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+============================================
+The object-lifetime debugging infrastructure
+============================================
+
+:Author: Thomas Gleixner
+
+Introduction
+============
+
+debugobjects is a generic infrastructure to track the life time of
+kernel objects and validate the operations on those.
+
+debugobjects is useful to check for the following error patterns:
+
+-  Activation of uninitialized objects
+
+-  Initialization of active objects
+
+-  Usage of freed/destroyed objects
+
+debugobjects is not changing the data structure of the real object so it
+can be compiled in with a minimal runtime impact and enabled on demand
+with a kernel command line option.
+
+Howto use debugobjects
+======================
+
+A kernel subsystem needs to provide a data structure which describes the
+object type and add calls into the debug code at appropriate places. The
+data structure to describe the object type needs at minimum the name of
+the object type. Optional functions can and should be provided to fixup
+detected problems so the kernel can continue to work and the debug
+information can be retrieved from a live system instead of hard core
+debugging with serial consoles and stack trace transcripts from the
+monitor.
+
+The debug calls provided by debugobjects are:
+
+-  debug_object_init
+
+-  debug_object_init_on_stack
+
+-  debug_object_activate
+
+-  debug_object_deactivate
+
+-  debug_object_destroy
+
+-  debug_object_free
+
+-  debug_object_assert_init
+
+Each of these functions takes the address of the real object and a
+pointer to the object type specific debug description structure.
+
+Each detected error is reported in the statistics and a limited number
+of errors are printk'ed including a full stack trace.
+
+The statistics are available via /sys/kernel/debug/debug_objects/stats.
+They provide information about the number of warnings and the number of
+successful fixups along with information about the usage of the internal
+tracking objects and the state of the internal tracking objects pool.
+
+Debug functions
+===============
+
+Debug object function reference
+-------------------------------
+
+.. kernel-doc:: lib/debugobjects.c
+   :export:
+
+debug_object_init
+-------------------
+
+This function is called whenever the initialization function of a real
+object is called.
+
+When the real object is already tracked by debugobjects it is checked,
+whether the object can be initialized. Initializing is not allowed for
+active and destroyed objects. When debugobjects detects an error, then
+it calls the fixup_init function of the object type description
+structure if provided by the caller. The fixup function can correct the
+problem before the real initialization of the object happens. E.g. it
+can deactivate an active object in order to prevent damage to the
+subsystem.
+
+When the real object is not yet tracked by debugobjects, debugobjects
+allocates a tracker object for the real object and sets the tracker
+object state to ODEBUG_STATE_INIT. It verifies that the object is not
+on the callers stack. If it is on the callers stack then a limited
+number of warnings including a full stack trace is printk'ed. The
+calling code must use debug_object_init_on_stack() and remove the
+object before leaving the function which allocated it. See next section.
+
+debug_object_init_on_stack
+------------------------------
+
+This function is called whenever the initialization function of a real
+object which resides on the stack is called.
+
+When the real object is already tracked by debugobjects it is checked,
+whether the object can be initialized. Initializing is not allowed for
+active and destroyed objects. When debugobjects detects an error, then
+it calls the fixup_init function of the object type description
+structure if provided by the caller. The fixup function can correct the
+problem before the real initialization of the object happens. E.g. it
+can deactivate an active object in order to prevent damage to the
+subsystem.
+
+When the real object is not yet tracked by debugobjects debugobjects
+allocates a tracker object for the real object and sets the tracker
+object state to ODEBUG_STATE_INIT. It verifies that the object is on
+the callers stack.
+
+An object which is on the stack must be removed from the tracker by
+calling debug_object_free() before the function which allocates the
+object returns. Otherwise we keep track of stale objects.
+
+debug_object_activate
+-----------------------
+
+This function is called whenever the activation function of a real
+object is called.
+
+When the real object is already tracked by debugobjects it is checked,
+whether the object can be activated. Activating is not allowed for
+active and destroyed objects. When debugobjects detects an error, then
+it calls the fixup_activate function of the object type description
+structure if provided by the caller. The fixup function can correct the
+problem before the real activation of the object happens. E.g. it can
+deactivate an active object in order to prevent damage to the subsystem.
+
+When the real object is not yet tracked by debugobjects then the
+fixup_activate function is called if available. This is necessary to
+allow the legitimate activation of statically allocated and initialized
+objects. The fixup function checks whether the object is valid and calls
+the debug_objects_init() function to initialize the tracking of this
+object.
+
+When the activation is legitimate, then the state of the associated
+tracker object is set to ODEBUG_STATE_ACTIVE.
+
+debug_object_deactivate
+-------------------------
+
+This function is called whenever the deactivation function of a real
+object is called.
+
+When the real object is tracked by debugobjects it is checked, whether
+the object can be deactivated. Deactivating is not allowed for untracked
+or destroyed objects.
+
+When the deactivation is legitimate, then the state of the associated
+tracker object is set to ODEBUG_STATE_INACTIVE.
+
+debug_object_destroy
+----------------------
+
+This function is called to mark an object destroyed. This is useful to
+prevent the usage of invalid objects, which are still available in
+memory: either statically allocated objects or objects which are freed
+later.
+
+When the real object is tracked by debugobjects it is checked, whether
+the object can be destroyed. Destruction is not allowed for active and
+destroyed objects. When debugobjects detects an error, then it calls the
+fixup_destroy function of the object type description structure if
+provided by the caller. The fixup function can correct the problem
+before the real destruction of the object happens. E.g. it can
+deactivate an active object in order to prevent damage to the subsystem.
+
+When the destruction is legitimate, then the state of the associated
+tracker object is set to ODEBUG_STATE_DESTROYED.
+
+debug_object_free
+-------------------
+
+This function is called before an object is freed.
+
+When the real object is tracked by debugobjects it is checked, whether
+the object can be freed. Free is not allowed for active objects. When
+debugobjects detects an error, then it calls the fixup_free function of
+the object type description structure if provided by the caller. The
+fixup function can correct the problem before the real free of the
+object happens. E.g. it can deactivate an active object in order to
+prevent damage to the subsystem.
+
+Note that debug_object_free removes the object from the tracker. Later
+usage of the object is detected by the other debug checks.
+
+debug_object_assert_init
+---------------------------
+
+This function is called to assert that an object has been initialized.
+
+When the real object is not tracked by debugobjects, it calls
+fixup_assert_init of the object type description structure provided by
+the caller, with the hardcoded object state ODEBUG_NOT_AVAILABLE. The
+fixup function can correct the problem by calling debug_object_init
+and other specific initializing functions.
+
+When the real object is already tracked by debugobjects it is ignored.
+
+Fixup functions
+===============
+
+Debug object type description structure
+---------------------------------------
+
+.. kernel-doc:: include/linux/debugobjects.h
+   :internal:
+
+fixup_init
+-----------
+
+This function is called from the debug code whenever a problem in
+debug_object_init is detected. The function takes the address of the
+object and the state which is currently recorded in the tracker.
+
+Called from debug_object_init when the object state is:
+
+-  ODEBUG_STATE_ACTIVE
+
+The function returns true when the fixup was successful, otherwise
+false. The return value is used to update the statistics.
+
+Note, that the function needs to call the debug_object_init() function
+again, after the damage has been repaired in order to keep the state
+consistent.
+
+fixup_activate
+---------------
+
+This function is called from the debug code whenever a problem in
+debug_object_activate is detected.
+
+Called from debug_object_activate when the object state is:
+
+-  ODEBUG_STATE_NOTAVAILABLE
+
+-  ODEBUG_STATE_ACTIVE
+
+The function returns true when the fixup was successful, otherwise
+false. The return value is used to update the statistics.
+
+Note that the function needs to call the debug_object_activate()
+function again after the damage has been repaired in order to keep the
+state consistent.
+
+The activation of statically initialized objects is a special case. When
+debug_object_activate() has no tracked object for this object address
+then fixup_activate() is called with object state
+ODEBUG_STATE_NOTAVAILABLE. The fixup function needs to check whether
+this is a legitimate case of a statically initialized object or not. In
+case it is it calls debug_object_init() and debug_object_activate()
+to make the object known to the tracker and marked active. In this case
+the function should return false because this is not a real fixup.
+
+fixup_destroy
+--------------
+
+This function is called from the debug code whenever a problem in
+debug_object_destroy is detected.
+
+Called from debug_object_destroy when the object state is:
+
+-  ODEBUG_STATE_ACTIVE
+
+The function returns true when the fixup was successful, otherwise
+false. The return value is used to update the statistics.
+
+fixup_free
+-----------
+
+This function is called from the debug code whenever a problem in
+debug_object_free is detected. Further it can be called from the debug
+checks in kfree/vfree, when an active object is detected from the
+debug_check_no_obj_freed() sanity checks.
+
+Called from debug_object_free() or debug_check_no_obj_freed() when
+the object state is:
+
+-  ODEBUG_STATE_ACTIVE
+
+The function returns true when the fixup was successful, otherwise
+false. The return value is used to update the statistics.
+
+fixup_assert_init
+-------------------
+
+This function is called from the debug code whenever a problem in
+debug_object_assert_init is detected.
+
+Called from debug_object_assert_init() with a hardcoded state
+ODEBUG_STATE_NOTAVAILABLE when the object is not found in the debug
+bucket.
+
+The function returns true when the fixup was successful, otherwise
+false. The return value is used to update the statistics.
+
+Note, this function should make sure debug_object_init() is called
+before returning.
+
+The handling of statically initialized objects is a special case. The
+fixup function should check if this is a legitimate case of a statically
+initialized object or not. In this case only debug_object_init()
+should be called to make the object known to the tracker. Then the
+function should return false because this is not a real fixup.
+
+Known Bugs And Assumptions
+==========================
+
+None (knock on wood).