gstreamer/docs/design/part-MT-refcounting.txt

Conventions for thread a safe API
---------------------------------

The GStreamer API is designed to be thread safe. This means that API functions can be
called from multiple threads at the same time. GStreamer internally uses threads to
perform the data passing and various asynchronous services such as the clock can also
use threads.

This design decision has implication for the usage of the API and the objects which
this document explains.

MT safety techniques
--------------------

Several design patterns are used to guarantee object consistency in GStreamer. This
is an overview of the methods used in various GStreamer subsystems.

Refcounting

  All object have a refcount associated with them. Each reference obtained to the
  object should increase the refcount and each reference lost should decrease the
  refcount.

  The refcounting is used to make sure that when another thread destroys the object,
  the ones which still hold a reference to the object do not read from invalid
  memory when accessing the object.

  It is a requirement that when two threads have a handle to an object, the
  refcount must be one, this means that when one thread passes an object to another
  thread it must increase the refcount. This requirement makes sure that one
  thread cannot suddenly dispose the object making the other thread crash when it
  tries to access the pointer to invalid memory.

Copy-on-write:

  All object have a refcount associated with them. Each reference obtained to the
  object should increase the refcount and each reference lost should decrease the
  refcount.

  Each thread having a refcount to the object can safely read from the object. but
  modifications made to the object should be preceeded with a _copy_on_write()
  function call. This function will check the refcount of the object and if the
  object is referenced by more than one instance, a copy is made of the object that
  is then by definition only referenced from the calling thread. This new copy
  is then modifyable without being visible to other refcount holders.

  This technique is used for information objects that, once created, never change
  their values. The lifetime of these objects is generally short, the objects are 
  usually simple and cheap to copy/create.

  The advantage of this method is that no reader/writers locks are needed. all
  threads can concurrently read but writes happen locally on a new copy. In most
  cases _copy_on_write() can avoid a real copy because the calling method is the
  only one holding a reference, wich makes read/writes very cheap.

  The drawback is that sometimes 1 needless copy can be done. This would happen
  when N threads call _copy_on_write() at the same time, all seeing that N references
  are held to the object. In this case 1 copy too many will be done. This is not
  a problem in any practical situation because the copy operation is fast.
 
Object locking:

  For objects that contain state information and generally have a longer lifetime,
  object locking is used to update the information contained in the object.

  All readers and writers acquire the lock before accessing the object. Only one
  thread is allowed access the protected structures at a time.

  Object locking is used for all objects extending from GstObject such as GstElement,
  GstPad.

Atomic operations

  Atomic operations are operations that are performed as one consistent operation
  even when executed by multiple threads. They do however not use the conventional
  aproach of using mutexes to protect the critical section but rely on CPU features
  and instructions.

  The advantages are mostly speed related since there are no heavyweight locks 
  involved. Most of this instructions also do not cause a context switch in case
  of concurrent access but use a retry mechanism or spinlocking.

  Disadvantages are that each of these instructions usually cause a cache flush
  on multi-CPU machines when two processors perform concurrent access.

  Atomic operations are generally used for refcounting and for the allocation of 
  small fixed size objects in a memchunk. They can also be used to implement a
  lockfree list or stack.
 

Objects
-------

* Locking involved:

    - atomic operations for refcounting
    - object locking

  All objects should have a lock associated with them. This lock is used to keep
  internal consistency when multiple threads call API function on the object.

  For objects that extend the GStreamer base object class this lock can be obtained with
  the macros GST_LOCK() and GST_UNLOCK(). For other object that do not extend from the
  base GstObject class these macros can be different.

* refcounting

  All new objects created have the FLOATING flag set. This means that the object is
  not owned or managed yet by anybody other than the one holding a reference to
  the object. The object in this state has a reference count of 1.

  Various object methods can take ownership of another object, this means that after 
  calling a method on object A with an object B as an argument, the object B is made
  sole property of object A. This means that after the method call you are not allowed
  to access the object anymore unless you keep an extra reference to the object.
  An example of such a method is the _bin_add() method. As soon as this function is
  called in a Bin, the element passed as an argument is owned by the bin and you
  are not allowed to access it anymore without taking a _ref() before adding it
  to the bin. The reason being that after the _bin_add() call disposing the bin
  also destroys the element.

  Taking ownership of an object happens trough the process of "sinking" the object.
  the _sink() method on an object will decrease the refcount of the object if the
  FLOATING flag is set. The act of taking ownership of an object is then performed
  as a _ref() followed by a _sink() call on the object.

  Sinking objects is very usefull because you don't have to worry about the lifetime
  of these sunken objects anymore since it is managed by the owner element.
  
* parent-child relations

  One can create parent child relationships with the _object_set_parent() method. This
  method sinks the object and assigns its parent property to that of the managing
  parent. 

  The child is said to have a weak link to the parent since the refcount of the parent
  is not increased in this process. This means that if the parent is disposed it has
  to unset itself as the parent of the object before disposing itself, else the child
  object holds a parent pointer to invalid memory.  
  
  The responsabilites for an object that sinks other objects are summarised as:

   - taking ownership of the object
     - call _object_set_parent() to set itself as the object parent, this call will
       _ref() and _sink() the object.
     - keep reference to object in a datastructure such as a list or array.

   - on dispose
     - call _object_unparent() to reset the parent property and unref the object.
     - remove the object from the list.


* Properties
  
  Most objects also expose state information with public properties in the object. Two
  types of properties might exist: accessible with or without holding the object lock.
  All properties should only be accessed with their corresponding macros.
  The public object properties are marked in the .h files with /*< public >*/. The
  public properties that require a lock to be held are marked with
  /*< public >*/ /* with <lock_type> */, where <lock_type> can be "LOCK" or "STATE_LOCK"
  to mark the type(s) of lock to be held.

  Example:

    in GstPad there is a public property "direction". It can be found in the section
    marked as public and requiring the LOCK to be held. There exists also a macro
    to access the property.

      struct _GstRealPad {
        ...
        /*< public >*/ /* with LOCK */
        ...
        GstPadDirection                direction;
        ...
      };

      #define GST_RPAD_DIRECTION(pad)         (GST_REAL_PAD_CAST(pad)->direction)
  
    Accessing the property is therefore allowed with the following code example:

      GST_LOCK (pad);
      direction = GST_RPAD_DIRECTION (pad);
      GST_UNLOCK (pad);

* Property lifetime

  All properties requiring a lock can change after releasing the associated lock.
  This means that as soon as long as you hold the lock, the state of the object
  regarding the locked properties is consistent with the information obtained. As
  soon as the lock is released, any values required from the properties might not
  be valid anymore and can as best be described as a snapshot of the state when
  the lock was held.

  This means that all properties that require access beyond the scope of the critial
  section should be copied or refcounted before releasing the lock.

  Example:

    the following example correctly gets the peer pad of an element. It is required
    to increase the refcount of the peer pad because as soon as the lock is released,
    the peer could be unreffed and disposed, making the pointer obtained in the critical
    section point to invalid memory.

      GST_LOCK (pad);
      peer = GST_RPAD_PEER (pad);
      if (peer)
        gst_object_ref (GST_OBJECT (peer));
      GST_UNLOCK (pad);
      ... use peer ...

      if (peer)
        gst_object_unref (GST_OBJECT (peer));

    Note that after releasing the lock the peer might not actually be the peer anymore
    of the pad. If you need to be sure it is, you need to extend the critical section
    to include the operations on the peer.

* Accessor methods

  For aplications it is encouraged to use the public methods of the object. Most usefull
  operations can be performed with the methods so it is seldom required to access the
  public fields manually.

  All accessor methods that return an object should increase the refcount of the returned
  object. The called should _unref() the object after usage. Each method should state this
  refcounting policy in the documentation.

* Accessing lists

  If the object property is a list, concurrent list iteration is needed to get the
  contents of the list. GStreamer uses the cookie mechanism to mark the last update 
  of a list. The list and the cookie are protected by the same lock. Each update to 
  a list requires the following actions:

   - acquire lock
   - update list
   - update cookie
   - release lock

  Updating the cookie is usually done by incrementing its value by one. Since cookies
  use guint32 its wraparound is for all practical reasons is not a problem.

  Iterating a list can safely be done by surrounding the list iteration with a
  lock/unlock of the lock.

  In some cases it is not a good idea to hold the lock for a long time while iterating
  the list. The state change code for a bin in GStreamer, for example, has to iterate
  over each element and perform a blocking call on each of them causing infinite bin
  locking. In this case the cookie can be used to iterate a list.

  Example:

     The following algorithm iterates a list and reverses the updates in the case a
     concurrent update was done to the list while iterating. The idea is that whenever
     we reacquire the lock, we check for updates to the cookie to decide if we are
     still iterating the right list.

     GST_LOCK (lock);
     /* grab list and cookie */
     cookie = object->list_cookie;
     list = object-list;
     while (list) {
       GstObject *item = GST_OBJECT (list->data);
       /* need to ref the item before releasing the lock */
       gst_object_ref (item);
       GST_UNLOCK (lock);

       ... use/change item here...

       /* release item here */
       gst_object_unref (item);
  
       GST_LOCK (lock);
       if (cookie != object->list_cookie) {
         /* handle rollback caused by concurrent modification 
	  * of the list here */

	 ...rollback changes to items...

	 /* grab new cookie and list */
	 cookie = object->list_cookie;
	 list = object->list;
       }
       else {
         list = g_list_next (list);
       }
     }
     GST_UNLOCK (lock);

* GstIterator

  GstIterator provides an easier way of retrieving elements in a concurrent list. 
  The followgin code example is equivalent to the previous example.
  
  Example:
    
    it = _get_iterator(object);
    while (!done) {
      switch (gst_iterator_next (it, &item)) {
        case GST_ITERATOR_OK:

          ... use/change item here...

          /* release item here */
          gst_object_unref (item);
	  break;
        case GST_ITERATOR_RESYNC:
          /* handle rollback caused by concurrent modification 
	   * of the list here */

	  ...rollback changes to items...

	  /* resync iterator to start again */
	  gst_iterator_resync (it);
	  break;
        case GST_ITERATOR_DONE:
	  done = TRUE;
	  break;
      }
    }
    gst_iterator_free (it);