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| 4 |
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| 5 | <book id="debug-objects-guide">
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| 6 | <bookinfo>
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| 7 | <title>Debug objects life time</title>
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| 8 |
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| 9 | <authorgroup>
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| 10 | <author>
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| 11 | <firstname>Thomas</firstname>
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| 12 | <surname>Gleixner</surname>
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| 13 | <affiliation>
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| 14 | <address>
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| 15 | <email>[email protected]</email>
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| 16 | </address>
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| 17 | </affiliation>
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| 18 | </author>
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| 19 | </authorgroup>
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| 20 |
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| 21 | <copyright>
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| 22 | <year>2008</year>
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| 23 | <holder>Thomas Gleixner</holder>
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| 24 | </copyright>
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| 25 |
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| 26 | <legalnotice>
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| 27 | <para>
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| 28 | This documentation is free software; you can redistribute
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| 29 | it and/or modify it under the terms of the GNU General Public
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| 30 | License version 2 as published by the Free Software Foundation.
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| 31 | </para>
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| 32 |
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| 33 | <para>
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| 34 | This program is distributed in the hope that it will be
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| 35 | useful, but WITHOUT ANY WARRANTY; without even the implied
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| 36 | warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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| 37 | See the GNU General Public License for more details.
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| 38 | </para>
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| 39 |
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| 40 | <para>
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| 41 | You should have received a copy of the GNU General Public
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| 42 | License along with this program; if not, write to the Free
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| 43 | Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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| 44 | MA 02111-1307 USA
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| 45 | </para>
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| 46 |
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| 47 | <para>
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| 48 | For more details see the file COPYING in the source
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| 49 | distribution of Linux.
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| 50 | </para>
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| 51 | </legalnotice>
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| 52 | </bookinfo>
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| 53 |
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| 54 | <toc></toc>
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| 55 |
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| 56 | <chapter id="intro">
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| 57 | <title>Introduction</title>
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| 58 | <para>
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| 59 | debugobjects is a generic infrastructure to track the life time
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| 60 | of kernel objects and validate the operations on those.
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| 61 | </para>
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| 62 | <para>
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| 63 | debugobjects is useful to check for the following error patterns:
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| 64 | <itemizedlist>
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| 65 | <listitem><para>Activation of uninitialized objects</para></listitem>
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| 66 | <listitem><para>Initialization of active objects</para></listitem>
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| 67 | <listitem><para>Usage of freed/destroyed objects</para></listitem>
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| 68 | </itemizedlist>
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| 69 | </para>
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| 70 | <para>
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| 71 | debugobjects is not changing the data structure of the real
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| 72 | object so it can be compiled in with a minimal runtime impact
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| 73 | and enabled on demand with a kernel command line option.
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| 74 | </para>
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| 75 | </chapter>
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| 76 |
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| 77 | <chapter id="howto">
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| 78 | <title>Howto use debugobjects</title>
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| 79 | <para>
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| 80 | A kernel subsystem needs to provide a data structure which
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| 81 | describes the object type and add calls into the debug code at
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| 82 | appropriate places. The data structure to describe the object
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| 83 | type needs at minimum the name of the object type. Optional
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| 84 | functions can and should be provided to fixup detected problems
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| 85 | so the kernel can continue to work and the debug information can
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| 86 | be retrieved from a live system instead of hard core debugging
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| 87 | with serial consoles and stack trace transcripts from the
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| 88 | monitor.
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| 89 | </para>
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| 90 | <para>
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| 91 | The debug calls provided by debugobjects are:
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| 92 | <itemizedlist>
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| 93 | <listitem><para>debug_object_init</para></listitem>
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| 94 | <listitem><para>debug_object_init_on_stack</para></listitem>
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| 95 | <listitem><para>debug_object_activate</para></listitem>
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| 96 | <listitem><para>debug_object_deactivate</para></listitem>
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| 97 | <listitem><para>debug_object_destroy</para></listitem>
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| 98 | <listitem><para>debug_object_free</para></listitem>
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| 99 | </itemizedlist>
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| 100 | Each of these functions takes the address of the real object and
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| 101 | a pointer to the object type specific debug description
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| 102 | structure.
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| 103 | </para>
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| 104 | <para>
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| 105 | Each detected error is reported in the statistics and a limited
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| 106 | number of errors are printk'ed including a full stack trace.
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| 107 | </para>
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| 108 | <para>
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| 109 | The statistics are available via debugfs/debug_objects/stats.
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| 110 | They provide information about the number of warnings and the
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| 111 | number of successful fixups along with information about the
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| 112 | usage of the internal tracking objects and the state of the
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| 113 | internal tracking objects pool.
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| 114 | </para>
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| 115 | </chapter>
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| 116 | <chapter id="debugfunctions">
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| 117 | <title>Debug functions</title>
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| 118 | <sect1 id="prototypes">
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| 119 | <title>Debug object function reference</title>
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| 120 | !Elib/debugobjects.c
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| 121 | </sect1>
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| 122 | <sect1 id="debug_object_init">
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| 123 | <title>debug_object_init</title>
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| 124 | <para>
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| 125 | This function is called whenever the initialization function
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| 126 | of a real object is called.
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| 127 | </para>
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| 128 | <para>
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| 129 | When the real object is already tracked by debugobjects it is
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| 130 | checked, whether the object can be initialized. Initializing
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| 131 | is not allowed for active and destroyed objects. When
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| 132 | debugobjects detects an error, then it calls the fixup_init
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| 133 | function of the object type description structure if provided
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| 134 | by the caller. The fixup function can correct the problem
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| 135 | before the real initialization of the object happens. E.g. it
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| 136 | can deactivate an active object in order to prevent damage to
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| 137 | the subsystem.
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| 138 | </para>
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| 139 | <para>
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| 140 | When the real object is not yet tracked by debugobjects,
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| 141 | debugobjects allocates a tracker object for the real object
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| 142 | and sets the tracker object state to ODEBUG_STATE_INIT. It
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| 143 | verifies that the object is not on the callers stack. If it is
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| 144 | on the callers stack then a limited number of warnings
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| 145 | including a full stack trace is printk'ed. The calling code
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| 146 | must use debug_object_init_on_stack() and remove the object
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| 147 | before leaving the function which allocated it. See next
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| 148 | section.
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| 149 | </para>
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| 150 | </sect1>
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| 151 |
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| 152 | <sect1 id="debug_object_init_on_stack">
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| 153 | <title>debug_object_init_on_stack</title>
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| 154 | <para>
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| 155 | This function is called whenever the initialization function
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| 156 | of a real object which resides on the stack is called.
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| 157 | </para>
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| 158 | <para>
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| 159 | When the real object is already tracked by debugobjects it is
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| 160 | checked, whether the object can be initialized. Initializing
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| 161 | is not allowed for active and destroyed objects. When
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| 162 | debugobjects detects an error, then it calls the fixup_init
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| 163 | function of the object type description structure if provided
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| 164 | by the caller. The fixup function can correct the problem
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| 165 | before the real initialization of the object happens. E.g. it
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| 166 | can deactivate an active object in order to prevent damage to
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| 167 | the subsystem.
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| 168 | </para>
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| 169 | <para>
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| 170 | When the real object is not yet tracked by debugobjects
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| 171 | debugobjects allocates a tracker object for the real object
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| 172 | and sets the tracker object state to ODEBUG_STATE_INIT. It
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| 173 | verifies that the object is on the callers stack.
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| 174 | </para>
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| 175 | <para>
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| 176 | An object which is on the stack must be removed from the
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| 177 | tracker by calling debug_object_free() before the function
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| 178 | which allocates the object returns. Otherwise we keep track of
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| 179 | stale objects.
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| 180 | </para>
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| 181 | </sect1>
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| 182 |
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| 183 | <sect1 id="debug_object_activate">
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| 184 | <title>debug_object_activate</title>
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| 185 | <para>
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| 186 | This function is called whenever the activation function of a
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| 187 | real object is called.
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| 188 | </para>
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| 189 | <para>
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| 190 | When the real object is already tracked by debugobjects it is
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| 191 | checked, whether the object can be activated. Activating is
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| 192 | not allowed for active and destroyed objects. When
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| 193 | debugobjects detects an error, then it calls the
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| 194 | fixup_activate function of the object type description
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| 195 | structure if provided by the caller. The fixup function can
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| 196 | correct the problem before the real activation of the object
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| 197 | happens. E.g. it can deactivate an active object in order to
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| 198 | prevent damage to the subsystem.
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| 199 | </para>
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| 200 | <para>
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| 201 | When the real object is not yet tracked by debugobjects then
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| 202 | the fixup_activate function is called if available. This is
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| 203 | necessary to allow the legitimate activation of statically
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| 204 | allocated and initialized objects. The fixup function checks
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| 205 | whether the object is valid and calls the debug_objects_init()
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| 206 | function to initialize the tracking of this object.
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| 207 | </para>
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| 208 | <para>
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| 209 | When the activation is legitimate, then the state of the
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| 210 | associated tracker object is set to ODEBUG_STATE_ACTIVE.
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| 211 | </para>
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| 212 | </sect1>
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| 213 |
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| 214 | <sect1 id="debug_object_deactivate">
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| 215 | <title>debug_object_deactivate</title>
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| 216 | <para>
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| 217 | This function is called whenever the deactivation function of
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| 218 | a real object is called.
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| 219 | </para>
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| 220 | <para>
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| 221 | When the real object is tracked by debugobjects it is checked,
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| 222 | whether the object can be deactivated. Deactivating is not
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| 223 | allowed for untracked or destroyed objects.
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| 224 | </para>
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| 225 | <para>
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| 226 | When the deactivation is legitimate, then the state of the
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| 227 | associated tracker object is set to ODEBUG_STATE_INACTIVE.
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| 228 | </para>
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| 229 | </sect1>
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| 230 |
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| 231 | <sect1 id="debug_object_destroy">
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| 232 | <title>debug_object_destroy</title>
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| 233 | <para>
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| 234 | This function is called to mark an object destroyed. This is
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| 235 | useful to prevent the usage of invalid objects, which are
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| 236 | still available in memory: either statically allocated objects
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| 237 | or objects which are freed later.
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| 238 | </para>
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| 239 | <para>
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| 240 | When the real object is tracked by debugobjects it is checked,
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| 241 | whether the object can be destroyed. Destruction is not
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| 242 | allowed for active and destroyed objects. When debugobjects
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| 243 | detects an error, then it calls the fixup_destroy function of
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| 244 | the object type description structure if provided by the
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| 245 | caller. The fixup function can correct the problem before the
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| 246 | real destruction of the object happens. E.g. it can deactivate
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| 247 | an active object in order to prevent damage to the subsystem.
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| 248 | </para>
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| 249 | <para>
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| 250 | When the destruction is legitimate, then the state of the
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| 251 | associated tracker object is set to ODEBUG_STATE_DESTROYED.
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| 252 | </para>
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| 253 | </sect1>
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| 254 |
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| 255 | <sect1 id="debug_object_free">
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| 256 | <title>debug_object_free</title>
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| 257 | <para>
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| 258 | This function is called before an object is freed.
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| 259 | </para>
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| 260 | <para>
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| 261 | When the real object is tracked by debugobjects it is checked,
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| 262 | whether the object can be freed. Free is not allowed for
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| 263 | active objects. When debugobjects detects an error, then it
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| 264 | calls the fixup_free function of the object type description
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| 265 | structure if provided by the caller. The fixup function can
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| 266 | correct the problem before the real free of the object
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| 267 | happens. E.g. it can deactivate an active object in order to
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| 268 | prevent damage to the subsystem.
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| 269 | </para>
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| 270 | <para>
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| 271 | Note that debug_object_free removes the object from the
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| 272 | tracker. Later usage of the object is detected by the other
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| 273 | debug checks.
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| 274 | </para>
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| 275 | </sect1>
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| 276 | </chapter>
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| 277 | <chapter id="fixupfunctions">
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| 278 | <title>Fixup functions</title>
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| 279 | <sect1 id="debug_obj_descr">
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| 280 | <title>Debug object type description structure</title>
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| 281 | !Iinclude/linux/debugobjects.h
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| 282 | </sect1>
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| 283 | <sect1 id="fixup_init">
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| 284 | <title>fixup_init</title>
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| 285 | <para>
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| 286 | This function is called from the debug code whenever a problem
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| 287 | in debug_object_init is detected. The function takes the
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| 288 | address of the object and the state which is currently
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| 289 | recorded in the tracker.
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| 290 | </para>
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| 291 | <para>
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| 292 | Called from debug_object_init when the object state is:
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| 293 | <itemizedlist>
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| 294 | <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
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| 295 | </itemizedlist>
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| 296 | </para>
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| 297 | <para>
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| 298 | The function returns 1 when the fixup was successful,
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| 299 | otherwise 0. The return value is used to update the
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| 300 | statistics.
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| 301 | </para>
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| 302 | <para>
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| 303 | Note, that the function needs to call the debug_object_init()
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| 304 | function again, after the damage has been repaired in order to
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| 305 | keep the state consistent.
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| 306 | </para>
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| 307 | </sect1>
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| 308 |
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| 309 | <sect1 id="fixup_activate">
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| 310 | <title>fixup_activate</title>
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| 311 | <para>
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| 312 | This function is called from the debug code whenever a problem
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| 313 | in debug_object_activate is detected.
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| 314 | </para>
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| 315 | <para>
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| 316 | Called from debug_object_activate when the object state is:
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| 317 | <itemizedlist>
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| 318 | <listitem><para>ODEBUG_STATE_NOTAVAILABLE</para></listitem>
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| 319 | <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
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| 320 | </itemizedlist>
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| 321 | </para>
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| 322 | <para>
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| 323 | The function returns 1 when the fixup was successful,
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| 324 | otherwise 0. The return value is used to update the
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| 325 | statistics.
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| 326 | </para>
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| 327 | <para>
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| 328 | Note that the function needs to call the debug_object_activate()
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| 329 | function again after the damage has been repaired in order to
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| 330 | keep the state consistent.
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| 331 | </para>
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| 332 | <para>
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| 333 | The activation of statically initialized objects is a special
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| 334 | case. When debug_object_activate() has no tracked object for
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| 335 | this object address then fixup_activate() is called with
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| 336 | object state ODEBUG_STATE_NOTAVAILABLE. The fixup function
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| 337 | needs to check whether this is a legitimate case of a
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| 338 | statically initialized object or not. In case it is it calls
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| 339 | debug_object_init() and debug_object_activate() to make the
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| 340 | object known to the tracker and marked active. In this case
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| 341 | the function should return 0 because this is not a real fixup.
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| 342 | </para>
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| 343 | </sect1>
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| 344 |
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| 345 | <sect1 id="fixup_destroy">
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| 346 | <title>fixup_destroy</title>
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| 347 | <para>
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| 348 | This function is called from the debug code whenever a problem
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| 349 | in debug_object_destroy is detected.
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| 350 | </para>
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| 351 | <para>
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| 352 | Called from debug_object_destroy when the object state is:
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| 353 | <itemizedlist>
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| 354 | <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
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| 355 | </itemizedlist>
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| 356 | </para>
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| 357 | <para>
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| 358 | The function returns 1 when the fixup was successful,
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| 359 | otherwise 0. The return value is used to update the
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| 360 | statistics.
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| 361 | </para>
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| 362 | </sect1>
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| 363 | <sect1 id="fixup_free">
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| 364 | <title>fixup_free</title>
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| 365 | <para>
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| 366 | This function is called from the debug code whenever a problem
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| 367 | in debug_object_free is detected. Further it can be called
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| 368 | from the debug checks in kfree/vfree, when an active object is
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| 369 | detected from the debug_check_no_obj_freed() sanity checks.
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| 370 | </para>
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| 371 | <para>
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| 372 | Called from debug_object_free() or debug_check_no_obj_freed()
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| 373 | when the object state is:
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| 374 | <itemizedlist>
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| 375 | <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
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| 376 | </itemizedlist>
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| 377 | </para>
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| 378 | <para>
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| 379 | The function returns 1 when the fixup was successful,
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| 380 | otherwise 0. The return value is used to update the
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| 381 | statistics.
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| 382 | </para>
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| 383 | </sect1>
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| 384 | </chapter>
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| 385 | <chapter id="bugs">
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| 386 | <title>Known Bugs And Assumptions</title>
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| 387 | <para>
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| 388 | None (knock on wood).
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| 389 | </para>
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| 390 | </chapter>
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| 391 | </book>
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