model: add deadlock detection

This is only a simple end-of-execution deadlock detection. If we have a
true deadlock, and no other threads are spinning, then our execution
will exit --- we can recognize this state.

For more complicated deadlocks involving other spinning threads, we need
to perform some sort of depth-first-search cycle detection.

diff --git a/model.cc b/model.cc
index ff1fdf932ee302fb6cc36c9199cd54c15f3bb50d..94620ea9d0400fcee239973705361210542040f5 100644 (file)
--- a/model.cc
+++ b/model.cc
@@ -244,6 +244,26 @@ void ModelChecker::wake_up_sleeping_actions(ModelAction * curr) {
        }
 }
 
+/**
+ * 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 *t = get_thread(int_to_id(i));
+               if (scheduler->is_enabled(t) != THREAD_DISABLED)
+                       return false;
+               else 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.
@@ -257,6 +277,8 @@ bool ModelChecker::next_execution()
 
        num_executions++;
 
+       if (is_deadlocked())
+               printf("ERROR: DEADLOCK\n");
        if (isfinalfeasible()) {
                printf("Earliest divergence point since last feasible execution:\n");
                if (earliest_diverge)

diff --git a/model.h b/model.h
index 3ef6ee3c6c90e4ae96eb6a7ac646de02648cfc85..6d702b0d94604d281a0cdea8331eb669e15a3704 100644 (file)
--- a/model.h
+++ b/model.h
@@ -115,6 +115,7 @@ public:
        void finish_execution();
        bool isfeasibleprefix();
        void set_assert() {asserted=true;}
+       bool is_deadlocked() const;
 
        /** @brief Alert the model-checker that an incorrectly-ordered
         * synchronization was made */