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model-checker : add variable deleted before to resolve a merge conflict
[simgrid.git] / src / mc / mc_dpor.c
index b10416e..3e1b700 100644 (file)
-/* Copyright (c) 2008 Martin Quinson, Cristian Rosa.
-   All rights reserved.                                          */
+/* Copyright (c) 2008-2013. Da SimGrid Team. All rights reserved.           */
 
 /* This program is free software; you can redistribute it and/or modify it
  * under the terms of the license (GNU LGPL) which comes with this package. */
 
 
 /* This program is free software; you can redistribute it and/or modify it
  * under the terms of the license (GNU LGPL) which comes with this package. */
 
-#include "private.h"
+#include "mc_private.h"
 
 XBT_LOG_NEW_DEFAULT_SUBCATEGORY(mc_dpor, mc,
 
 XBT_LOG_NEW_DEFAULT_SUBCATEGORY(mc_dpor, mc,
-                               "Logging specific to MC DPOR exploration");
+                                "Logging specific to MC DPOR exploration");
+
+xbt_dynar_t visited_states;
+xbt_dict_t first_enabled_state;
+
+static void visited_state_free(mc_visited_state_t state){
+  if(state){
+    MC_free_snapshot(state->system_state);
+    xbt_free(state);
+  }
+}
+
+static void visited_state_free_voidp(void *s){
+  visited_state_free((mc_visited_state_t) * (void **) s);
+}
+
+static mc_visited_state_t visited_state_new(){
+
+  mc_visited_state_t new_state = NULL;
+  new_state = xbt_new0(s_mc_visited_state_t, 1);
+  new_state->heap_bytes_used = mmalloc_get_bytes_used(std_heap);
+  new_state->nb_processes = xbt_swag_size(simix_global->process_list);
+  new_state->system_state = MC_take_snapshot();
+  new_state->num = mc_stats->expanded_states - 1;
+
+  return new_state;
+  
+}
+
+/* Dichotomic search in visited_states dynar. 
+ * States are ordered by the number of processes then the number of bytes used in std_heap */
+
+static int is_visited_state(){
+
+  if(_sg_mc_visited == 0)
+    return -1;
+
+  int raw_mem_set = (mmalloc_get_current_heap() == raw_heap);
+
+  MC_SET_RAW_MEM;
+  mc_visited_state_t new_state = visited_state_new();
+  MC_UNSET_RAW_MEM;
+  
+  if(xbt_dynar_is_empty(visited_states)){
+
+    MC_SET_RAW_MEM;
+    xbt_dynar_push(visited_states, &new_state); 
+    MC_UNSET_RAW_MEM;
+
+    if(raw_mem_set)
+      MC_SET_RAW_MEM;
+
+    return -1;
+
+  }else{
+
+    MC_SET_RAW_MEM;
+    
+    size_t current_bytes_used = new_state->heap_bytes_used;
+    int current_nb_processes = new_state->nb_processes;
+
+    unsigned int cursor = 0;
+    int previous_cursor = 0, next_cursor = 0;
+    int start = 0;
+    int end = xbt_dynar_length(visited_states) - 1;
+
+    mc_visited_state_t state_test = NULL;
+    size_t bytes_used_test;
+    int nb_processes_test;
+    int same_processes_and_bytes_not_found = 1;
+
+    while(start <= end && same_processes_and_bytes_not_found){
+      cursor = (start + end) / 2;
+      state_test = (mc_visited_state_t)xbt_dynar_get_as(visited_states, cursor, mc_visited_state_t);
+      bytes_used_test = state_test->heap_bytes_used;
+      nb_processes_test = state_test->nb_processes;
+      if(nb_processes_test < current_nb_processes)
+        start = cursor + 1;
+      if(nb_processes_test > current_nb_processes)
+        end = cursor - 1; 
+      if(nb_processes_test == current_nb_processes){
+        if(bytes_used_test < current_bytes_used)
+          start = cursor + 1;
+        if(bytes_used_test > current_bytes_used)
+          end = cursor - 1;
+        if(bytes_used_test == current_bytes_used){
+          same_processes_and_bytes_not_found = 0;
+          if(snapshot_compare(new_state->system_state, state_test->system_state) == 0){
+            xbt_dynar_remove_at(visited_states, cursor, NULL);
+            xbt_dynar_insert_at(visited_states, cursor, &new_state);
+            XBT_DEBUG("State %d already visited ! (equal to state %d)", new_state->num, state_test->num);
+            if(raw_mem_set)
+              MC_SET_RAW_MEM;
+            else
+              MC_UNSET_RAW_MEM;
+            return state_test->num;
+          }else{
+            /* Search another state with same number of bytes used in std_heap */
+            previous_cursor = cursor - 1;
+            while(previous_cursor >= 0){
+              state_test = (mc_visited_state_t)xbt_dynar_get_as(visited_states, previous_cursor, mc_visited_state_t);
+              bytes_used_test = state_test->system_state->heap_bytes_used;
+              if(bytes_used_test != current_bytes_used)
+                break;
+              if(snapshot_compare(new_state->system_state, state_test->system_state) == 0){
+                xbt_dynar_remove_at(visited_states, previous_cursor, NULL);
+                xbt_dynar_insert_at(visited_states, previous_cursor, &new_state);
+                XBT_DEBUG("State %d already visited ! (equal to state %d)", new_state->num, state_test->num);
+                if(raw_mem_set)
+                  MC_SET_RAW_MEM;
+                else
+                  MC_UNSET_RAW_MEM;
+                return state_test->num;
+              }
+              previous_cursor--;
+            }
+            next_cursor = cursor + 1;
+            while(next_cursor < xbt_dynar_length(visited_states)){
+              state_test = (mc_visited_state_t)xbt_dynar_get_as(visited_states, next_cursor, mc_visited_state_t);
+              bytes_used_test = state_test->system_state->heap_bytes_used;
+              if(bytes_used_test != current_bytes_used)
+                break;
+              if(snapshot_compare(new_state->system_state, state_test->system_state) == 0){
+                xbt_dynar_remove_at(visited_states, next_cursor, NULL);
+                xbt_dynar_insert_at(visited_states, next_cursor, &new_state);
+                XBT_DEBUG("State %d already visited ! (equal to state %d)", new_state->num, state_test->num);
+                if(raw_mem_set)
+                  MC_SET_RAW_MEM;
+                else
+                  MC_UNSET_RAW_MEM;
+                return state_test->num;
+              }
+              next_cursor++;
+            }
+          }   
+        }
+      }
+    }
+
+    state_test = (mc_visited_state_t)xbt_dynar_get_as(visited_states, cursor, mc_visited_state_t);
+    bytes_used_test = state_test->heap_bytes_used;
+
+    if(bytes_used_test < current_bytes_used)
+      xbt_dynar_insert_at(visited_states, cursor + 1, &new_state);
+    else
+      xbt_dynar_insert_at(visited_states, cursor, &new_state);
+
+    if(xbt_dynar_length(visited_states) > _sg_mc_visited){
+      int min = mc_stats->expanded_states;
+      unsigned int cursor2 = 0;
+      unsigned int index = 0;
+      xbt_dynar_foreach(visited_states, cursor2, state_test){
+        if(state_test->num < min){
+          index = cursor2;
+          min = state_test->num;
+        }
+      }
+      xbt_dynar_remove_at(visited_states, index, NULL);
+    }
+    
+    MC_UNSET_RAW_MEM;
+
+    if(raw_mem_set)
+      MC_SET_RAW_MEM;
+    
+    return -1;
+    
+  }
+}
 
 /**
  *  \brief Initialize the DPOR exploration algorithm
  */
 void MC_dpor_init()
 {
 
 /**
  *  \brief Initialize the DPOR exploration algorithm
  */
 void MC_dpor_init()
 {
+  
+  int raw_mem_set = (mmalloc_get_current_heap() == raw_heap);
+
   mc_state_t initial_state = NULL;
   mc_state_t initial_state = NULL;
-  mc_transition_t trans, trans_tmp = NULL;
-  xbt_setset_cursor_t cursor = NULL;
+  smx_process_t process;
   
   /* Create the initial state and push it into the exploration stack */
   MC_SET_RAW_MEM;
   
   /* Create the initial state and push it into the exploration stack */
   MC_SET_RAW_MEM;
+
   initial_state = MC_state_new();
   initial_state = MC_state_new();
-  xbt_fifo_unshift(mc_stack, initial_state);
+  visited_states = xbt_dynar_new(sizeof(mc_visited_state_t), visited_state_free_voidp);
+
+  first_enabled_state = xbt_dict_new_homogeneous(&xbt_free_f);
+
   MC_UNSET_RAW_MEM;
 
   MC_UNSET_RAW_MEM;
 
-  /* Schedule all the processes to detect the transitions of the initial state */
-  DEBUG0("**************************************************"); 
-  DEBUG0("Initial state");
+  XBT_DEBUG("**************************************************");
+  XBT_DEBUG("Initial state");
 
 
-  while(xbt_swag_size(simix_global->process_to_run)){
-    MC_schedule_enabled_processes();
-    MC_execute_surf_actions();
-  }
+  /* Wait for requests (schedules processes) */
+  MC_wait_for_requests();
 
   MC_SET_RAW_MEM;
 
   MC_SET_RAW_MEM;
-  MC_trans_compute_enabled(initial_state->enabled_transitions,
-                           initial_state->transitions);
-
-  /* Fill the interleave set of the initial state with an enabled process */
-  trans = xbt_setset_set_choose(initial_state->enabled_transitions);  
-  if(trans){
-    DEBUG1("Choosing process %s for next interleave", trans->process->name);
-    xbt_setset_foreach(initial_state->enabled_transitions, cursor, trans_tmp){
-      if(trans_tmp->process == trans->process)
-        xbt_setset_set_insert(initial_state->interleave, trans_tmp);    
+  /* Get an enabled process and insert it in the interleave set of the initial state */
+  xbt_swag_foreach(process, simix_global->process_list){
+    if(MC_process_is_enabled(process)){
+      MC_state_interleave_process(initial_state, process);
+      if(mc_reduce_kind != e_mc_reduce_none)
+        break;
     }
   }
     }
   }
+
+  xbt_fifo_unshift(mc_stack_safety, initial_state);
+
+  /* To ensure the soundness of DPOR, we have to keep a list of 
+     processes which are still enabled at each step of the exploration. 
+     If max depth is reached, we interleave them in the state in which they have 
+     been enabled for the first time. */
+  xbt_swag_foreach(process, simix_global->process_list){
+    if(MC_process_is_enabled(process)){
+      char *key = bprintf("%lu", process->pid);
+      char *data = bprintf("%d", xbt_fifo_size(mc_stack_safety));
+      xbt_dict_set(first_enabled_state, key, data, NULL);
+      xbt_free(key);
+    }
+  }
+
   MC_UNSET_RAW_MEM;
 
   MC_UNSET_RAW_MEM;
 
-  /* Update Statistics */
-  mc_stats->state_size += xbt_setset_set_size(initial_state->enabled_transitions);
+  if(raw_mem_set)
+    MC_SET_RAW_MEM;
+  else
+    MC_UNSET_RAW_MEM;
+  
 }
 
 
 /**
 }
 
 
 /**
- *     \brief Perform the model-checking operation using a depth-first search exploration
+ *   \brief Perform the model-checking operation using a depth-first search exploration
  *         with Dynamic Partial Order Reductions
  */
 void MC_dpor(void)
 {
  *         with Dynamic Partial Order Reductions
  */
 void MC_dpor(void)
 {
-  mc_transition_t trans, trans_tmp = NULL;  
-  mc_state_t next_state = NULL;
-  xbt_setset_cursor_t cursor = NULL;
-  
-  while(xbt_fifo_size(mc_stack) > 0){
-  
-    DEBUG0("**************************************************");
 
 
-    /* FIXME: Think about what happen if there are no transitions but there are
-       some actions on the models. (ex. all the processes do a sleep(0) in a round). */
+  char *req_str;
+  int value;
+  smx_simcall_t req = NULL, prev_req = NULL;
+  mc_state_t state = NULL, prev_state = NULL, next_state = NULL, restore_state=NULL;
+  smx_process_t process = NULL;
+  xbt_fifo_item_t item = NULL;
+  int pos;
+  int visited_state;
+  int max_depth_reached = 0;
+
+  while (xbt_fifo_size(mc_stack_safety) > 0) {
 
     /* Get current state */
 
     /* Get current state */
-    mc_current_state = (mc_state_t) xbt_fifo_get_item_content(xbt_fifo_get_first_item(mc_stack));
-   
-    /* If there are transitions to execute and the maximun depth has not been reached 
+    state = (mc_state_t) 
+      xbt_fifo_get_item_content(xbt_fifo_get_first_item(mc_stack_safety));
+
+    XBT_DEBUG("**************************************************");
+    XBT_DEBUG("Exploration depth=%d (state=%p)(%u interleave)",
+              xbt_fifo_size(mc_stack_safety), state,
+              MC_state_interleave_size(state));
+
+    /* Update statistics */
+    mc_stats->visited_states++;
+
+    /* If there are processes to interleave and the maximum depth has not been reached
        then perform one step of the exploration algorithm */
        then perform one step of the exploration algorithm */
-    if(xbt_setset_set_size(mc_current_state->interleave) > 0 && xbt_fifo_size(mc_stack) < MAX_DEPTH){
+    if (xbt_fifo_size(mc_stack_safety) <= _sg_mc_max_depth &&
+        (req = MC_state_get_request(state, &value))) {
 
 
-      DEBUG4("Exploration detph=%d (state=%p)(%d interleave) (%d enabled)", xbt_fifo_size(mc_stack),
-        mc_current_state, xbt_setset_set_size(mc_current_state->interleave),
-        xbt_setset_set_size(mc_current_state->enabled_transitions));
-      
-      /* Update statistics */
-      mc_stats->visited_states++;
+      /* Debug information */
+      if(XBT_LOG_ISENABLED(mc_dpor, xbt_log_priority_debug)){
+        req_str = MC_request_to_string(req, value);
+        XBT_DEBUG("Execute: %s", req_str);
+        xbt_free(req_str);
+      }
+        
+      req_str = MC_request_get_dot_output(req, value);
+
+      MC_state_set_executed_request(state, req, value);
       mc_stats->executed_transitions++;
       mc_stats->executed_transitions++;
-      
-      /* Choose a transition to execute from the interleave set of the current
-         state, and create the data structures for the new expanded state in the
-         exploration stack. */
-      MC_SET_RAW_MEM;
-      trans = xbt_setset_set_extract(mc_current_state->interleave);
 
 
-      /* Add the transition in the done set of the current state */
-      xbt_setset_set_insert(mc_current_state->done, trans);
-      
-      next_state = MC_state_new();
-      xbt_fifo_unshift(mc_stack, next_state);
-      
-      /* Set it as the executed transition of the current state */
-      mc_current_state->executed_transition = trans;
+      MC_SET_RAW_MEM;
+      char *key = bprintf("%lu", req->issuer->pid);
+      xbt_dict_remove(first_enabled_state, key); 
+      xbt_free(key);
       MC_UNSET_RAW_MEM;
 
       MC_UNSET_RAW_MEM;
 
-      /* Execute the selected transition by scheduling it's associated process.
-         Then schedule every process that got ready to run due to the execution
-         of the transition */
-      DEBUG1("Executing transition %s", trans->name);
-      SIMIX_process_schedule(trans->process);
-      MC_execute_surf_actions();
-      
-      while(xbt_swag_size(simix_global->process_to_run)){
-        MC_schedule_enabled_processes();
-        MC_execute_surf_actions();
-      }
-      
-      /* Calculate the enabled transitions set of the next state */
+      /* Answer the request */
+      SIMIX_simcall_pre(req, value); /* After this call req is no longer usefull */
+
+      /* Wait for requests (schedules processes) */
+      MC_wait_for_requests();
+
+      /* Create the new expanded state */
       MC_SET_RAW_MEM;
 
       MC_SET_RAW_MEM;
 
-      xbt_setset_foreach(mc_current_state->transitions, cursor, trans_tmp){
-        if(trans_tmp->process != trans->process){
-          xbt_setset_set_insert(next_state->transitions, trans_tmp);
+      next_state = MC_state_new();
+
+      if((visited_state = is_visited_state()) == -1){
+     
+        /* Get an enabled process and insert it in the interleave set of the next state */
+        xbt_swag_foreach(process, simix_global->process_list){
+          if(MC_process_is_enabled(process)){
+            MC_state_interleave_process(next_state, process);
+            if(mc_reduce_kind != e_mc_reduce_none)
+              break;
+          }
         }
         }
+
+        if(_sg_mc_checkpoint && ((xbt_fifo_size(mc_stack_safety) + 1) % _sg_mc_checkpoint == 0)){
+          next_state->system_state = MC_take_snapshot();
+        }
+
+        if(dot_output != NULL)
+          fprintf(dot_output, "\"%d\" -> \"%d\" [%s];\n", state->num, next_state->num, req_str);
+
+      }else{
+
+        if(dot_output != NULL)
+          fprintf(dot_output, "\"%d\" -> \"%d\" [%s];\n", state->num, visited_state, req_str);
+
       }
       }
-      
-      MC_trans_compute_enabled(next_state->enabled_transitions, next_state->transitions);
-      
-      /* Choose one transition to interleave from the enabled transition set */      
-      trans = xbt_setset_set_choose(next_state->enabled_transitions);
-      if(trans){
-        DEBUG1("Choosing process %s for next interleave", trans->process->name);
-        xbt_setset_foreach(next_state->enabled_transitions, cursor, trans_tmp){
-          if(trans_tmp->process == trans->process)
-            xbt_setset_set_insert(next_state->interleave, trans_tmp);    
+
+      xbt_fifo_unshift(mc_stack_safety, next_state);
+
+      /* Insert in dict all enabled processes, if not included yet */
+      xbt_swag_foreach(process, simix_global->process_list){
+        if(MC_process_is_enabled(process)){
+          char *key = bprintf("%lu", process->pid);
+          if(xbt_dict_get_or_null(first_enabled_state, key) == NULL){
+            char *data = bprintf("%d", xbt_fifo_size(mc_stack_safety));
+            xbt_dict_set(first_enabled_state, key, data, NULL); 
+          }
+          xbt_free(key);
         }
       }
         }
       }
+
       MC_UNSET_RAW_MEM;
 
       MC_UNSET_RAW_MEM;
 
-      /* Update Statistics */
-      mc_stats->state_size += xbt_setset_set_size(next_state->enabled_transitions);
+      xbt_free(req_str);
 
       /* Let's loop again */
 
       /* Let's loop again */
-      
-    /* The interleave set is empty or the maximum depth is reached, let's back-track */
-    }else{
-      DEBUG0("There are no more transitions to interleave.");
 
 
-  
-      /* Check for deadlocks */
-      xbt_setset_substract(mc_current_state->transitions, mc_current_state->done);
-      if(xbt_setset_set_size(mc_current_state->transitions) > 0){
-        INFO0("**************************");
-        INFO0("*** DEAD-LOCK DETECTED ***");
-        INFO0("**************************");
-        INFO0("Locked transitions:");
-        xbt_setset_foreach(mc_current_state->transitions, cursor, trans){
-          INFO1("%s", trans->name);
+      /* The interleave set is empty or the maximum depth is reached, let's back-track */
+    } else {
+
+      if(xbt_fifo_size(mc_stack_safety) > _sg_mc_max_depth){  
+
+        XBT_WARN("/!\\ Max depth reached ! /!\\ ");
+
+        /* Interleave enabled processes in the state in which they have been enabled for the first time */
+        xbt_swag_foreach(process, simix_global->process_list){
+          if(MC_process_is_enabled(process)){
+            char *key = bprintf("%lu", process->pid);
+            enabled = (int)strtoul(xbt_dict_get_or_null(first_enabled_state, key), 0, 10);
+            xbt_free(key);
+            mc_state_t state_test = NULL;
+            xbt_fifo_item_t item = NULL;
+            int cursor = xbt_fifo_size(mc_stack_safety);
+            xbt_fifo_foreach(mc_stack_safety, item, state_test, mc_state_t){
+              if(cursor-- == enabled){ 
+                if(!MC_state_process_is_done(state_test, process)){ 
+                  MC_state_interleave_process(state_test, process);
+                  break;
+                }
+              }
+            } 
+          }
         }
 
         }
 
-        INFO0("Counter-example execution trace:");
-        MC_dump_stack(mc_stack);
-        
-        MC_print_statistics(mc_stats);
-        xbt_abort();
+        max_depth_reached = 1;
+
+      }else{
+
+        XBT_DEBUG("There are no more processes to interleave.");
+
+        /* Trash the current state, no longer needed */
+        MC_SET_RAW_MEM;
+        xbt_fifo_shift(mc_stack_safety);
+        MC_state_delete(state);
+        MC_UNSET_RAW_MEM;
+
+        max_depth_reached = 0;
       }
       }
-              
-      mc_transition_t q = NULL;
-      xbt_fifo_item_t item = NULL;
-      mc_state_t state = NULL;
       
       
-      //MC_show_stack(mc_stack);
-      
-      /* Trash the current state, no longer needed */
+      /* Check for deadlocks */
+      if(MC_deadlock_check()){
+        MC_show_deadlock(NULL);
+        return;
+      }
+
       MC_SET_RAW_MEM;
       MC_SET_RAW_MEM;
-      xbt_fifo_shift(mc_stack);
-      MC_state_delete(mc_current_state);
-      
       /* Traverse the stack backwards until a state with a non empty interleave
          set is found, deleting all the states that have it empty in the way.
       /* Traverse the stack backwards until a state with a non empty interleave
          set is found, deleting all the states that have it empty in the way.
-         For each deleted state, check if the transition that has generated it 
-         (from it's predecesor state), depends on any other previous transition 
+         For each deleted state, check if the request that has generated it 
+         (from it's predecesor state), depends on any other previous request 
          executed before it. If it does then add it to the interleave set of the
          executed before it. If it does then add it to the interleave set of the
-         state that executed that previous transition. */
-      while((mc_current_state = xbt_fifo_shift(mc_stack)) != NULL){
-        q = mc_current_state->executed_transition;
-        xbt_fifo_foreach(mc_stack, item, state, mc_state_t){
-          if(MC_transition_depend(q, state->executed_transition)){
-            DEBUG3("Dependence found at state %p (%p,%p)", state, state->executed_transition, q); 
-            xbt_setset_foreach(state->enabled_transitions, cursor, trans){
-              if((trans->process == q->process) && !xbt_setset_set_belongs(state->done, trans)){
-                DEBUG2("Unexplored interleaving found at state %p (%p)", state, trans);
-
-                xbt_setset_foreach(state->enabled_transitions, cursor, trans){
-                  if(trans->process == q->process)
-                    xbt_setset_set_insert(state->interleave, trans);    
-                }
-                /* FIXME: hack to make it work*/
-                trans = q;
-                break;
+         state that executed that previous request. */
+      
+      while ((state = xbt_fifo_shift(mc_stack_safety)) != NULL) {
+        if(mc_reduce_kind != e_mc_reduce_none){
+          if((xbt_fifo_size(mc_stack_safety) == _sg_mc_max_depth) && max_depth_reached){
+            req = MC_state_get_request(state, &value);
+            MC_state_set_executed_request(state, req, value);
+          }
+          req = MC_state_get_internal_request(state);
+          xbt_fifo_foreach(mc_stack_safety, item, prev_state, mc_state_t) {
+            if(MC_request_depend(req, MC_state_get_internal_request(prev_state))){
+              if(XBT_LOG_ISENABLED(mc_dpor, xbt_log_priority_debug)){
+                XBT_DEBUG("Dependent Transitions:");
+                prev_req = MC_state_get_executed_request(prev_state, &value);
+                req_str = MC_request_to_string(prev_req, value);
+                XBT_DEBUG("%s (state=%p)", req_str, prev_state);
+                xbt_free(req_str);
+                prev_req = MC_state_get_executed_request(state, &value);
+                req_str = MC_request_to_string(prev_req, value);
+                XBT_DEBUG("%s (state=%p)", req_str, state);
+                xbt_free(req_str);              
               }
               }
-            }
-            if(trans)
+
+              if(!MC_state_process_is_done(prev_state, req->issuer))
+                MC_state_interleave_process(prev_state, req->issuer);
+              else
+                XBT_DEBUG("Process %p is in done set", req->issuer);
+
               break;
               break;
+
+            }else if(req->issuer == MC_state_get_internal_request(prev_state)->issuer){
+
+              XBT_DEBUG("Simcall %d and %d with same issuer", req->call, MC_state_get_internal_request(prev_state)->call);
+              break;
+
+            }
           }
         }
           }
         }
-        if(xbt_setset_set_size(mc_current_state->interleave) > 0){
+             
+        if (MC_state_interleave_size(state) && xbt_fifo_size(mc_stack_safety) < _sg_mc_max_depth) {
           /* We found a back-tracking point, let's loop */
           /* We found a back-tracking point, let's loop */
-          xbt_fifo_unshift(mc_stack, mc_current_state);
-          DEBUG1("Back-tracking to depth %d", xbt_fifo_size(mc_stack));
-          MC_replay(mc_stack);
-          MC_UNSET_RAW_MEM;
+          XBT_DEBUG("Back-tracking to depth %d", xbt_fifo_size(mc_stack_safety) + 1);
+          if(_sg_mc_checkpoint){
+            if(state->system_state != NULL){
+              MC_restore_snapshot(state->system_state);
+              xbt_fifo_unshift(mc_stack_safety, state);
+              MC_UNSET_RAW_MEM;
+            }else{
+              pos = xbt_fifo_size(mc_stack_safety);
+              item = xbt_fifo_get_first_item(mc_stack_safety);
+              while(pos>0){
+                restore_state = (mc_state_t) xbt_fifo_get_item_content(item);
+                if(restore_state->system_state != NULL){
+                  break;
+                }else{
+                  item = xbt_fifo_get_next_item(item);
+                  pos--;
+                }
+              }
+              MC_restore_snapshot(restore_state->system_state);
+              xbt_fifo_unshift(mc_stack_safety, state);
+              MC_UNSET_RAW_MEM;
+              MC_replay(mc_stack_safety, pos);
+            }
+          }else{
+            xbt_fifo_unshift(mc_stack_safety, state);
+            MC_UNSET_RAW_MEM;
+            MC_replay(mc_stack_safety, -1);
+          }
+          XBT_DEBUG("Back-tracking to depth %d", xbt_fifo_size(mc_stack_safety));
           break;
           break;
-        }else{
-          MC_state_delete(mc_current_state);
-        }        
+        } else {
+          MC_state_delete(state);
+        }
       }
       }
+      MC_UNSET_RAW_MEM;
     }
   }
     }
   }
-  DEBUG0("We are done");
-  /* We are done, show the statistics and return */
   MC_print_statistics(mc_stats);
   MC_print_statistics(mc_stats);
-}
\ No newline at end of file
+  MC_UNSET_RAW_MEM;  
+
+  return;
+}
+
+
+
+