sim: Add hooks to implement event reference counting

We currently only support deleting an event if it is triggered and not
re-scheduled. This is fine for most native code. However, there are
cases where Python needs to count references to make sure that the
Python object stays live while the native object is live.

Generalise the mechanism used to implement by adding reference
counting hooks to the event base class:

  * Event::acquire() / Event::acquireImpl()
  * Event::release() / Event::releaseImpl()

These calls can be used to implement both reference counting and the
existing AutoDelete functionality. The default implementation in Event
maintains backwards compatibility with the existing AutoDelete feature
by ignoring acquireImpl() and deleting the event on releaseImpl() if
it isn't scheduled anymore.

Since AutoDelete functionality is no longer the only way events can be
managed, this change introduces the new Managed flag. This flag
activates automatic memory management. The acquireImpl()/releaseImpl()
methods are only called from acquire()/release() it is set. To
maintain backwards compatibility, AutoDelete is used as an alias for
Managed.

Change-Id: I5637984c906a9d44c22780712cf1c521b8297149
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3221
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

src/sim/eventq.cc

@@ -227,7 +227,7 @@ EventQueue::serviceOne()
 
         event->process();
         if (event->isExitEvent()) {
-            assert(!event->flags.isSet(Event::AutoDelete) ||
+            assert(!event->flags.isSet(Event::Managed) ||
                    !event->flags.isSet(Event::IsMainQueue)); // would be silly
             return event;
         }
@@ -235,8 +235,7 @@ EventQueue::serviceOne()
         event->flags.clear(Event::Squashed);
     }
 
-    if (event->flags.isSet(Event::AutoDelete) && !event->scheduled())
-        delete event;
+    event->release();
 
     return NULL;
 }

src/sim/eventq.hh

@@ -99,7 +99,8 @@ class EventBase
     static const FlagsType PublicWrite   = 0x001d; // public writable flags
     static const FlagsType Squashed      = 0x0001; // has been squashed
     static const FlagsType Scheduled     = 0x0002; // has been scheduled
-    static const FlagsType AutoDelete    = 0x0004; // delete after dispatch
+    static const FlagsType Managed       = 0x0004; // Use life cycle manager
+    static const FlagsType AutoDelete    = Managed; // delete after dispatch
     /**
      * This used to be AutoSerialize. This value can't be reused
      * without changing the checkpoint version since the flag field
@@ -282,6 +283,55 @@ class Event : public EventBase, public Serializable
     // This function isn't really useful if TRACING_ON is not defined
     virtual void trace(const char *action);     //!< trace event activity
 
+  protected: /* Memory management */
+    /**
+     * @{
+     * Memory management hooks for events that have the Managed flag set
+     *
+     * Events can use automatic memory management by setting the
+     * Managed flag. The default implementation automatically deletes
+     * events once they have been removed from the event queue. This
+     * typically happens when events are descheduled or have been
+     * triggered and not rescheduled.
+     *
+     * The methods below may be overridden by events that need custom
+     * memory management. For example, events exported to Python need
+     * to impement reference counting to ensure that the Python
+     * implementation of the event is kept alive while it lives in the
+     * event queue.
+     *
+     * @note Memory managers are responsible for implementing
+     * reference counting (by overriding both acquireImpl() and
+     * releaseImpl()) or checking if an event is no longer scheduled
+     * in releaseImpl() before deallocating it.
+     */
+
+    /**
+     * Managed event scheduled and being held in the event queue.
+     */
+    void acquire()
+    {
+        if (flags.isSet(Event::Managed))
+            acquireImpl();
+    }
+
+    /**
+     * Managed event removed from the event queue.
+     */
+    void release() {
+        if (flags.isSet(Event::Managed))
+            releaseImpl();
+    }
+
+    virtual void acquireImpl() {}
+
+    virtual void releaseImpl() {
+        if (!scheduled())
+            delete this;
+    }
+
+    /** @} */
+
   public:
 
     /*
@@ -340,7 +390,8 @@ class Event : public EventBase, public Serializable
     bool isExitEvent() const { return flags.isSet(IsExitEvent); }
 
     /// Check whether this event will auto-delete
-    bool isAutoDelete() const { return flags.isSet(AutoDelete); }
+    bool isManaged() const { return flags.isSet(Managed); }
+    bool isAutoDelete() const { return isManaged(); }
 
     /// Get the time that the event is scheduled
     Tick when() const { return _when; }

src/sim/eventq_impl.hh

@@ -59,6 +59,7 @@ EventQueue::schedule(Event *event, Tick when, bool global)
         insert(event);
     }
     event->flags.set(Event::Scheduled);
+    event->acquire();
 
     if (DTRACE(Event))
         event->trace("scheduled");
@@ -79,8 +80,7 @@ EventQueue::deschedule(Event *event)
     if (DTRACE(Event))
         event->trace("descheduled");
 
-    if (event->flags.isSet(Event::AutoDelete))
-        delete event;
+    event->release();
 }
 
 inline void
@@ -91,8 +91,11 @@ EventQueue::reschedule(Event *event, Tick when, bool always)
     assert(event->initialized());
     assert(!inParallelMode || this == curEventQueue());
 
-    if (event->scheduled())
+    if (event->scheduled()) {
         remove(event);
+    } else {
+        event->acquire();
+    }
 
     event->setWhen(when, this);
     insert(event);