392 lines
14 KiB
XML
392 lines
14 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<book id="debug-objects-guide">
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<bookinfo>
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<title>Debug objects life time</title>
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<authorgroup>
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<author>
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<firstname>Thomas</firstname>
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<surname>Gleixner</surname>
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<affiliation>
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<address>
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<email>tglx@linutronix.de</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<copyright>
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<year>2008</year>
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<holder>Thomas Gleixner</holder>
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</copyright>
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<legalnotice>
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<para>
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This documentation is free software; you can redistribute
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it and/or modify it under the terms of the GNU General Public
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License version 2 as published by the Free Software Foundation.
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</para>
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<para>
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This program is distributed in the hope that it will be
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useful, but WITHOUT ANY WARRANTY; without even the implied
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warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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See the GNU General Public License for more details.
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</para>
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<para>
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You should have received a copy of the GNU General Public
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License along with this program; if not, write to the Free
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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MA 02111-1307 USA
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</para>
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<para>
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For more details see the file COPYING in the source
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distribution of Linux.
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</para>
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</legalnotice>
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</bookinfo>
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<toc></toc>
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<chapter id="intro">
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<title>Introduction</title>
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<para>
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debugobjects is a generic infrastructure to track the life time
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of kernel objects and validate the operations on those.
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</para>
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<para>
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debugobjects is useful to check for the following error patterns:
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<itemizedlist>
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<listitem><para>Activation of uninitialized objects</para></listitem>
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<listitem><para>Initialization of active objects</para></listitem>
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<listitem><para>Usage of freed/destroyed objects</para></listitem>
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</itemizedlist>
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</para>
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<para>
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debugobjects is not changing the data structure of the real
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object so it can be compiled in with a minimal runtime impact
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and enabled on demand with a kernel command line option.
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</para>
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</chapter>
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<chapter id="howto">
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<title>Howto use debugobjects</title>
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<para>
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A kernel subsystem needs to provide a data structure which
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describes the object type and add calls into the debug code at
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appropriate places. The data structure to describe the object
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type needs at minimum the name of the object type. Optional
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functions can and should be provided to fixup detected problems
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so the kernel can continue to work and the debug information can
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be retrieved from a live system instead of hard core debugging
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with serial consoles and stack trace transcripts from the
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monitor.
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</para>
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<para>
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The debug calls provided by debugobjects are:
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<itemizedlist>
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<listitem><para>debug_object_init</para></listitem>
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<listitem><para>debug_object_init_on_stack</para></listitem>
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<listitem><para>debug_object_activate</para></listitem>
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<listitem><para>debug_object_deactivate</para></listitem>
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<listitem><para>debug_object_destroy</para></listitem>
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<listitem><para>debug_object_free</para></listitem>
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</itemizedlist>
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Each of these functions takes the address of the real object and
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a pointer to the object type specific debug description
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structure.
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</para>
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<para>
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Each detected error is reported in the statistics and a limited
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number of errors are printk'ed including a full stack trace.
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</para>
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<para>
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The statistics are available via /sys/kernel/debug/debug_objects/stats.
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They provide information about the number of warnings and the
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number of successful fixups along with information about the
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usage of the internal tracking objects and the state of the
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internal tracking objects pool.
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</para>
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</chapter>
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<chapter id="debugfunctions">
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<title>Debug functions</title>
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<sect1 id="prototypes">
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<title>Debug object function reference</title>
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!Elib/debugobjects.c
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</sect1>
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<sect1 id="debug_object_init">
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<title>debug_object_init</title>
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<para>
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This function is called whenever the initialization function
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of a real object is called.
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</para>
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<para>
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When the real object is already tracked by debugobjects it is
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checked, whether the object can be initialized. Initializing
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is not allowed for active and destroyed objects. When
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debugobjects detects an error, then it calls the fixup_init
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function of the object type description structure if provided
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by the caller. The fixup function can correct the problem
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before the real initialization of the object happens. E.g. it
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can deactivate an active object in order to prevent damage to
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the subsystem.
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</para>
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<para>
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When the real object is not yet tracked by debugobjects,
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debugobjects allocates a tracker object for the real object
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and sets the tracker object state to ODEBUG_STATE_INIT. It
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verifies that the object is not on the callers stack. If it is
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on the callers stack then a limited number of warnings
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including a full stack trace is printk'ed. The calling code
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must use debug_object_init_on_stack() and remove the object
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before leaving the function which allocated it. See next
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section.
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</para>
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</sect1>
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<sect1 id="debug_object_init_on_stack">
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<title>debug_object_init_on_stack</title>
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<para>
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This function is called whenever the initialization function
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of a real object which resides on the stack is called.
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</para>
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<para>
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When the real object is already tracked by debugobjects it is
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checked, whether the object can be initialized. Initializing
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is not allowed for active and destroyed objects. When
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debugobjects detects an error, then it calls the fixup_init
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function of the object type description structure if provided
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by the caller. The fixup function can correct the problem
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before the real initialization of the object happens. E.g. it
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can deactivate an active object in order to prevent damage to
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the subsystem.
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</para>
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<para>
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When the real object is not yet tracked by debugobjects
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debugobjects allocates a tracker object for the real object
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and sets the tracker object state to ODEBUG_STATE_INIT. It
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verifies that the object is on the callers stack.
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</para>
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<para>
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An object which is on the stack must be removed from the
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tracker by calling debug_object_free() before the function
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which allocates the object returns. Otherwise we keep track of
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stale objects.
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</para>
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</sect1>
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<sect1 id="debug_object_activate">
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<title>debug_object_activate</title>
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<para>
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This function is called whenever the activation function of a
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real object is called.
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</para>
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<para>
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When the real object is already tracked by debugobjects it is
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checked, whether the object can be activated. Activating is
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not allowed for active and destroyed objects. When
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debugobjects detects an error, then it calls the
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fixup_activate function of the object type description
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structure if provided by the caller. The fixup function can
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correct the problem before the real activation of the object
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happens. E.g. it can deactivate an active object in order to
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prevent damage to the subsystem.
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</para>
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<para>
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When the real object is not yet tracked by debugobjects then
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the fixup_activate function is called if available. This is
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necessary to allow the legitimate activation of statically
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allocated and initialized objects. The fixup function checks
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whether the object is valid and calls the debug_objects_init()
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function to initialize the tracking of this object.
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</para>
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<para>
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When the activation is legitimate, then the state of the
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associated tracker object is set to ODEBUG_STATE_ACTIVE.
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</para>
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</sect1>
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<sect1 id="debug_object_deactivate">
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<title>debug_object_deactivate</title>
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<para>
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This function is called whenever the deactivation function of
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a real object is called.
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</para>
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<para>
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When the real object is tracked by debugobjects it is checked,
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whether the object can be deactivated. Deactivating is not
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allowed for untracked or destroyed objects.
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</para>
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<para>
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When the deactivation is legitimate, then the state of the
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associated tracker object is set to ODEBUG_STATE_INACTIVE.
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</para>
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</sect1>
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<sect1 id="debug_object_destroy">
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<title>debug_object_destroy</title>
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<para>
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This function is called to mark an object destroyed. This is
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useful to prevent the usage of invalid objects, which are
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still available in memory: either statically allocated objects
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or objects which are freed later.
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</para>
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<para>
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When the real object is tracked by debugobjects it is checked,
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whether the object can be destroyed. Destruction is not
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allowed for active and destroyed objects. When debugobjects
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detects an error, then it calls the fixup_destroy function of
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the object type description structure if provided by the
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caller. The fixup function can correct the problem before the
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real destruction of the object happens. E.g. it can deactivate
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an active object in order to prevent damage to the subsystem.
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</para>
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<para>
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When the destruction is legitimate, then the state of the
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associated tracker object is set to ODEBUG_STATE_DESTROYED.
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</para>
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</sect1>
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<sect1 id="debug_object_free">
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<title>debug_object_free</title>
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<para>
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This function is called before an object is freed.
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</para>
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<para>
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When the real object is tracked by debugobjects it is checked,
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whether the object can be freed. Free is not allowed for
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active objects. When debugobjects detects an error, then it
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calls the fixup_free function of the object type description
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structure if provided by the caller. The fixup function can
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correct the problem before the real free of the object
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happens. E.g. it can deactivate an active object in order to
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prevent damage to the subsystem.
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</para>
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<para>
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Note that debug_object_free removes the object from the
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tracker. Later usage of the object is detected by the other
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debug checks.
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</para>
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</sect1>
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</chapter>
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<chapter id="fixupfunctions">
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<title>Fixup functions</title>
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<sect1 id="debug_obj_descr">
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<title>Debug object type description structure</title>
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!Iinclude/linux/debugobjects.h
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</sect1>
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<sect1 id="fixup_init">
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<title>fixup_init</title>
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<para>
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This function is called from the debug code whenever a problem
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in debug_object_init is detected. The function takes the
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address of the object and the state which is currently
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recorded in the tracker.
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</para>
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<para>
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Called from debug_object_init when the object state is:
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<itemizedlist>
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<listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
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</itemizedlist>
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</para>
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<para>
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The function returns 1 when the fixup was successful,
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otherwise 0. The return value is used to update the
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statistics.
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</para>
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<para>
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Note, that the function needs to call the debug_object_init()
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function again, after the damage has been repaired in order to
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keep the state consistent.
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</para>
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</sect1>
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<sect1 id="fixup_activate">
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<title>fixup_activate</title>
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<para>
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This function is called from the debug code whenever a problem
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in debug_object_activate is detected.
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</para>
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<para>
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Called from debug_object_activate when the object state is:
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<itemizedlist>
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<listitem><para>ODEBUG_STATE_NOTAVAILABLE</para></listitem>
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<listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
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</itemizedlist>
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</para>
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<para>
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The function returns 1 when the fixup was successful,
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otherwise 0. The return value is used to update the
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statistics.
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</para>
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<para>
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Note that the function needs to call the debug_object_activate()
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function again after the damage has been repaired in order to
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keep the state consistent.
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</para>
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<para>
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The activation of statically initialized objects is a special
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case. When debug_object_activate() has no tracked object for
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this object address then fixup_activate() is called with
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object state ODEBUG_STATE_NOTAVAILABLE. The fixup function
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needs to check whether this is a legitimate case of a
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statically initialized object or not. In case it is it calls
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debug_object_init() and debug_object_activate() to make the
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object known to the tracker and marked active. In this case
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the function should return 0 because this is not a real fixup.
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</para>
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</sect1>
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<sect1 id="fixup_destroy">
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<title>fixup_destroy</title>
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<para>
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This function is called from the debug code whenever a problem
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in debug_object_destroy is detected.
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</para>
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<para>
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Called from debug_object_destroy when the object state is:
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<itemizedlist>
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<listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
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</itemizedlist>
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</para>
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<para>
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The function returns 1 when the fixup was successful,
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otherwise 0. The return value is used to update the
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statistics.
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</para>
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</sect1>
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<sect1 id="fixup_free">
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<title>fixup_free</title>
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<para>
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This function is called from the debug code whenever a problem
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in debug_object_free is detected. Further it can be called
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from the debug checks in kfree/vfree, when an active object is
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detected from the debug_check_no_obj_freed() sanity checks.
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</para>
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<para>
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Called from debug_object_free() or debug_check_no_obj_freed()
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when the object state is:
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<itemizedlist>
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<listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
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</itemizedlist>
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</para>
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<para>
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The function returns 1 when the fixup was successful,
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otherwise 0. The return value is used to update the
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statistics.
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</para>
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</sect1>
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</chapter>
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<chapter id="bugs">
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<title>Known Bugs And Assumptions</title>
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<para>
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None (knock on wood).
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</para>
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</chapter>
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</book>
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