| 1 | <?xml version="1.0" encoding="UTF-8"?>
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| 2 | <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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| 3 | "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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| 4 |
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| 5 | <book id="lk-hacking-guide">
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| 6 | <bookinfo>
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| 7 | <title>Unreliable Guide To Hacking The Linux Kernel</title>
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| 8 |
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| 9 | <authorgroup>
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| 10 | <author>
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| 11 | <firstname>Rusty</firstname>
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| 12 | <surname>Russell</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>2005</year>
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| 23 | <holder>Rusty Russell</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 as published by the Free Software Foundation; either
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| 31 | version 2 of the License, or (at your option) any later
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| 32 | version.
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| 33 | </para>
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| 34 |
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| 35 | <para>
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| 36 | This program is distributed in the hope that it will be
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| 37 | useful, but WITHOUT ANY WARRANTY; without even the implied
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| 38 | warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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| 39 | See the GNU General Public License for more details.
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| 40 | </para>
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| 41 |
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| 42 | <para>
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| 43 | You should have received a copy of the GNU General Public
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| 44 | License along with this program; if not, write to the Free
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| 45 | Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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| 46 | MA 02111-1307 USA
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| 47 | </para>
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| 48 |
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| 49 | <para>
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| 50 | For more details see the file COPYING in the source
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| 51 | distribution of Linux.
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| 52 | </para>
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| 53 | </legalnotice>
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| 54 |
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| 55 | <releaseinfo>
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| 56 | This is the first release of this document as part of the kernel tarball.
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| 57 | </releaseinfo>
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| 58 |
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| 59 | </bookinfo>
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| 60 |
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| 61 | <toc></toc>
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| 62 |
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| 63 | <chapter id="introduction">
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| 64 | <title>Introduction</title>
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| 65 | <para>
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| 66 | Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux
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| 67 | Kernel Hacking. This document describes the common routines and
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| 68 | general requirements for kernel code: its goal is to serve as a
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| 69 | primer for Linux kernel development for experienced C
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| 70 | programmers. I avoid implementation details: that's what the
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| 71 | code is for, and I ignore whole tracts of useful routines.
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| 72 | </para>
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| 73 | <para>
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| 74 | Before you read this, please understand that I never wanted to
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| 75 | write this document, being grossly under-qualified, but I always
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| 76 | wanted to read it, and this was the only way. I hope it will
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| 77 | grow into a compendium of best practice, common starting points
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| 78 | and random information.
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| 79 | </para>
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| 80 | </chapter>
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| 81 |
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| 82 | <chapter id="basic-players">
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| 83 | <title>The Players</title>
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| 84 |
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| 85 | <para>
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| 86 | At any time each of the CPUs in a system can be:
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| 87 | </para>
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| 88 |
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| 89 | <itemizedlist>
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| 90 | <listitem>
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| 91 | <para>
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| 92 | not associated with any process, serving a hardware interrupt;
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| 93 | </para>
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| 94 | </listitem>
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| 95 |
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| 96 | <listitem>
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| 97 | <para>
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| 98 | not associated with any process, serving a softirq or tasklet;
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| 99 | </para>
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| 100 | </listitem>
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| 101 |
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| 102 | <listitem>
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| 103 | <para>
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| 104 | running in kernel space, associated with a process (user context);
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| 105 | </para>
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| 106 | </listitem>
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| 107 |
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| 108 | <listitem>
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| 109 | <para>
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| 110 | running a process in user space.
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| 111 | </para>
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| 112 | </listitem>
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| 113 | </itemizedlist>
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| 114 |
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| 115 | <para>
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| 116 | There is an ordering between these. The bottom two can preempt
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| 117 | each other, but above that is a strict hierarchy: each can only be
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| 118 | preempted by the ones above it. For example, while a softirq is
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| 119 | running on a CPU, no other softirq will preempt it, but a hardware
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| 120 | interrupt can. However, any other CPUs in the system execute
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| 121 | independently.
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| 122 | </para>
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| 123 |
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| 124 | <para>
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| 125 | We'll see a number of ways that the user context can block
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| 126 | interrupts, to become truly non-preemptable.
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| 127 | </para>
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| 128 |
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| 129 | <sect1 id="basics-usercontext">
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| 130 | <title>User Context</title>
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| 131 |
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| 132 | <para>
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| 133 | User context is when you are coming in from a system call or other
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| 134 | trap: like userspace, you can be preempted by more important tasks
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| 135 | and by interrupts. You can sleep, by calling
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| 136 | <function>schedule()</function>.
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| 137 | </para>
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| 138 |
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| 139 | <note>
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| 140 | <para>
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| 141 | You are always in user context on module load and unload,
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| 142 | and on operations on the block device layer.
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| 143 | </para>
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| 144 | </note>
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| 145 |
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| 146 | <para>
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| 147 | In user context, the <varname>current</varname> pointer (indicating
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| 148 | the task we are currently executing) is valid, and
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| 149 | <function>in_interrupt()</function>
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| 150 | (<filename>include/linux/interrupt.h</filename>) is <returnvalue>false
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| 151 | </returnvalue>.
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| 152 | </para>
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| 153 |
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| 154 | <caution>
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| 155 | <para>
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| 156 | Beware that if you have preemption or softirqs disabled
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| 157 | (see below), <function>in_interrupt()</function> will return a
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| 158 | false positive.
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| 159 | </para>
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| 160 | </caution>
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| 161 | </sect1>
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| 162 |
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| 163 | <sect1 id="basics-hardirqs">
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| 164 | <title>Hardware Interrupts (Hard IRQs)</title>
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| 165 |
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| 166 | <para>
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| 167 | Timer ticks, <hardware>network cards</hardware> and
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| 168 | <hardware>keyboard</hardware> are examples of real
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| 169 | hardware which produce interrupts at any time. The kernel runs
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| 170 | interrupt handlers, which services the hardware. The kernel
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| 171 | guarantees that this handler is never re-entered: if the same
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| 172 | interrupt arrives, it is queued (or dropped). Because it
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| 173 | disables interrupts, this handler has to be fast: frequently it
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| 174 | simply acknowledges the interrupt, marks a 'software interrupt'
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| 175 | for execution and exits.
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| 176 | </para>
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| 177 |
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| 178 | <para>
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| 179 | You can tell you are in a hardware interrupt, because
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| 180 | <function>in_irq()</function> returns <returnvalue>true</returnvalue>.
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| 181 | </para>
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| 182 | <caution>
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| 183 | <para>
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| 184 | Beware that this will return a false positive if interrupts are disabled
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| 185 | (see below).
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| 186 | </para>
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| 187 | </caution>
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| 188 | </sect1>
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| 189 |
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| 190 | <sect1 id="basics-softirqs">
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| 191 | <title>Software Interrupt Context: Softirqs and Tasklets</title>
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| 192 |
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| 193 | <para>
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| 194 | Whenever a system call is about to return to userspace, or a
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| 195 | hardware interrupt handler exits, any 'software interrupts'
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| 196 | which are marked pending (usually by hardware interrupts) are
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| 197 | run (<filename>kernel/softirq.c</filename>).
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| 198 | </para>
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| 199 |
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| 200 | <para>
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| 201 | Much of the real interrupt handling work is done here. Early in
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| 202 | the transition to <acronym>SMP</acronym>, there were only 'bottom
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| 203 | halves' (BHs), which didn't take advantage of multiple CPUs. Shortly
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| 204 | after we switched from wind-up computers made of match-sticks and snot,
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| 205 | we abandoned this limitation and switched to 'softirqs'.
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| 206 | </para>
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| 207 |
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| 208 | <para>
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| 209 | <filename class="headerfile">include/linux/interrupt.h</filename> lists the
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| 210 | different softirqs. A very important softirq is the
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| 211 | timer softirq (<filename
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| 212 | class="headerfile">include/linux/timer.h</filename>): you can
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| 213 | register to have it call functions for you in a given length of
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| 214 | time.
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| 215 | </para>
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| 216 |
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| 217 | <para>
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| 218 | Softirqs are often a pain to deal with, since the same softirq
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| 219 | will run simultaneously on more than one CPU. For this reason,
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| 220 | tasklets (<filename
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| 221 | class="headerfile">include/linux/interrupt.h</filename>) are more
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| 222 | often used: they are dynamically-registrable (meaning you can have
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| 223 | as many as you want), and they also guarantee that any tasklet
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| 224 | will only run on one CPU at any time, although different tasklets
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| 225 | can run simultaneously.
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| 226 | </para>
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| 227 | <caution>
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| 228 | <para>
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| 229 | The name 'tasklet' is misleading: they have nothing to do with 'tasks',
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| 230 | and probably more to do with some bad vodka Alexey Kuznetsov had at the
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| 231 | time.
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| 232 | </para>
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| 233 | </caution>
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| 234 |
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| 235 | <para>
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| 236 | You can tell you are in a softirq (or tasklet)
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| 237 | using the <function>in_softirq()</function> macro
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| 238 | (<filename class="headerfile">include/linux/interrupt.h</filename>).
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| 239 | </para>
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| 240 | <caution>
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| 241 | <para>
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| 242 | Beware that this will return a false positive if a bh lock (see below)
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| 243 | is held.
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| 244 | </para>
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| 245 | </caution>
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| 246 | </sect1>
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| 247 | </chapter>
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| 248 |
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| 249 | <chapter id="basic-rules">
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| 250 | <title>Some Basic Rules</title>
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| 251 |
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| 252 | <variablelist>
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| 253 | <varlistentry>
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| 254 | <term>No memory protection</term>
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| 255 | <listitem>
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| 256 | <para>
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| 257 | If you corrupt memory, whether in user context or
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| 258 | interrupt context, the whole machine will crash. Are you
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| 259 | sure you can't do what you want in userspace?
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| 260 | </para>
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| 261 | </listitem>
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| 262 | </varlistentry>
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| 263 |
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| 264 | <varlistentry>
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| 265 | <term>No floating point or <acronym>MMX</acronym></term>
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| 266 | <listitem>
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| 267 | <para>
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| 268 | The <acronym>FPU</acronym> context is not saved; even in user
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| 269 | context the <acronym>FPU</acronym> state probably won't
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| 270 | correspond with the current process: you would mess with some
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| 271 | user process' <acronym>FPU</acronym> state. If you really want
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| 272 | to do this, you would have to explicitly save/restore the full
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| 273 | <acronym>FPU</acronym> state (and avoid context switches). It
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| 274 | is generally a bad idea; use fixed point arithmetic first.
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| 275 | </para>
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| 276 | </listitem>
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| 277 | </varlistentry>
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| 278 |
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| 279 | <varlistentry>
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| 280 | <term>A rigid stack limit</term>
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| 281 | <listitem>
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| 282 | <para>
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| 283 | Depending on configuration options the kernel stack is about 3K to 6K for most 32-bit architectures: it's
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| 284 | about 14K on most 64-bit archs, and often shared with interrupts
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| 285 | so you can't use it all. Avoid deep recursion and huge local
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| 286 | arrays on the stack (allocate them dynamically instead).
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| 287 | </para>
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| 288 | </listitem>
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| 289 | </varlistentry>
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| 290 |
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| 291 | <varlistentry>
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| 292 | <term>The Linux kernel is portable</term>
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| 293 | <listitem>
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| 294 | <para>
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| 295 | Let's keep it that way. Your code should be 64-bit clean,
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| 296 | and endian-independent. You should also minimize CPU
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| 297 | specific stuff, e.g. inline assembly should be cleanly
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| 298 | encapsulated and minimized to ease porting. Generally it
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| 299 | should be restricted to the architecture-dependent part of
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| 300 | the kernel tree.
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| 301 | </para>
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| 302 | </listitem>
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| 303 | </varlistentry>
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| 304 | </variablelist>
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| 305 | </chapter>
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| 306 |
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| 307 | <chapter id="ioctls">
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| 308 | <title>ioctls: Not writing a new system call</title>
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| 309 |
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| 310 | <para>
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| 311 | A system call generally looks like this
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| 312 | </para>
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| 313 |
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| 314 | <programlisting>
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| 315 | asmlinkage long sys_mycall(int arg)
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| 316 | {
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| 317 | return 0;
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| 318 | }
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| 319 | </programlisting>
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| 320 |
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| 321 | <para>
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| 322 | First, in most cases you don't want to create a new system call.
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| 323 | You create a character device and implement an appropriate ioctl
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| 324 | for it. This is much more flexible than system calls, doesn't have
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| 325 | to be entered in every architecture's
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| 326 | <filename class="headerfile">include/asm/unistd.h</filename> and
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| 327 | <filename>arch/kernel/entry.S</filename> file, and is much more
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| 328 | likely to be accepted by Linus.
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| 329 | </para>
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| 330 |
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| 331 | <para>
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| 332 | If all your routine does is read or write some parameter, consider
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| 333 | implementing a <function>sysfs</function> interface instead.
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| 334 | </para>
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| 335 |
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| 336 | <para>
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| 337 | Inside the ioctl you're in user context to a process. When a
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| 338 | error occurs you return a negated errno (see
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| 339 | <filename class="headerfile">include/linux/errno.h</filename>),
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| 340 | otherwise you return <returnvalue>0</returnvalue>.
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| 341 | </para>
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| 342 |
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| 343 | <para>
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| 344 | After you slept you should check if a signal occurred: the
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| 345 | Unix/Linux way of handling signals is to temporarily exit the
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| 346 | system call with the <constant>-ERESTARTSYS</constant> error. The
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| 347 | system call entry code will switch back to user context, process
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| 348 | the signal handler and then your system call will be restarted
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| 349 | (unless the user disabled that). So you should be prepared to
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| 350 | process the restart, e.g. if you're in the middle of manipulating
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| 351 | some data structure.
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| 352 | </para>
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| 353 |
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| 354 | <programlisting>
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| 355 | if (signal_pending(current))
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| 356 | return -ERESTARTSYS;
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| 357 | </programlisting>
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| 358 |
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| 359 | <para>
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| 360 | If you're doing longer computations: first think userspace. If you
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| 361 | <emphasis>really</emphasis> want to do it in kernel you should
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| 362 | regularly check if you need to give up the CPU (remember there is
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| 363 | cooperative multitasking per CPU). Idiom:
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| 364 | </para>
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| 365 |
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| 366 | <programlisting>
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| 367 | cond_resched(); /* Will sleep */
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| 368 | </programlisting>
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| 369 |
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| 370 | <para>
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| 371 | A short note on interface design: the UNIX system call motto is
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| 372 | "Provide mechanism not policy".
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| 373 | </para>
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| 374 | </chapter>
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| 375 |
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| 376 | <chapter id="deadlock-recipes">
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| 377 | <title>Recipes for Deadlock</title>
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| 378 |
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| 379 | <para>
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| 380 | You cannot call any routines which may sleep, unless:
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| 381 | </para>
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| 382 | <itemizedlist>
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| 383 | <listitem>
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| 384 | <para>
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| 385 | You are in user context.
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| 386 | </para>
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| 387 | </listitem>
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| 388 |
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| 389 | <listitem>
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| 390 | <para>
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| 391 | You do not own any spinlocks.
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| 392 | </para>
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| 393 | </listitem>
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| 394 |
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| 395 | <listitem>
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| 396 | <para>
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| 397 | You have interrupts enabled (actually, Andi Kleen says
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| 398 | that the scheduling code will enable them for you, but
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| 399 | that's probably not what you wanted).
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| 400 | </para>
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| 401 | </listitem>
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| 402 | </itemizedlist>
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| 403 |
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| 404 | <para>
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| 405 | Note that some functions may sleep implicitly: common ones are
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| 406 | the user space access functions (*_user) and memory allocation
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| 407 | functions without <symbol>GFP_ATOMIC</symbol>.
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| 408 | </para>
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| 409 |
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| 410 | <para>
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| 411 | You should always compile your kernel
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| 412 | <symbol>CONFIG_DEBUG_ATOMIC_SLEEP</symbol> on, and it will warn
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| 413 | you if you break these rules. If you <emphasis>do</emphasis> break
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| 414 | the rules, you will eventually lock up your box.
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| 415 | </para>
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| 416 |
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| 417 | <para>
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| 418 | Really.
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| 419 | </para>
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| 420 | </chapter>
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| 421 |
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| 422 | <chapter id="common-routines">
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| 423 | <title>Common Routines</title>
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| 424 |
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| 425 | <sect1 id="routines-printk">
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| 426 | <title>
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| 427 | <function>printk()</function>
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| 428 | <filename class="headerfile">include/linux/kernel.h</filename>
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| 429 | </title>
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| 430 |
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| 431 | <para>
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| 432 | <function>printk()</function> feeds kernel messages to the
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| 433 | console, dmesg, and the syslog daemon. It is useful for debugging
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| 434 | and reporting errors, and can be used inside interrupt context,
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| 435 | but use with caution: a machine which has its console flooded with
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| 436 | printk messages is unusable. It uses a format string mostly
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| 437 | compatible with ANSI C printf, and C string concatenation to give
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| 438 | it a first "priority" argument:
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| 439 | </para>
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| 440 |
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| 441 | <programlisting>
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| 442 | printk(KERN_INFO "i = %u\n", i);
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| 443 | </programlisting>
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| 444 |
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| 445 | <para>
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| 446 | See <filename class="headerfile">include/linux/kernel.h</filename>;
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| 447 | for other KERN_ values; these are interpreted by syslog as the
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| 448 | level. Special case: for printing an IP address use
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| 449 | </para>
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| 450 |
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| 451 | <programlisting>
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| 452 | __be32 ipaddress;
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| 453 | printk(KERN_INFO "my ip: %pI4\n", &ipaddress);
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| 454 | </programlisting>
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| 455 |
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| 456 | <para>
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| 457 | <function>printk()</function> internally uses a 1K buffer and does
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| 458 | not catch overruns. Make sure that will be enough.
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| 459 | </para>
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| 460 |
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| 461 | <note>
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| 462 | <para>
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| 463 | You will know when you are a real kernel hacker
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| 464 | when you start typoing printf as printk in your user programs :)
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| 465 | </para>
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| 466 | </note>
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| 467 |
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| 468 |
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| 469 |
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| 470 | <note>
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| 471 | <para>
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| 472 | Another sidenote: the original Unix Version 6 sources had a
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| 473 | comment on top of its printf function: "Printf should not be
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| 474 | used for chit-chat". You should follow that advice.
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| 475 | </para>
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| 476 | </note>
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| 477 | </sect1>
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| 478 |
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| 479 | <sect1 id="routines-copy">
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| 480 | <title>
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| 481 | <function>copy_[to/from]_user()</function>
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| 482 | /
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| 483 | <function>get_user()</function>
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| 484 | /
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| 485 | <function>put_user()</function>
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| 486 | <filename class="headerfile">include/asm/uaccess.h</filename>
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| 487 | </title>
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| 488 |
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| 489 | <para>
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| 490 | <emphasis>[SLEEPS]</emphasis>
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| 491 | </para>
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| 492 |
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| 493 | <para>
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| 494 | <function>put_user()</function> and <function>get_user()</function>
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| 495 | are used to get and put single values (such as an int, char, or
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| 496 | long) from and to userspace. A pointer into userspace should
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| 497 | never be simply dereferenced: data should be copied using these
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| 498 | routines. Both return <constant>-EFAULT</constant> or 0.
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| 499 | </para>
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| 500 | <para>
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| 501 | <function>copy_to_user()</function> and
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| 502 | <function>copy_from_user()</function> are more general: they copy
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| 503 | an arbitrary amount of data to and from userspace.
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| 504 | <caution>
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| 505 | <para>
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| 506 | Unlike <function>put_user()</function> and
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| 507 | <function>get_user()</function>, they return the amount of
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| 508 | uncopied data (ie. <returnvalue>0</returnvalue> still means
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| 509 | success).
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| 510 | </para>
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| 511 | </caution>
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| 512 | [Yes, this moronic interface makes me cringe. The flamewar comes up every year or so. --RR.]
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| 513 | </para>
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| 514 | <para>
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| 515 | The functions may sleep implicitly. This should never be called
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| 516 | outside user context (it makes no sense), with interrupts
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| 517 | disabled, or a spinlock held.
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| 518 | </para>
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| 519 | </sect1>
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| 520 |
|
| 521 | <sect1 id="routines-kmalloc">
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| 522 | <title><function>kmalloc()</function>/<function>kfree()</function>
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| 523 | <filename class="headerfile">include/linux/slab.h</filename></title>
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| 524 |
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| 525 | <para>
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| 526 | <emphasis>[MAY SLEEP: SEE BELOW]</emphasis>
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| 527 | </para>
|
| 528 |
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| 529 | <para>
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| 530 | These routines are used to dynamically request pointer-aligned
|
| 531 | chunks of memory, like malloc and free do in userspace, but
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| 532 | <function>kmalloc()</function> takes an extra flag word.
|
| 533 | Important values:
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| 534 | </para>
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| 535 |
|
| 536 | <variablelist>
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| 537 | <varlistentry>
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| 538 | <term>
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| 539 | <constant>
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| 540 | GFP_KERNEL
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| 541 | </constant>
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| 542 | </term>
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| 543 | <listitem>
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| 544 | <para>
|
| 545 | May sleep and swap to free memory. Only allowed in user
|
| 546 | context, but is the most reliable way to allocate memory.
|
| 547 | </para>
|
| 548 | </listitem>
|
| 549 | </varlistentry>
|
| 550 |
|
| 551 | <varlistentry>
|
| 552 | <term>
|
| 553 | <constant>
|
| 554 | GFP_ATOMIC
|
| 555 | </constant>
|
| 556 | </term>
|
| 557 | <listitem>
|
| 558 | <para>
|
| 559 | Don't sleep. Less reliable than <constant>GFP_KERNEL</constant>,
|
| 560 | but may be called from interrupt context. You should
|
| 561 | <emphasis>really</emphasis> have a good out-of-memory
|
| 562 | error-handling strategy.
|
| 563 | </para>
|
| 564 | </listitem>
|
| 565 | </varlistentry>
|
| 566 |
|
| 567 | <varlistentry>
|
| 568 | <term>
|
| 569 | <constant>
|
| 570 | GFP_DMA
|
| 571 | </constant>
|
| 572 | </term>
|
| 573 | <listitem>
|
| 574 | <para>
|
| 575 | Allocate ISA DMA lower than 16MB. If you don't know what that
|
| 576 | is you don't need it. Very unreliable.
|
| 577 | </para>
|
| 578 | </listitem>
|
| 579 | </varlistentry>
|
| 580 | </variablelist>
|
| 581 |
|
| 582 | <para>
|
| 583 | If you see a <errorname>sleeping function called from invalid
|
| 584 | context</errorname> warning message, then maybe you called a
|
| 585 | sleeping allocation function from interrupt context without
|
| 586 | <constant>GFP_ATOMIC</constant>. You should really fix that.
|
| 587 | Run, don't walk.
|
| 588 | </para>
|
| 589 |
|
| 590 | <para>
|
| 591 | If you are allocating at least <constant>PAGE_SIZE</constant>
|
| 592 | (<filename class="headerfile">include/asm/page.h</filename>) bytes,
|
| 593 | consider using <function>__get_free_pages()</function>
|
| 594 |
|
| 595 | (<filename class="headerfile">include/linux/mm.h</filename>). It
|
| 596 | takes an order argument (0 for page sized, 1 for double page, 2
|
| 597 | for four pages etc.) and the same memory priority flag word as
|
| 598 | above.
|
| 599 | </para>
|
| 600 |
|
| 601 | <para>
|
| 602 | If you are allocating more than a page worth of bytes you can use
|
| 603 | <function>vmalloc()</function>. It'll allocate virtual memory in
|
| 604 | the kernel map. This block is not contiguous in physical memory,
|
| 605 | but the <acronym>MMU</acronym> makes it look like it is for you
|
| 606 | (so it'll only look contiguous to the CPUs, not to external device
|
| 607 | drivers). If you really need large physically contiguous memory
|
| 608 | for some weird device, you have a problem: it is poorly supported
|
| 609 | in Linux because after some time memory fragmentation in a running
|
| 610 | kernel makes it hard. The best way is to allocate the block early
|
| 611 | in the boot process via the <function>alloc_bootmem()</function>
|
| 612 | routine.
|
| 613 | </para>
|
| 614 |
|
| 615 | <para>
|
| 616 | Before inventing your own cache of often-used objects consider
|
| 617 | using a slab cache in
|
| 618 | <filename class="headerfile">include/linux/slab.h</filename>
|
| 619 | </para>
|
| 620 | </sect1>
|
| 621 |
|
| 622 | <sect1 id="routines-current">
|
| 623 | <title><function>current</function>
|
| 624 | <filename class="headerfile">include/asm/current.h</filename></title>
|
| 625 |
|
| 626 | <para>
|
| 627 | This global variable (really a macro) contains a pointer to
|
| 628 | the current task structure, so is only valid in user context.
|
| 629 | For example, when a process makes a system call, this will
|
| 630 | point to the task structure of the calling process. It is
|
| 631 | <emphasis>not NULL</emphasis> in interrupt context.
|
| 632 | </para>
|
| 633 | </sect1>
|
| 634 |
|
| 635 | <sect1 id="routines-udelay">
|
| 636 | <title><function>mdelay()</function>/<function>udelay()</function>
|
| 637 | <filename class="headerfile">include/asm/delay.h</filename>
|
| 638 | <filename class="headerfile">include/linux/delay.h</filename>
|
| 639 | </title>
|
| 640 |
|
| 641 | <para>
|
| 642 | The <function>udelay()</function> and <function>ndelay()</function> functions can be used for small pauses.
|
| 643 | Do not use large values with them as you risk
|
| 644 | overflow - the helper function <function>mdelay()</function> is useful
|
| 645 | here, or consider <function>msleep()</function>.
|
| 646 | </para>
|
| 647 | </sect1>
|
| 648 |
|
| 649 | <sect1 id="routines-endian">
|
| 650 | <title><function>cpu_to_be32()</function>/<function>be32_to_cpu()</function>/<function>cpu_to_le32()</function>/<function>le32_to_cpu()</function>
|
| 651 | <filename class="headerfile">include/asm/byteorder.h</filename>
|
| 652 | </title>
|
| 653 |
|
| 654 | <para>
|
| 655 | The <function>cpu_to_be32()</function> family (where the "32" can
|
| 656 | be replaced by 64 or 16, and the "be" can be replaced by "le") are
|
| 657 | the general way to do endian conversions in the kernel: they
|
| 658 | return the converted value. All variations supply the reverse as
|
| 659 | well: <function>be32_to_cpu()</function>, etc.
|
| 660 | </para>
|
| 661 |
|
| 662 | <para>
|
| 663 | There are two major variations of these functions: the pointer
|
| 664 | variation, such as <function>cpu_to_be32p()</function>, which take
|
| 665 | a pointer to the given type, and return the converted value. The
|
| 666 | other variation is the "in-situ" family, such as
|
| 667 | <function>cpu_to_be32s()</function>, which convert value referred
|
| 668 | to by the pointer, and return void.
|
| 669 | </para>
|
| 670 | </sect1>
|
| 671 |
|
| 672 | <sect1 id="routines-local-irqs">
|
| 673 | <title><function>local_irq_save()</function>/<function>local_irq_restore()</function>
|
| 674 | <filename class="headerfile">include/linux/irqflags.h</filename>
|
| 675 | </title>
|
| 676 |
|
| 677 | <para>
|
| 678 | These routines disable hard interrupts on the local CPU, and
|
| 679 | restore them. They are reentrant; saving the previous state in
|
| 680 | their one <varname>unsigned long flags</varname> argument. If you
|
| 681 | know that interrupts are enabled, you can simply use
|
| 682 | <function>local_irq_disable()</function> and
|
| 683 | <function>local_irq_enable()</function>.
|
| 684 | </para>
|
| 685 | </sect1>
|
| 686 |
|
| 687 | <sect1 id="routines-softirqs">
|
| 688 | <title><function>local_bh_disable()</function>/<function>local_bh_enable()</function>
|
| 689 | <filename class="headerfile">include/linux/interrupt.h</filename></title>
|
| 690 |
|
| 691 | <para>
|
| 692 | These routines disable soft interrupts on the local CPU, and
|
| 693 | restore them. They are reentrant; if soft interrupts were
|
| 694 | disabled before, they will still be disabled after this pair
|
| 695 | of functions has been called. They prevent softirqs and tasklets
|
| 696 | from running on the current CPU.
|
| 697 | </para>
|
| 698 | </sect1>
|
| 699 |
|
| 700 | <sect1 id="routines-processorids">
|
| 701 | <title><function>smp_processor_id</function>()
|
| 702 | <filename class="headerfile">include/asm/smp.h</filename></title>
|
| 703 |
|
| 704 | <para>
|
| 705 | <function>get_cpu()</function> disables preemption (so you won't
|
| 706 | suddenly get moved to another CPU) and returns the current
|
| 707 | processor number, between 0 and <symbol>NR_CPUS</symbol>. Note
|
| 708 | that the CPU numbers are not necessarily continuous. You return
|
| 709 | it again with <function>put_cpu()</function> when you are done.
|
| 710 | </para>
|
| 711 | <para>
|
| 712 | If you know you cannot be preempted by another task (ie. you are
|
| 713 | in interrupt context, or have preemption disabled) you can use
|
| 714 | smp_processor_id().
|
| 715 | </para>
|
| 716 | </sect1>
|
| 717 |
|
| 718 | <sect1 id="routines-init">
|
| 719 | <title><type>__init</type>/<type>__exit</type>/<type>__initdata</type>
|
| 720 | <filename class="headerfile">include/linux/init.h</filename></title>
|
| 721 |
|
| 722 | <para>
|
| 723 | After boot, the kernel frees up a special section; functions
|
| 724 | marked with <type>__init</type> and data structures marked with
|
| 725 | <type>__initdata</type> are dropped after boot is complete: similarly
|
| 726 | modules discard this memory after initialization. <type>__exit</type>
|
| 727 | is used to declare a function which is only required on exit: the
|
| 728 | function will be dropped if this file is not compiled as a module.
|
| 729 | See the header file for use. Note that it makes no sense for a function
|
| 730 | marked with <type>__init</type> to be exported to modules with
|
| 731 | <function>EXPORT_SYMBOL()</function> - this will break.
|
| 732 | </para>
|
| 733 |
|
| 734 | </sect1>
|
| 735 |
|
| 736 | <sect1 id="routines-init-again">
|
| 737 | <title><function>__initcall()</function>/<function>module_init()</function>
|
| 738 | <filename class="headerfile">include/linux/init.h</filename></title>
|
| 739 | <para>
|
| 740 | Many parts of the kernel are well served as a module
|
| 741 | (dynamically-loadable parts of the kernel). Using the
|
| 742 | <function>module_init()</function> and
|
| 743 | <function>module_exit()</function> macros it is easy to write code
|
| 744 | without #ifdefs which can operate both as a module or built into
|
| 745 | the kernel.
|
| 746 | </para>
|
| 747 |
|
| 748 | <para>
|
| 749 | The <function>module_init()</function> macro defines which
|
| 750 | function is to be called at module insertion time (if the file is
|
| 751 | compiled as a module), or at boot time: if the file is not
|
| 752 | compiled as a module the <function>module_init()</function> macro
|
| 753 | becomes equivalent to <function>__initcall()</function>, which
|
| 754 | through linker magic ensures that the function is called on boot.
|
| 755 | </para>
|
| 756 |
|
| 757 | <para>
|
| 758 | The function can return a negative error number to cause
|
| 759 | module loading to fail (unfortunately, this has no effect if
|
| 760 | the module is compiled into the kernel). This function is
|
| 761 | called in user context with interrupts enabled, so it can sleep.
|
| 762 | </para>
|
| 763 | </sect1>
|
| 764 |
|
| 765 | <sect1 id="routines-moduleexit">
|
| 766 | <title> <function>module_exit()</function>
|
| 767 | <filename class="headerfile">include/linux/init.h</filename> </title>
|
| 768 |
|
| 769 | <para>
|
| 770 | This macro defines the function to be called at module removal
|
| 771 | time (or never, in the case of the file compiled into the
|
| 772 | kernel). It will only be called if the module usage count has
|
| 773 | reached zero. This function can also sleep, but cannot fail:
|
| 774 | everything must be cleaned up by the time it returns.
|
| 775 | </para>
|
| 776 |
|
| 777 | <para>
|
| 778 | Note that this macro is optional: if it is not present, your
|
| 779 | module will not be removable (except for 'rmmod -f').
|
| 780 | </para>
|
| 781 | </sect1>
|
| 782 |
|
| 783 | <sect1 id="routines-module-use-counters">
|
| 784 | <title> <function>try_module_get()</function>/<function>module_put()</function>
|
| 785 | <filename class="headerfile">include/linux/module.h</filename></title>
|
| 786 |
|
| 787 | <para>
|
| 788 | These manipulate the module usage count, to protect against
|
| 789 | removal (a module also can't be removed if another module uses one
|
| 790 | of its exported symbols: see below). Before calling into module
|
| 791 | code, you should call <function>try_module_get()</function> on
|
| 792 | that module: if it fails, then the module is being removed and you
|
| 793 | should act as if it wasn't there. Otherwise, you can safely enter
|
| 794 | the module, and call <function>module_put()</function> when you're
|
| 795 | finished.
|
| 796 | </para>
|
| 797 |
|
| 798 | <para>
|
| 799 | Most registerable structures have an
|
| 800 | <structfield>owner</structfield> field, such as in the
|
| 801 | <structname>file_operations</structname> structure. Set this field
|
| 802 | to the macro <symbol>THIS_MODULE</symbol>.
|
| 803 | </para>
|
| 804 | </sect1>
|
| 805 |
|
| 806 |
|
| 807 | </chapter>
|
| 808 |
|
| 809 | <chapter id="queues">
|
| 810 | <title>Wait Queues
|
| 811 | <filename class="headerfile">include/linux/wait.h</filename>
|
| 812 | </title>
|
| 813 | <para>
|
| 814 | <emphasis>[SLEEPS]</emphasis>
|
| 815 | </para>
|
| 816 |
|
| 817 | <para>
|
| 818 | A wait queue is used to wait for someone to wake you up when a
|
| 819 | certain condition is true. They must be used carefully to ensure
|
| 820 | there is no race condition. You declare a
|
| 821 | <type>wait_queue_head_t</type>, and then processes which want to
|
| 822 | wait for that condition declare a <type>wait_queue_t</type>
|
| 823 | referring to themselves, and place that in the queue.
|
| 824 | </para>
|
| 825 |
|
| 826 | <sect1 id="queue-declaring">
|
| 827 | <title>Declaring</title>
|
| 828 |
|
| 829 | <para>
|
| 830 | You declare a <type>wait_queue_head_t</type> using the
|
| 831 | <function>DECLARE_WAIT_QUEUE_HEAD()</function> macro, or using the
|
| 832 | <function>init_waitqueue_head()</function> routine in your
|
| 833 | initialization code.
|
| 834 | </para>
|
| 835 | </sect1>
|
| 836 |
|
| 837 | <sect1 id="queue-waitqueue">
|
| 838 | <title>Queuing</title>
|
| 839 |
|
| 840 | <para>
|
| 841 | Placing yourself in the waitqueue is fairly complex, because you
|
| 842 | must put yourself in the queue before checking the condition.
|
| 843 | There is a macro to do this:
|
| 844 | <function>wait_event_interruptible()</function>
|
| 845 |
|
| 846 | <filename class="headerfile">include/linux/wait.h</filename> The
|
| 847 | first argument is the wait queue head, and the second is an
|
| 848 | expression which is evaluated; the macro returns
|
| 849 | <returnvalue>0</returnvalue> when this expression is true, or
|
| 850 | <returnvalue>-ERESTARTSYS</returnvalue> if a signal is received.
|
| 851 | The <function>wait_event()</function> version ignores signals.
|
| 852 | </para>
|
| 853 |
|
| 854 | </sect1>
|
| 855 |
|
| 856 | <sect1 id="queue-waking">
|
| 857 | <title>Waking Up Queued Tasks</title>
|
| 858 |
|
| 859 | <para>
|
| 860 | Call <function>wake_up()</function>
|
| 861 |
|
| 862 | <filename class="headerfile">include/linux/wait.h</filename>;,
|
| 863 | which will wake up every process in the queue. The exception is
|
| 864 | if one has <constant>TASK_EXCLUSIVE</constant> set, in which case
|
| 865 | the remainder of the queue will not be woken. There are other variants
|
| 866 | of this basic function available in the same header.
|
| 867 | </para>
|
| 868 | </sect1>
|
| 869 | </chapter>
|
| 870 |
|
| 871 | <chapter id="atomic-ops">
|
| 872 | <title>Atomic Operations</title>
|
| 873 |
|
| 874 | <para>
|
| 875 | Certain operations are guaranteed atomic on all platforms. The
|
| 876 | first class of operations work on <type>atomic_t</type>
|
| 877 |
|
| 878 | <filename class="headerfile">include/asm/atomic.h</filename>; this
|
| 879 | contains a signed integer (at least 32 bits long), and you must use
|
| 880 | these functions to manipulate or read atomic_t variables.
|
| 881 | <function>atomic_read()</function> and
|
| 882 | <function>atomic_set()</function> get and set the counter,
|
| 883 | <function>atomic_add()</function>,
|
| 884 | <function>atomic_sub()</function>,
|
| 885 | <function>atomic_inc()</function>,
|
| 886 | <function>atomic_dec()</function>, and
|
| 887 | <function>atomic_dec_and_test()</function> (returns
|
| 888 | <returnvalue>true</returnvalue> if it was decremented to zero).
|
| 889 | </para>
|
| 890 |
|
| 891 | <para>
|
| 892 | Yes. It returns <returnvalue>true</returnvalue> (i.e. != 0) if the
|
| 893 | atomic variable is zero.
|
| 894 | </para>
|
| 895 |
|
| 896 | <para>
|
| 897 | Note that these functions are slower than normal arithmetic, and
|
| 898 | so should not be used unnecessarily.
|
| 899 | </para>
|
| 900 |
|
| 901 | <para>
|
| 902 | The second class of atomic operations is atomic bit operations on an
|
| 903 | <type>unsigned long</type>, defined in
|
| 904 |
|
| 905 | <filename class="headerfile">include/linux/bitops.h</filename>. These
|
| 906 | operations generally take a pointer to the bit pattern, and a bit
|
| 907 | number: 0 is the least significant bit.
|
| 908 | <function>set_bit()</function>, <function>clear_bit()</function>
|
| 909 | and <function>change_bit()</function> set, clear, and flip the
|
| 910 | given bit. <function>test_and_set_bit()</function>,
|
| 911 | <function>test_and_clear_bit()</function> and
|
| 912 | <function>test_and_change_bit()</function> do the same thing,
|
| 913 | except return true if the bit was previously set; these are
|
| 914 | particularly useful for atomically setting flags.
|
| 915 | </para>
|
| 916 |
|
| 917 | <para>
|
| 918 | It is possible to call these operations with bit indices greater
|
| 919 | than BITS_PER_LONG. The resulting behavior is strange on big-endian
|
| 920 | platforms though so it is a good idea not to do this.
|
| 921 | </para>
|
| 922 | </chapter>
|
| 923 |
|
| 924 | <chapter id="symbols">
|
| 925 | <title>Symbols</title>
|
| 926 |
|
| 927 | <para>
|
| 928 | Within the kernel proper, the normal linking rules apply
|
| 929 | (ie. unless a symbol is declared to be file scope with the
|
| 930 | <type>static</type> keyword, it can be used anywhere in the
|
| 931 | kernel). However, for modules, a special exported symbol table is
|
| 932 | kept which limits the entry points to the kernel proper. Modules
|
| 933 | can also export symbols.
|
| 934 | </para>
|
| 935 |
|
| 936 | <sect1 id="sym-exportsymbols">
|
| 937 | <title><function>EXPORT_SYMBOL()</function>
|
| 938 | <filename class="headerfile">include/linux/export.h</filename></title>
|
| 939 |
|
| 940 | <para>
|
| 941 | This is the classic method of exporting a symbol: dynamically
|
| 942 | loaded modules will be able to use the symbol as normal.
|
| 943 | </para>
|
| 944 | </sect1>
|
| 945 |
|
| 946 | <sect1 id="sym-exportsymbols-gpl">
|
| 947 | <title><function>EXPORT_SYMBOL_GPL()</function>
|
| 948 | <filename class="headerfile">include/linux/export.h</filename></title>
|
| 949 |
|
| 950 | <para>
|
| 951 | Similar to <function>EXPORT_SYMBOL()</function> except that the
|
| 952 | symbols exported by <function>EXPORT_SYMBOL_GPL()</function> can
|
| 953 | only be seen by modules with a
|
| 954 | <function>MODULE_LICENSE()</function> that specifies a GPL
|
| 955 | compatible license. It implies that the function is considered
|
| 956 | an internal implementation issue, and not really an interface.
|
| 957 | </para>
|
| 958 | </sect1>
|
| 959 | </chapter>
|
| 960 |
|
| 961 | <chapter id="conventions">
|
| 962 | <title>Routines and Conventions</title>
|
| 963 |
|
| 964 | <sect1 id="conventions-doublelinkedlist">
|
| 965 | <title>Double-linked lists
|
| 966 | <filename class="headerfile">include/linux/list.h</filename></title>
|
| 967 |
|
| 968 | <para>
|
| 969 | There used to be three sets of linked-list routines in the kernel
|
| 970 | headers, but this one is the winner. If you don't have some
|
| 971 | particular pressing need for a single list, it's a good choice.
|
| 972 | </para>
|
| 973 |
|
| 974 | <para>
|
| 975 | In particular, <function>list_for_each_entry</function> is useful.
|
| 976 | </para>
|
| 977 | </sect1>
|
| 978 |
|
| 979 | <sect1 id="convention-returns">
|
| 980 | <title>Return Conventions</title>
|
| 981 |
|
| 982 | <para>
|
| 983 | For code called in user context, it's very common to defy C
|
| 984 | convention, and return <returnvalue>0</returnvalue> for success,
|
| 985 | and a negative error number
|
| 986 | (eg. <returnvalue>-EFAULT</returnvalue>) for failure. This can be
|
| 987 | unintuitive at first, but it's fairly widespread in the kernel.
|
| 988 | </para>
|
| 989 |
|
| 990 | <para>
|
| 991 | Using <function>ERR_PTR()</function>
|
| 992 |
|
| 993 | <filename class="headerfile">include/linux/err.h</filename>; to
|
| 994 | encode a negative error number into a pointer, and
|
| 995 | <function>IS_ERR()</function> and <function>PTR_ERR()</function>
|
| 996 | to get it back out again: avoids a separate pointer parameter for
|
| 997 | the error number. Icky, but in a good way.
|
| 998 | </para>
|
| 999 | </sect1>
|
| 1000 |
|
| 1001 | <sect1 id="conventions-borkedcompile">
|
| 1002 | <title>Breaking Compilation</title>
|
| 1003 |
|
| 1004 | <para>
|
| 1005 | Linus and the other developers sometimes change function or
|
| 1006 | structure names in development kernels; this is not done just to
|
| 1007 | keep everyone on their toes: it reflects a fundamental change
|
| 1008 | (eg. can no longer be called with interrupts on, or does extra
|
| 1009 | checks, or doesn't do checks which were caught before). Usually
|
| 1010 | this is accompanied by a fairly complete note to the linux-kernel
|
| 1011 | mailing list; search the archive. Simply doing a global replace
|
| 1012 | on the file usually makes things <emphasis>worse</emphasis>.
|
| 1013 | </para>
|
| 1014 | </sect1>
|
| 1015 |
|
| 1016 | <sect1 id="conventions-initialising">
|
| 1017 | <title>Initializing structure members</title>
|
| 1018 |
|
| 1019 | <para>
|
| 1020 | The preferred method of initializing structures is to use
|
| 1021 | designated initialisers, as defined by ISO C99, eg:
|
| 1022 | </para>
|
| 1023 | <programlisting>
|
| 1024 | static struct block_device_operations opt_fops = {
|
| 1025 | .open = opt_open,
|
| 1026 | .release = opt_release,
|
| 1027 | .ioctl = opt_ioctl,
|
| 1028 | .check_media_change = opt_media_change,
|
| 1029 | };
|
| 1030 | </programlisting>
|
| 1031 | <para>
|
| 1032 | This makes it easy to grep for, and makes it clear which
|
| 1033 | structure fields are set. You should do this because it looks
|
| 1034 | cool.
|
| 1035 | </para>
|
| 1036 | </sect1>
|
| 1037 |
|
| 1038 | <sect1 id="conventions-gnu-extns">
|
| 1039 | <title>GNU Extensions</title>
|
| 1040 |
|
| 1041 | <para>
|
| 1042 | GNU Extensions are explicitly allowed in the Linux kernel.
|
| 1043 | Note that some of the more complex ones are not very well
|
| 1044 | supported, due to lack of general use, but the following are
|
| 1045 | considered standard (see the GCC info page section "C
|
| 1046 | Extensions" for more details - Yes, really the info page, the
|
| 1047 | man page is only a short summary of the stuff in info).
|
| 1048 | </para>
|
| 1049 | <itemizedlist>
|
| 1050 | <listitem>
|
| 1051 | <para>
|
| 1052 | Inline functions
|
| 1053 | </para>
|
| 1054 | </listitem>
|
| 1055 | <listitem>
|
| 1056 | <para>
|
| 1057 | Statement expressions (ie. the ({ and }) constructs).
|
| 1058 | </para>
|
| 1059 | </listitem>
|
| 1060 | <listitem>
|
| 1061 | <para>
|
| 1062 | Declaring attributes of a function / variable / type
|
| 1063 | (__attribute__)
|
| 1064 | </para>
|
| 1065 | </listitem>
|
| 1066 | <listitem>
|
| 1067 | <para>
|
| 1068 | typeof
|
| 1069 | </para>
|
| 1070 | </listitem>
|
| 1071 | <listitem>
|
| 1072 | <para>
|
| 1073 | Zero length arrays
|
| 1074 | </para>
|
| 1075 | </listitem>
|
| 1076 | <listitem>
|
| 1077 | <para>
|
| 1078 | Macro varargs
|
| 1079 | </para>
|
| 1080 | </listitem>
|
| 1081 | <listitem>
|
| 1082 | <para>
|
| 1083 | Arithmetic on void pointers
|
| 1084 | </para>
|
| 1085 | </listitem>
|
| 1086 | <listitem>
|
| 1087 | <para>
|
| 1088 | Non-Constant initializers
|
| 1089 | </para>
|
| 1090 | </listitem>
|
| 1091 | <listitem>
|
| 1092 | <para>
|
| 1093 | Assembler Instructions (not outside arch/ and include/asm/)
|
| 1094 | </para>
|
| 1095 | </listitem>
|
| 1096 | <listitem>
|
| 1097 | <para>
|
| 1098 | Function names as strings (__func__).
|
| 1099 | </para>
|
| 1100 | </listitem>
|
| 1101 | <listitem>
|
| 1102 | <para>
|
| 1103 | __builtin_constant_p()
|
| 1104 | </para>
|
| 1105 | </listitem>
|
| 1106 | </itemizedlist>
|
| 1107 |
|
| 1108 | <para>
|
| 1109 | Be wary when using long long in the kernel, the code gcc generates for
|
| 1110 | it is horrible and worse: division and multiplication does not work
|
| 1111 | on i386 because the GCC runtime functions for it are missing from
|
| 1112 | the kernel environment.
|
| 1113 | </para>
|
| 1114 |
|
| 1115 |
|
| 1116 | </sect1>
|
| 1117 |
|
| 1118 | <sect1 id="conventions-cplusplus">
|
| 1119 | <title>C++</title>
|
| 1120 |
|
| 1121 | <para>
|
| 1122 | Using C++ in the kernel is usually a bad idea, because the
|
| 1123 | kernel does not provide the necessary runtime environment
|
| 1124 | and the include files are not tested for it. It is still
|
| 1125 | possible, but not recommended. If you really want to do
|
| 1126 | this, forget about exceptions at least.
|
| 1127 | </para>
|
| 1128 | </sect1>
|
| 1129 |
|
| 1130 | <sect1 id="conventions-ifdef">
|
| 1131 | <title>#if</title>
|
| 1132 |
|
| 1133 | <para>
|
| 1134 | It is generally considered cleaner to use macros in header files
|
| 1135 | (or at the top of .c files) to abstract away functions rather than
|
| 1136 | using `#if' pre-processor statements throughout the source code.
|
| 1137 | </para>
|
| 1138 | </sect1>
|
| 1139 | </chapter>
|
| 1140 |
|
| 1141 | <chapter id="submitting">
|
| 1142 | <title>Putting Your Stuff in the Kernel</title>
|
| 1143 |
|
| 1144 | <para>
|
| 1145 | In order to get your stuff into shape for official inclusion, or
|
| 1146 | even to make a neat patch, there's administrative work to be
|
| 1147 | done:
|
| 1148 | </para>
|
| 1149 | <itemizedlist>
|
| 1150 | <listitem>
|
| 1151 | <para>
|
| 1152 | Figure out whose pond you've been pissing in. Look at the top of
|
| 1153 | the source files, inside the <filename>MAINTAINERS</filename>
|
| 1154 | file, and last of all in the <filename>CREDITS</filename> file.
|
| 1155 | You should coordinate with this person to make sure you're not
|
| 1156 | duplicating effort, or trying something that's already been
|
| 1157 | rejected.
|
| 1158 | </para>
|
| 1159 |
|
| 1160 | <para>
|
| 1161 | Make sure you put your name and EMail address at the top of
|
| 1162 | any files you create or mangle significantly. This is the
|
| 1163 | first place people will look when they find a bug, or when
|
| 1164 | <emphasis>they</emphasis> want to make a change.
|
| 1165 | </para>
|
| 1166 | </listitem>
|
| 1167 |
|
| 1168 | <listitem>
|
| 1169 | <para>
|
| 1170 | Usually you want a configuration option for your kernel hack.
|
| 1171 | Edit <filename>Kconfig</filename> in the appropriate directory.
|
| 1172 | The Config language is simple to use by cut and paste, and there's
|
| 1173 | complete documentation in
|
| 1174 | <filename>Documentation/kbuild/kconfig-language.txt</filename>.
|
| 1175 | </para>
|
| 1176 |
|
| 1177 | <para>
|
| 1178 | In your description of the option, make sure you address both the
|
| 1179 | expert user and the user who knows nothing about your feature. Mention
|
| 1180 | incompatibilities and issues here. <emphasis> Definitely
|
| 1181 | </emphasis> end your description with <quote> if in doubt, say N
|
| 1182 | </quote> (or, occasionally, `Y'); this is for people who have no
|
| 1183 | idea what you are talking about.
|
| 1184 | </para>
|
| 1185 | </listitem>
|
| 1186 |
|
| 1187 | <listitem>
|
| 1188 | <para>
|
| 1189 | Edit the <filename>Makefile</filename>: the CONFIG variables are
|
| 1190 | exported here so you can usually just add a "obj-$(CONFIG_xxx) +=
|
| 1191 | xxx.o" line. The syntax is documented in
|
| 1192 | <filename>Documentation/kbuild/makefiles.txt</filename>.
|
| 1193 | </para>
|
| 1194 | </listitem>
|
| 1195 |
|
| 1196 | <listitem>
|
| 1197 | <para>
|
| 1198 | Put yourself in <filename>CREDITS</filename> if you've done
|
| 1199 | something noteworthy, usually beyond a single file (your name
|
| 1200 | should be at the top of the source files anyway).
|
| 1201 | <filename>MAINTAINERS</filename> means you want to be consulted
|
| 1202 | when changes are made to a subsystem, and hear about bugs; it
|
| 1203 | implies a more-than-passing commitment to some part of the code.
|
| 1204 | </para>
|
| 1205 | </listitem>
|
| 1206 |
|
| 1207 | <listitem>
|
| 1208 | <para>
|
| 1209 | Finally, don't forget to read <filename>Documentation/SubmittingPatches</filename>
|
| 1210 | and possibly <filename>Documentation/SubmittingDrivers</filename>.
|
| 1211 | </para>
|
| 1212 | </listitem>
|
| 1213 | </itemizedlist>
|
| 1214 | </chapter>
|
| 1215 |
|
| 1216 | <chapter id="cantrips">
|
| 1217 | <title>Kernel Cantrips</title>
|
| 1218 |
|
| 1219 | <para>
|
| 1220 | Some favorites from browsing the source. Feel free to add to this
|
| 1221 | list.
|
| 1222 | </para>
|
| 1223 |
|
| 1224 | <para>
|
| 1225 | <filename>arch/x86/include/asm/delay.h:</filename>
|
| 1226 | </para>
|
| 1227 | <programlisting>
|
| 1228 | #define ndelay(n) (__builtin_constant_p(n) ? \
|
| 1229 | ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \
|
| 1230 | __ndelay(n))
|
| 1231 | </programlisting>
|
| 1232 |
|
| 1233 | <para>
|
| 1234 | <filename>include/linux/fs.h</filename>:
|
| 1235 | </para>
|
| 1236 | <programlisting>
|
| 1237 | /*
|
| 1238 | * Kernel pointers have redundant information, so we can use a
|
| 1239 | * scheme where we can return either an error code or a dentry
|
| 1240 | * pointer with the same return value.
|
| 1241 | *
|
| 1242 | * This should be a per-architecture thing, to allow different
|
| 1243 | * error and pointer decisions.
|
| 1244 | */
|
| 1245 | #define ERR_PTR(err) ((void *)((long)(err)))
|
| 1246 | #define PTR_ERR(ptr) ((long)(ptr))
|
| 1247 | #define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000))
|
| 1248 | </programlisting>
|
| 1249 |
|
| 1250 | <para>
|
| 1251 | <filename>arch/x86/include/asm/uaccess_32.h:</filename>
|
| 1252 | </para>
|
| 1253 |
|
| 1254 | <programlisting>
|
| 1255 | #define copy_to_user(to,from,n) \
|
| 1256 | (__builtin_constant_p(n) ? \
|
| 1257 | __constant_copy_to_user((to),(from),(n)) : \
|
| 1258 | __generic_copy_to_user((to),(from),(n)))
|
| 1259 | </programlisting>
|
| 1260 |
|
| 1261 | <para>
|
| 1262 | <filename>arch/sparc/kernel/head.S:</filename>
|
| 1263 | </para>
|
| 1264 |
|
| 1265 | <programlisting>
|
| 1266 | /*
|
| 1267 | * Sun people can't spell worth damn. "compatability" indeed.
|
| 1268 | * At least we *know* we can't spell, and use a spell-checker.
|
| 1269 | */
|
| 1270 |
|
| 1271 | /* Uh, actually Linus it is I who cannot spell. Too much murky
|
| 1272 | * Sparc assembly will do this to ya.
|
| 1273 | */
|
| 1274 | C_LABEL(cputypvar):
|
| 1275 | .asciz "compatibility"
|
| 1276 |
|
| 1277 | /* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */
|
| 1278 | .align 4
|
| 1279 | C_LABEL(cputypvar_sun4m):
|
| 1280 | .asciz "compatible"
|
| 1281 | </programlisting>
|
| 1282 |
|
| 1283 | <para>
|
| 1284 | <filename>arch/sparc/lib/checksum.S:</filename>
|
| 1285 | </para>
|
| 1286 |
|
| 1287 | <programlisting>
|
| 1288 | /* Sun, you just can't beat me, you just can't. Stop trying,
|
| 1289 | * give up. I'm serious, I am going to kick the living shit
|
| 1290 | * out of you, game over, lights out.
|
| 1291 | */
|
| 1292 | </programlisting>
|
| 1293 | </chapter>
|
| 1294 |
|
| 1295 | <chapter id="credits">
|
| 1296 | <title>Thanks</title>
|
| 1297 |
|
| 1298 | <para>
|
| 1299 | Thanks to Andi Kleen for the idea, answering my questions, fixing
|
| 1300 | my mistakes, filling content, etc. Philipp Rumpf for more spelling
|
| 1301 | and clarity fixes, and some excellent non-obvious points. Werner
|
| 1302 | Almesberger for giving me a great summary of
|
| 1303 | <function>disable_irq()</function>, and Jes Sorensen and Andrea
|
| 1304 | Arcangeli added caveats. Michael Elizabeth Chastain for checking
|
| 1305 | and adding to the Configure section. |
| 1306 | bit; I didn't do it! Telsa Gwynne for teaching me DocBook.
|
| 1307 | </para>
|
| 1308 | </chapter>
|
| 1309 | </book>
|
| 1310 |
|