)[i];
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;

                        /* Only consider 'write' actions */
                        if (!act->is_write() || act == curr)
                                continue;

                        /* Don't consider more than one seq_cst write if we are a seq_cst read. */
                        if (!curr->is_seqcst() || (!act->is_seqcst() && (last_seq_cst == NULL || !act->happens_before(last_seq_cst))) || act == last_seq_cst) {
                                if (!curr->get_sleep_flag() || curr->is_seqcst() || sleep_can_read_from(curr, act)) {
                                        DEBUG("Adding action to may_read_from:\n");
                                        if (DBG_ENABLED()) {
                                                act->print();
                                                curr->print();
                                        }
                                        curr->get_node()->add_read_from(act);
                                }
                        }

                        /* Include at most one act per-thread that "happens before" curr */
                        if (act->happens_before(curr)) {
                                initialized = true;
                                break;
                        }
                }
        }

        if (!initialized) {
                /** @todo Need a more informative way of reporting errors. */
                printf("ERROR: may read from uninitialized atomic\n");
                set_assert();
        }

        if (DBG_ENABLED() || !initialized) {
                printf("Reached read action:\n");
                curr->print();
                printf("Printing may_read_from\n");
                curr->get_node()->print_may_read_from();
                printf("End printing may_read_from\n");
        }
}

bool ModelChecker::sleep_can_read_from(ModelAction * curr, const ModelAction *write) {
        while(true) {
                Node *prevnode=write->get_node()->get_parent();

                bool thread_sleep=prevnode->enabled_status(curr->get_tid())==THREAD_SLEEP_SET;
                if (write->is_release()&&thread_sleep)
                        return true;
                if (!write->is_rmw()) {
                        return false;
                }
                if (write->get_reads_from()==NULL)
                        return true;
                write=write->get_reads_from();
        }
}

static void print_list(action_list_t *list)
{
        action_list_t::iterator it;

        printf("---------------------------------------------------------------------\n");
        printf("Trace:\n");
        unsigned int hash=0;

        for (it = list->begin(); it != list->end(); it++) {
                (*it)->print();
                hash=hash^(hash<<3)^((*it)->hash());
        }
        printf("HASH %u\n", hash);
        printf("---------------------------------------------------------------------\n");
}

#if SUPPORT_MOD_ORDER_DUMP
void ModelChecker::dumpGraph(char *filename) {
        char buffer[200];
        sprintf(buffer, "%s.dot",filename);
        FILE *file=fopen(buffer, "w");
        fprintf(file, "digraph %s {\n",filename);
        mo_graph->dumpNodes(file);
        ModelAction ** thread_array=(ModelAction **)model_calloc(1, sizeof(ModelAction *)*get_num_threads());

        for (action_list_t::iterator it = action_trace->begin(); it != action_trace->end(); it++) {
                ModelAction *action=*it;
                if (action->is_read()) {
                        fprintf(file, "N%u [label=\"%u, T%u\"];\n", action->get_seq_number(),action->get_seq_number(), action->get_tid());
                        if (action->get_reads_from()!=NULL)
                                fprintf(file, "N%u -> N%u[label=\"rf\", color=red];\n", action->get_seq_number(), action->get_reads_from()->get_seq_number());
                }
                if (thread_array[action->get_tid()] != NULL) {
                        fprintf(file, "N%u -> N%u[label=\"sb\", color=blue];\n", thread_array[action->get_tid()]->get_seq_number(), action->get_seq_number());
                }

                thread_array[action->get_tid()]=action;
        }
        fprintf(file,"}\n");
        model_free(thread_array);
        fclose(file);
}
#endif

void ModelChecker::print_summary()
{
        printf("\n");
        printf("Number of executions: %d\n", num_executions);
        printf("Number of feasible executions: %d\n", num_feasible_executions);
        printf("Total nodes created: %d\n", node_stack->get_total_nodes());

#if SUPPORT_MOD_ORDER_DUMP
        scheduler->print();
        char buffername[100];
        sprintf(buffername, "exec%04u", num_executions);
        mo_graph->dumpGraphToFile(buffername);
        sprintf(buffername, "graph%04u", num_executions);
        dumpGraph(buffername);
#endif

        if (!isfinalfeasible())
                printf("INFEASIBLE EXECUTION!\n");
        print_list(action_trace);
        printf("\n");
}

/**
 * Add a Thread to the system for the first time. Should only be called once
 * per thread.
 * @param t The Thread to add
 */
void ModelChecker::add_thread(Thread *t)
{
        thread_map->put(id_to_int(t->get_id()), t);
        scheduler->add_thread(t);
}

/**
 * Removes a thread from the scheduler.
 * @param the thread to remove.
 */
void ModelChecker::remove_thread(Thread *t)
{
        scheduler->remove_thread(t);
}

/**
 * @brief Get a Thread reference by its ID
 * @param tid The Thread's ID
 * @return A Thread reference
 */
Thread * ModelChecker::get_thread(thread_id_t tid) const
{
        return thread_map->get(id_to_int(tid));
}

/**
 * @brief Get a reference to the Thread in which a ModelAction was executed
 * @param act The ModelAction
 * @return A Thread reference
 */
Thread * ModelChecker::get_thread(ModelAction *act) const
{
        return get_thread(act->get_tid());
}

/**
 * @brief Check if a Thread is currently enabled
 * @param t The Thread to check
 * @return True if the Thread is currently enabled
 */
bool ModelChecker::is_enabled(Thread *t) const
{
        return scheduler->is_enabled(t);
}

/**
 * @brief Check if a Thread is currently enabled
 * @param tid The ID of the Thread to check
 * @return True if the Thread is currently enabled
 */
bool ModelChecker::is_enabled(thread_id_t tid) const
{
        return scheduler->is_enabled(tid);
}

/**
 * Switch from a user-context to the "master thread" context (a.k.a. system
 * context). This switch is made with the intention of exploring a particular
 * model-checking action (described by a ModelAction object). Must be called
 * from a user-thread context.
 *
 * @param act The current action that will be explored. May be NULL only if
 * trace is exiting via an assertion (see ModelChecker::set_assert and
 * ModelChecker::has_asserted).
 * @return Return status from the 'swap' call (i.e., success/fail, 0/-1)
 */
int ModelChecker::switch_to_master(ModelAction *act)
{
        DBG();
        Thread *old = thread_current();
        set_current_action(act);
        old->set_state(THREAD_READY);
        return Thread::swap(old, &system_context);
}

/**
 * Takes the next step in the execution, if possible.
 * @return Returns true (success) if a step was taken and false otherwise.
 */
bool ModelChecker::take_step() {
        if (has_asserted())
                return false;

        Thread *curr = priv->current_action ? get_thread(priv->current_action) : NULL;
        if (curr) {
                if (curr->get_state() == THREAD_READY) {
                        ASSERT(priv->current_action);

                        priv->nextThread = check_current_action(priv->current_action);
                        priv->current_action = NULL;

                        if (curr->is_blocked() || curr->is_complete())
                                scheduler->remove_thread(curr);
                } else {
                        ASSERT(false);
                }
        }
        Thread *next = scheduler->next_thread(priv->nextThread);

        /* Infeasible -> don't take any more steps */
        if (!isfeasible())
                return false;

        if (params.bound != 0) {
                if (priv->used_sequence_numbers > params.bound) {
                        return false;
                }
        }

        DEBUG("(%d, %d)\n", curr ? id_to_int(curr->get_id()) : -1,
                        next ? id_to_int(next->get_id()) : -1);

        /*
         * Launch end-of-execution release sequence fixups only when there are:
         *
         * (1) no more user threads to run (or when execution replay chooses
         *     the 'model_thread')
         * (2) pending release sequences
         * (3) pending assertions (i.e., data races)
         * (4) no pending promises
         */
        if (!pending_rel_seqs->empty() && (!next || next->is_model_thread()) &&
                        isfinalfeasible() && !unrealizedraces.empty()) {
                printf("*** WARNING: release sequence fixup action (%zu pending release seuqences) ***\n",
                                pending_rel_seqs->size());
                ModelAction *fixup = new ModelAction(MODEL_FIXUP_RELSEQ,
                                std::memory_order_seq_cst, NULL, VALUE_NONE,
                                model_thread);
                set_current_action(fixup);
                return true;
        }

        /* next == NULL -> don't take any more steps */
        if (!next)
                return false;

        next->set_state(THREAD_RUNNING);

        if (next->get_pending() != NULL) {
                /* restart a pending action */
                set_current_action(next->get_pending());
                next->set_pending(NULL);
                next->set_state(THREAD_READY);
                return true;
        }

        /* Return false only if swap fails with an error */
        return (Thread::swap(&system_context, next) == 0);
}

/** Runs the current execution until threre are no more steps to take. */
void ModelChecker::finish_execution() {
        DBG();

        while (take_step());
}