path: root/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 implications 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 shared objects 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.

  Refcounting is also used to ensure that mutable data structures are only
  modified when they are owned by the calling code.

  It is a requirement that when two threads have a handle on an object, the
  refcount must be more than 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.

Shared data structures and writability:

  All objects 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 preceded with a
  _get_writable() 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 modifiable 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 _get_writable() can avoid a real copy because the calling method is the
  only one holding a reference, which makes read/write very cheap.

  The drawback is that sometimes 1 needless copy can be done. This would happen
  when N threads call _get_writable() at the same time, all seeing that N
  references are held on 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.
 
Mutable substructures:

  Special techniques are necessary to ensure the consistency of compound shared
  objects. As mentioned above, shared objects need to have a reference count of
  1 if they are to be modified. Implicit in this assumption is that all parts of
  the shared object belong only to the object. For example, a GstStructure in
  one GstCaps object should not belong to any other GstCaps object. This
  condition suggests a parent-child relationship: structures can only be added
  to parent object if they do not already have a parent object.

  In addition, these substructures must not be modified while more than one code
  segment has a reference on the parent object. For example, if the user creates
  a GstStructure, adds it to a GstCaps, and the GstCaps is then referenced by
  other code segments, the GstStructure should then become immutable, so that
  changes to that data structure do not affect other parts of the code. This
  means that the child is only mutable when the parent's reference count is 1,
  as well as when the child structure has no parent.

  The general solution to this problem is to include a field in child structures
  pointing to the parent's atomic reference count. When set to NULL, this
  indicates that the child has no parent. Otherwise, procedures that modify the
  child structure must check if the parent's refcount is 1, and otherwise must
  cause an error to be signaled.

  Note that this is an internal implementation detail; application or plugin
  code that calls _get_writable() on an object is guaranteed to receive an
  object of refcount 1, which must then be writable. The only trick is that a
  pointer to a child structure of an object is only valid while the calling code
  has a reference on the parent object, because the parent is the owner of the
  child.

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.

  Object locking can be done with recursive locks or regular mutexes. Object
  locks in GStreamer are implemented with mutexes which cause deadlocks when
  locked recursively from the same thread. This is done because regular mutexes
  are cheaper.

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 these 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.

Compare and swap

  As part of the atomic operations, compare-and-swap (CAS) can be used to access
  or update a single property or pointer in an object without having to take a
  lock.

  This technique is currently not used in GStreamer but might be added in the
  future in performance critical places.
 

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_OBJECT_LOCK() and GST_OBJECT_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 through 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.

  The float/sink process is very useful when initializing elements that will
  then be placed under control of a parent. The floating ref keeps the object
  alive until it is parented, and once the object is parented you can forget
  about it.

  also see part-relations.txt
  
* parent-child relations

  One can create parent-child relationships with the _object_set_parent()
  method. This method refs and 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 responsibilities 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.

  also see part-relations.txt

* 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" or any other 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_OBJECT_LOCK (pad);
      direction = GST_RPAD_DIRECTION (pad);
      GST_OBJECT_UNLOCK (pad);

* Property lifetime

  All properties requiring a lock can change after releasing the associated
  lock. This means that 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 acquired 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.

  Most object provide a _get_<property>() method to get a copy or refcounted
  instance of the property value. The caller should not wory about any locks
  but should unref/free the object after usage.

  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_OBJECT_LOCK (pad);
      peer = GST_RPAD_PEER (pad);
      if (peer)
        gst_object_ref (GST_OBJECT (peer));
      GST_OBJECT_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.

    The following code is equivalent to the above but with using the functions
    to access object properties.

      peer = gst_pad_get_peer (pad);
      if (peer) {
        ... use peer ...

        gst_object_unref (GST_OBJECT (peer));
      }

  Example:

    Accessing the name of an object makes a copy of the name. The caller of the
    function should g_free() the name after usage.

      GST_OBJECT_LOCK (object)
      name = g_strdup (GST_OBJECT_NAME (object));
      GST_OBJECT_UNLOCK (object)
      ... use name ...

      g_free (name);

    or:

      name = gst_object_get_name (object);

      ... use name ...

      g_free (name);
    

* Accessor methods

  For aplications it is encouraged to use the public methods of the object. Most
  useful 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 caller 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
  potentially 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_OBJECT_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_OBJECT_UNLOCK (lock);

       ... use/change item here...

       /* release item here */
       gst_object_unref (item);
  
       GST_OBJECT_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_OBJECT_UNLOCK (lock);

* GstIterator

  GstIterator provides an easier way of retrieving elements in a concurrent
  list. The following 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);