debugobjects: add documentation

Add a DocBook for debugobjects.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Greg KH <greg@kroah.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 391 insertions, 0 deletions

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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<book id="debug-objects-guide">
+ <bookinfo>
+  <title>Debug objects life time</title>
+
+  <authorgroup>
+   <author>
+    <firstname>Thomas</firstname>
+    <surname>Gleixner</surname>
+    <affiliation>
+     <address>
+      <email>tglx@linutronix.de</email>
+     </address>
+    </affiliation>
+   </author>
+  </authorgroup>
+
+  <copyright>
+   <year>2008</year>
+   <holder>Thomas Gleixner</holder>
+  </copyright>
+
+  <legalnotice>
+   <para>
+     This documentation is free software; you can redistribute
+     it and/or modify it under the terms of the GNU General Public
+     License version 2 as published by the Free Software Foundation.
+   </para>
+
+   <para>
+     This program is distributed in the hope that it will be
+     useful, but WITHOUT ANY WARRANTY; without even the implied
+     warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+     See the GNU General Public License for more details.
+   </para>
+
+   <para>
+     You should have received a copy of the GNU General Public
+     License along with this program; if not, write to the Free
+     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+     MA 02111-1307 USA
+   </para>
+
+   <para>
+     For more details see the file COPYING in the source
+     distribution of Linux.
+   </para>
+  </legalnotice>
+ </bookinfo>
+
+<toc></toc>
+
+  <chapter id="intro">
+    <title>Introduction</title>
+    <para>
+      debugobjects is a generic infrastructure to track the life time
+      of kernel objects and validate the operations on those.
+    </para>
+    <para>
+      debugobjects is useful to check for the following error patterns:
+	<itemizedlist>
+	  <listitem><para>Activation of uninitialized objects</para></listitem>
+	  <listitem><para>Initialization of active objects</para></listitem>
+	  <listitem><para>Usage of freed/destroyed objects</para></listitem>
+	</itemizedlist>
+    </para>
+    <para>
+      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.
+    </para>
+  </chapter>
+
+  <chapter id="howto">
+    <title>Howto use debugobjects</title>
+    <para>
+      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.
+    </para>
+    <para>
+      The debug calls provided by debugobjects are:
+      <itemizedlist>
+	<listitem><para>debug_object_init</para></listitem>
+	<listitem><para>debug_object_init_on_stack</para></listitem>
+	<listitem><para>debug_object_activate</para></listitem>
+	<listitem><para>debug_object_deactivate</para></listitem>
+	<listitem><para>debug_object_destroy</para></listitem>
+	<listitem><para>debug_object_free</para></listitem>
+      </itemizedlist>
+      Each of these functions takes the address of the real object and
+      a pointer to the object type specific debug description
+      structure.
+    </para>
+    <para>
+      Each detected error is reported in the statistics and a limited
+      number of errors are printk'ed including a full stack trace.
+    </para>
+    <para>
+      The statistics are available via debugfs/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.
+    </para>
+  </chapter>
+  <chapter id="debugfunctions">
+    <title>Debug functions</title>
+    <sect1 id="prototypes">
+      <title>Debug object function reference</title>
+!Elib/debugobjects.c
+    </sect1>
+    <sect1 id="debug_object_init">
+      <title>debug_object_init</title>
+      <para>
+	This function is called whenever the initialization function
+	of a real object is called.
+      </para>
+      <para>
+	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.
+      </para>
+      <para>
+	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.
+      </para>
+    </sect1>
+
+    <sect1 id="debug_object_init_on_stack">
+      <title>debug_object_init_on_stack</title>
+      <para>
+	This function is called whenever the initialization function
+	of a real object which resides on the stack is called.
+      </para>
+      <para>
+	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.
+      </para>
+      <para>
+	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.
+      </para>
+      <para>
+	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.
+      </para>
+    </sect1>
+
+    <sect1 id="debug_object_activate">
+      <title>debug_object_activate</title>
+      <para>
+	This function is called whenever the activation function of a
+	real object is called.
+      </para>
+      <para>
+	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.
+      </para>
+      <para>
+	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.
+      </para>
+      <para>
+	When the activation is legitimate, then the state of the
+	associated tracker object is set to ODEBUG_STATE_ACTIVE.
+      </para>
+    </sect1>
+
+    <sect1 id="debug_object_deactivate">
+      <title>debug_object_deactivate</title>
+      <para>
+	This function is called whenever the deactivation function of
+	a real object is called.
+      </para>
+      <para>
+	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.
+      </para>
+      <para>
+	When the deactivation is legitimate, then the state of the
+	associated tracker object is set to ODEBUG_STATE_INACTIVE.
+      </para>
+    </sect1>
+
+    <sect1 id="debug_object_destroy">
+      <title>debug_object_destroy</title>
+      <para>
+	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.
+      </para>
+      <para>
+	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.
+      </para>
+      <para>
+	When the destruction is legitimate, then the state of the
+	associated tracker object is set to ODEBUG_STATE_DESTROYED.
+      </para>
+    </sect1>
+
+    <sect1 id="debug_object_free">
+      <title>debug_object_free</title>
+      <para>
+	This function is called before an object is freed.
+      </para>
+      <para>
+	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.
+      </para>
+      <para>
+	Note that debug_object_free removes the object from the
+	tracker. Later usage of the object is detected by the other
+	debug checks.
+      </para>
+    </sect1>
+  </chapter>
+  <chapter id="fixupfunctions">
+    <title>Fixup functions</title>
+    <sect1 id="debug_obj_descr">
+      <title>Debug object type description structure</title>
+!Iinclude/linux/debugobjects.h
+    </sect1>
+    <sect1 id="fixup_init">
+      <title>fixup_init</title>
+      <para>
+	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.
+      </para>
+      <para>
+	Called from debug_object_init when the object state is:
+	<itemizedlist>
+	  <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
+	</itemizedlist>
+      </para>
+      <para>
+	The function returns 1 when the fixup was successful,
+	otherwise 0. The return value is used to update the
+	statistics.
+      </para>
+      <para>
+	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.
+      </para>
+    </sect1>
+
+    <sect1 id="fixup_activate">
+      <title>fixup_activate</title>
+      <para>
+	This function is called from the debug code whenever a problem
+	in debug_object_activate is detected.
+      </para>
+      <para>
+	Called from debug_object_activate when the object state is:
+	<itemizedlist>
+	  <listitem><para>ODEBUG_STATE_NOTAVAILABLE</para></listitem>
+	  <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
+	</itemizedlist>
+      </para>
+      <para>
+	The function returns 1 when the fixup was successful,
+	otherwise 0. The return value is used to update the
+	statistics.
+      </para>
+      <para>
+	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.
+      </para>
+      <para>
+	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 0 because this is not a real fixup.
+      </para>
+    </sect1>
+
+    <sect1 id="fixup_destroy">
+      <title>fixup_destroy</title>
+      <para>
+	This function is called from the debug code whenever a problem
+	in debug_object_destroy is detected.
+      </para>
+      <para>
+	Called from debug_object_destroy when the object state is:
+	<itemizedlist>
+	  <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
+	</itemizedlist>
+      </para>
+      <para>
+	The function returns 1 when the fixup was successful,
+	otherwise 0. The return value is used to update the
+	statistics.
+      </para>
+    </sect1>
+    <sect1 id="fixup_free">
+      <title>fixup_free</title>
+      <para>
+	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.
+      </para>
+      <para>
+	Called from debug_object_free() or debug_check_no_obj_freed()
+	when the object state is:
+	<itemizedlist>
+	  <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
+	</itemizedlist>
+      </para>
+      <para>
+	The function returns 1 when the fixup was successful,
+	otherwise 0. The return value is used to update the
+	statistics.
+      </para>
+    </sect1>
+  </chapter>
+  <chapter id="bugs">
+    <title>Known Bugs And Assumptions</title>
+    <para>
+	None (knock on wood).
+    </para>
+  </chapter>
+</book>