docs-rst: convert kernel-hacking to ReST

Use pandoc to convert documentation to ReST by calling
Documentation/sphinx/tmplcvt script.

- Manually adjusted to use ..note and ..warning
- Minor fixes for it to be parsed without errors
- Use **bold** for emphasis.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

1 files changed, 0 insertions, 1312 deletions

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-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
-	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
-
-<book id="lk-hacking-guide">
- <bookinfo>
-  <title>Unreliable Guide To Hacking The Linux Kernel</title>
-  
-  <authorgroup>
-   <author>
-    <firstname>Rusty</firstname>
-    <surname>Russell</surname>
-    <affiliation>
-     <address>
-      <email>rusty@rustcorp.com.au</email>
-     </address>
-    </affiliation>
-   </author>
-  </authorgroup>
-
-  <copyright>
-   <year>2005</year>
-   <holder>Rusty Russell</holder>
-  </copyright>
-
-  <legalnotice>
-   <para>
-    This documentation is free software; you can redistribute
-    it and/or modify it under the terms of the GNU General Public
-    License as published by the Free Software Foundation; either
-    version 2 of the License, or (at your option) any later
-    version.
-   </para>
-   
-   <para>
-    This program is distributed in the hope that it will be
-    useful, but WITHOUT ANY WARRANTY; without even the implied
-    warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
-    See the GNU General Public License for more details.
-   </para>
-   
-   <para>
-    You should have received a copy of the GNU General Public
-    License along with this program; if not, write to the Free
-    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
-    MA 02111-1307 USA
-   </para>
-   
-   <para>
-    For more details see the file COPYING in the source
-    distribution of Linux.
-   </para>
-  </legalnotice>
-
-  <releaseinfo>
-   This is the first release of this document as part of the kernel tarball.
-  </releaseinfo>
-
- </bookinfo>
-
- <toc></toc>
-
- <chapter id="introduction">
-  <title>Introduction</title>
-  <para>
-   Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux
-   Kernel Hacking.  This document describes the common routines and
-   general requirements for kernel code: its goal is to serve as a
-   primer for Linux kernel development for experienced C
-   programmers.  I avoid implementation details: that's what the
-   code is for, and I ignore whole tracts of useful routines.
-  </para>
-  <para>
-   Before you read this, please understand that I never wanted to
-   write this document, being grossly under-qualified, but I always
-   wanted to read it, and this was the only way.  I hope it will
-   grow into a compendium of best practice, common starting points
-   and random information.
-  </para>
- </chapter>
-
- <chapter id="basic-players">
-  <title>The Players</title>
-
-  <para>
-   At any time each of the CPUs in a system can be:
-  </para>
-
-  <itemizedlist>
-   <listitem>
-    <para>
-     not associated with any process, serving a hardware interrupt;
-    </para>
-   </listitem>
-
-   <listitem>
-    <para>
-     not associated with any process, serving a softirq or tasklet;
-    </para>
-   </listitem>
-
-   <listitem>
-    <para>
-     running in kernel space, associated with a process (user context);
-    </para>
-   </listitem>
-
-   <listitem>
-    <para>
-     running a process in user space.
-    </para>
-   </listitem>
-  </itemizedlist>
-
-  <para>
-   There is an ordering between these.  The bottom two can preempt
-   each other, but above that is a strict hierarchy: each can only be
-   preempted by the ones above it.  For example, while a softirq is
-   running on a CPU, no other softirq will preempt it, but a hardware
-   interrupt can.  However, any other CPUs in the system execute
-   independently.
-  </para>
-
-  <para>
-   We'll see a number of ways that the user context can block
-   interrupts, to become truly non-preemptable.
-  </para>
-  
-  <sect1 id="basics-usercontext">
-   <title>User Context</title>
-
-   <para>
-    User context is when you are coming in from a system call or other
-    trap: like userspace, you can be preempted by more important tasks
-    and by interrupts.  You can sleep, by calling
-    <function>schedule()</function>.
-   </para>
-
-   <note>
-    <para>
-     You are always in user context on module load and unload,
-     and on operations on the block device layer.
-    </para>
-   </note>
-
-   <para>
-    In user context, the <varname>current</varname> pointer (indicating 
-    the task we are currently executing) is valid, and
-    <function>in_interrupt()</function>
-    (<filename>include/linux/interrupt.h</filename>) is <returnvalue>false
-    </returnvalue>.  
-   </para>
-
-   <caution>
-    <para>
-     Beware that if you have preemption or softirqs disabled
-     (see below), <function>in_interrupt()</function> will return a 
-     false positive.
-    </para>
-   </caution>
-  </sect1>
-
-  <sect1 id="basics-hardirqs">
-   <title>Hardware Interrupts (Hard IRQs)</title>
-
-   <para>
-    Timer ticks, <hardware>network cards</hardware> and 
-    <hardware>keyboard</hardware> are examples of real
-    hardware which produce interrupts at any time.  The kernel runs
-    interrupt handlers, which services the hardware.  The kernel
-    guarantees that this handler is never re-entered: if the same
-    interrupt arrives, it is queued (or dropped).  Because it
-    disables interrupts, this handler has to be fast: frequently it
-    simply acknowledges the interrupt, marks a 'software interrupt'
-    for execution and exits.
-   </para>
-
-   <para>
-    You can tell you are in a hardware interrupt, because 
-    <function>in_irq()</function> returns <returnvalue>true</returnvalue>.  
-   </para>
-   <caution>
-    <para>
-     Beware that this will return a false positive if interrupts are disabled 
-     (see below).
-    </para>
-   </caution>
-  </sect1>
-
-  <sect1 id="basics-softirqs">
-   <title>Software Interrupt Context: Softirqs and Tasklets</title>
-
-   <para>
-    Whenever a system call is about to return to userspace, or a
-    hardware interrupt handler exits, any 'software interrupts'
-    which are marked pending (usually by hardware interrupts) are
-    run (<filename>kernel/softirq.c</filename>).
-   </para>
-
-   <para>
-    Much of the real interrupt handling work is done here.  Early in
-    the transition to <acronym>SMP</acronym>, there were only 'bottom
-    halves' (BHs), which didn't take advantage of multiple CPUs.  Shortly 
-    after we switched from wind-up computers made of match-sticks and snot,
-    we abandoned this limitation and switched to 'softirqs'.
-   </para>
-
-   <para>
-    <filename class="headerfile">include/linux/interrupt.h</filename> lists the
-    different softirqs.  A very important softirq is the
-    timer softirq (<filename
-    class="headerfile">include/linux/timer.h</filename>): you can
-    register to have it call functions for you in a given length of
-    time.
-   </para>
-
-   <para>
-    Softirqs are often a pain to deal with, since the same softirq
-    will run simultaneously on more than one CPU.  For this reason,
-    tasklets (<filename
-    class="headerfile">include/linux/interrupt.h</filename>) are more
-    often used: they are dynamically-registrable (meaning you can have
-    as many as you want), and they also guarantee that any tasklet
-    will only run on one CPU at any time, although different tasklets
-    can run simultaneously.
-   </para>
-   <caution>
-    <para>
-     The name 'tasklet' is misleading: they have nothing to do with 'tasks',
-     and probably more to do with some bad vodka Alexey Kuznetsov had at the 
-     time.
-    </para>
-   </caution>
-
-   <para>
-    You can tell you are in a softirq (or tasklet)
-    using the <function>in_softirq()</function> macro 
-    (<filename class="headerfile">include/linux/interrupt.h</filename>).
-   </para>
-   <caution>
-    <para>
-     Beware that this will return a false positive if a bh lock (see below)
-     is held.
-    </para>
-   </caution>
-  </sect1>
- </chapter>
-
- <chapter id="basic-rules">
-  <title>Some Basic Rules</title>
-
-  <variablelist>
-   <varlistentry>
-    <term>No memory protection</term>
-    <listitem>
-     <para>
-      If you corrupt memory, whether in user context or
-      interrupt context, the whole machine will crash.  Are you
-      sure you can't do what you want in userspace?
-     </para>
-    </listitem>
-   </varlistentry>
-
-   <varlistentry>
-    <term>No floating point or <acronym>MMX</acronym></term>
-    <listitem>
-     <para>
-      The <acronym>FPU</acronym> context is not saved; even in user
-      context the <acronym>FPU</acronym> state probably won't
-      correspond with the current process: you would mess with some
-      user process' <acronym>FPU</acronym> state.  If you really want
-      to do this, you would have to explicitly save/restore the full
-      <acronym>FPU</acronym> state (and avoid context switches).  It
-      is generally a bad idea; use fixed point arithmetic first.
-     </para>
-    </listitem>
-   </varlistentry>
-
-   <varlistentry>
-    <term>A rigid stack limit</term>
-    <listitem>
-     <para>
-      Depending on configuration options the kernel stack is about 3K to 6K for most 32-bit architectures: it's
-      about 14K on most 64-bit archs, and often shared with interrupts
-      so you can't use it all.  Avoid deep recursion and huge local
-      arrays on the stack (allocate them dynamically instead).
-     </para>
-    </listitem>
-   </varlistentry>
-
-   <varlistentry>
-    <term>The Linux kernel is portable</term>
-    <listitem>
-     <para>
-      Let's keep it that way.  Your code should be 64-bit clean,
-      and endian-independent.  You should also minimize CPU
-      specific stuff, e.g. inline assembly should be cleanly
-      encapsulated and minimized to ease porting.  Generally it
-      should be restricted to the architecture-dependent part of
-      the kernel tree.
-     </para>
-    </listitem>
-   </varlistentry>
-  </variablelist>
- </chapter>
-
- <chapter id="ioctls">
-  <title>ioctls: Not writing a new system call</title>
-
-  <para>
-   A system call generally looks like this
-  </para>
-
-  <programlisting>
-asmlinkage long sys_mycall(int arg)
-{
-        return 0; 
-}
-  </programlisting>
-
-  <para>
-   First, in most cases you don't want to create a new system call.
-   You create a character device and implement an appropriate ioctl
-   for it.  This is much more flexible than system calls, doesn't have
-   to be entered in every architecture's
-   <filename class="headerfile">include/asm/unistd.h</filename> and
-   <filename>arch/kernel/entry.S</filename> file, and is much more
-   likely to be accepted by Linus.
-  </para>
-
-  <para>
-   If all your routine does is read or write some parameter, consider
-   implementing a <function>sysfs</function> interface instead.
-  </para>
-
-  <para>
-   Inside the ioctl you're in user context to a process.  When a
-   error occurs you return a negated errno (see
-   <filename class="headerfile">include/linux/errno.h</filename>),
-   otherwise you return <returnvalue>0</returnvalue>.
-  </para>
-
-  <para>
-   After you slept you should check if a signal occurred: the
-   Unix/Linux way of handling signals is to temporarily exit the
-   system call with the <constant>-ERESTARTSYS</constant> error.  The
-   system call entry code will switch back to user context, process
-   the signal handler and then your system call will be restarted
-   (unless the user disabled that).  So you should be prepared to
-   process the restart, e.g. if you're in the middle of manipulating
-   some data structure.
-  </para>
-
-  <programlisting>
-if (signal_pending(current))
-        return -ERESTARTSYS;
-  </programlisting>
-
-  <para>
-   If you're doing longer computations: first think userspace. If you
-   <emphasis>really</emphasis> want to do it in kernel you should
-   regularly check if you need to give up the CPU (remember there is
-   cooperative multitasking per CPU).  Idiom:
-  </para>
-
-  <programlisting>
-cond_resched(); /* Will sleep */ 
-  </programlisting>
-
-  <para>
-   A short note on interface design: the UNIX system call motto is
-   "Provide mechanism not policy".
-  </para>
- </chapter>
-
- <chapter id="deadlock-recipes">
-  <title>Recipes for Deadlock</title>
-
-  <para>
-   You cannot call any routines which may sleep, unless:
-  </para>
-  <itemizedlist>
-   <listitem>
-    <para>
-     You are in user context.
-    </para>
-   </listitem>
-
-   <listitem>
-    <para>
-     You do not own any spinlocks.
-    </para>
-   </listitem>
-
-   <listitem>
-    <para>
-     You have interrupts enabled (actually, Andi Kleen says
-     that the scheduling code will enable them for you, but
-     that's probably not what you wanted).
-    </para>
-   </listitem>
-  </itemizedlist>
-
-  <para>
-   Note that some functions may sleep implicitly: common ones are
-   the user space access functions (*_user) and memory allocation
-   functions without <symbol>GFP_ATOMIC</symbol>.
-  </para>
-
-  <para>
-   You should always compile your kernel
-   <symbol>CONFIG_DEBUG_ATOMIC_SLEEP</symbol> on, and it will warn
-   you if you break these rules.  If you <emphasis>do</emphasis> break
-   the rules, you will eventually lock up your box.
-  </para>
-
-  <para>
-   Really.
-  </para>
- </chapter>
-
- <chapter id="common-routines">
-  <title>Common Routines</title>
-
-  <sect1 id="routines-printk">
-   <title>
-    <function>printk()</function>
-    <filename class="headerfile">include/linux/kernel.h</filename>
-   </title>
-
-   <para>
-    <function>printk()</function> feeds kernel messages to the
-    console, dmesg, and the syslog daemon.  It is useful for debugging
-    and reporting errors, and can be used inside interrupt context,
-    but use with caution: a machine which has its console flooded with
-    printk messages is unusable.  It uses a format string mostly
-    compatible with ANSI C printf, and C string concatenation to give
-    it a first "priority" argument:
-   </para>
-
-   <programlisting>
-printk(KERN_INFO "i = %u\n", i);
-   </programlisting>
-
-   <para>
-    See <filename class="headerfile">include/linux/kernel.h</filename>;
-    for other KERN_ values; these are interpreted by syslog as the
-    level.  Special case: for printing an IP address use
-   </para>
-
-   <programlisting>
-__be32 ipaddress;
-printk(KERN_INFO "my ip: %pI4\n", &amp;ipaddress);
-   </programlisting>
-
-   <para>
-    <function>printk()</function> internally uses a 1K buffer and does
-    not catch overruns.  Make sure that will be enough.
-   </para>
-
-   <note>
-    <para>
-     You will know when you are a real kernel hacker
-     when you start typoing printf as printk in your user programs :)
-    </para>
-   </note>
-
-   <!--- From the Lions book reader department --> 
-
-   <note>
-    <para>
-     Another sidenote: the original Unix Version 6 sources had a
-     comment on top of its printf function: "Printf should not be
-     used for chit-chat".  You should follow that advice.
-    </para>
-   </note>
-  </sect1>
-
-  <sect1 id="routines-copy">
-   <title>
-    <function>copy_[to/from]_user()</function>
-    /
-    <function>get_user()</function>
-    /
-    <function>put_user()</function>
-    <filename class="headerfile">include/linux/uaccess.h</filename>
-   </title>  
-
-   <para>
-    <emphasis>[SLEEPS]</emphasis>
-   </para>
-
-   <para>
-    <function>put_user()</function> and <function>get_user()</function>
-    are used to get and put single values (such as an int, char, or
-    long) from and to userspace.  A pointer into userspace should
-    never be simply dereferenced: data should be copied using these
-    routines.  Both return <constant>-EFAULT</constant> or 0.
-   </para>
-   <para>
-    <function>copy_to_user()</function> and
-    <function>copy_from_user()</function> are more general: they copy
-    an arbitrary amount of data to and from userspace.
-    <caution>
-     <para>
-      Unlike <function>put_user()</function> and
-      <function>get_user()</function>, they return the amount of
-      uncopied data (ie. <returnvalue>0</returnvalue> still means
-      success).
-     </para>
-    </caution>
-    [Yes, this moronic interface makes me cringe.  The flamewar comes up every year or so. --RR.]
-   </para>
-   <para>
-    The functions may sleep implicitly. This should never be called
-    outside user context (it makes no sense), with interrupts
-    disabled, or a spinlock held.
-   </para>
-  </sect1>
-
-  <sect1 id="routines-kmalloc">
-   <title><function>kmalloc()</function>/<function>kfree()</function>
-    <filename class="headerfile">include/linux/slab.h</filename></title>
-
-   <para>
-    <emphasis>[MAY SLEEP: SEE BELOW]</emphasis>
-   </para>
-
-   <para>
-    These routines are used to dynamically request pointer-aligned
-    chunks of memory, like malloc and free do in userspace, but
-    <function>kmalloc()</function> takes an extra flag word.
-    Important values:
-   </para>
-
-   <variablelist>
-    <varlistentry>
-     <term>
-      <constant>
-       GFP_KERNEL
-      </constant>
-     </term>
-     <listitem>
-      <para>
-       May sleep and swap to free memory. Only allowed in user
-       context, but is the most reliable way to allocate memory.
-      </para>
-     </listitem>
-    </varlistentry>
-    
-    <varlistentry>
-     <term>
-      <constant>
-       GFP_ATOMIC
-      </constant>
-     </term>
-     <listitem>
-      <para>
-       Don't sleep. Less reliable than <constant>GFP_KERNEL</constant>,
-       but may be called from interrupt context. You should
-       <emphasis>really</emphasis> have a good out-of-memory
-       error-handling strategy.
-      </para>
-     </listitem>
-    </varlistentry>
-    
-    <varlistentry>
-     <term>
-      <constant>
-       GFP_DMA
-      </constant>
-     </term>
-     <listitem>
-      <para>
-       Allocate ISA DMA lower than 16MB. If you don't know what that
-       is you don't need it.  Very unreliable.
-      </para>
-     </listitem>
-    </varlistentry>
-   </variablelist>
-
-   <para>
-    If you see a <errorname>sleeping function called from invalid
-    context</errorname> warning message, then maybe you called a
-    sleeping allocation function from interrupt context without
-    <constant>GFP_ATOMIC</constant>.  You should really fix that.
-    Run, don't walk.
-   </para>
-
-   <para>
-    If you are allocating at least <constant>PAGE_SIZE</constant>
-    (<filename class="headerfile">include/asm/page.h</filename>) bytes,
-    consider using <function>__get_free_pages()</function>
-
-    (<filename class="headerfile">include/linux/mm.h</filename>).  It
-    takes an order argument (0 for page sized, 1 for double page, 2
-    for four pages etc.) and the same memory priority flag word as
-    above.
-   </para>
-
-   <para>
-    If you are allocating more than a page worth of bytes you can use
-    <function>vmalloc()</function>.  It'll allocate virtual memory in
-    the kernel map.  This block is not contiguous in physical memory,
-    but the <acronym>MMU</acronym> makes it look like it is for you
-    (so it'll only look contiguous to the CPUs, not to external device
-    drivers).  If you really need large physically contiguous memory
-    for some weird device, you have a problem: it is poorly supported
-    in Linux because after some time memory fragmentation in a running
-    kernel makes it hard.  The best way is to allocate the block early
-    in the boot process via the <function>alloc_bootmem()</function>
-    routine.
-   </para>
-
-   <para>
-    Before inventing your own cache of often-used objects consider
-    using a slab cache in
-    <filename class="headerfile">include/linux/slab.h</filename>
-   </para>
-  </sect1>
-
-  <sect1 id="routines-current">
-   <title><function>current</function>
-    <filename class="headerfile">include/asm/current.h</filename></title>
-
-   <para>
-    This global variable (really a macro) contains a pointer to
-    the current task structure, so is only valid in user context.
-    For example, when a process makes a system call, this will
-    point to the task structure of the calling process.  It is
-    <emphasis>not NULL</emphasis> in interrupt context.
-   </para>
-  </sect1>
-
-  <sect1 id="routines-udelay">
-   <title><function>mdelay()</function>/<function>udelay()</function>
-     <filename class="headerfile">include/asm/delay.h</filename>
-     <filename class="headerfile">include/linux/delay.h</filename>
-   </title>
-
-   <para>
-    The <function>udelay()</function> and <function>ndelay()</function> functions can be used for small pauses.
-    Do not use large values with them as you risk
-    overflow - the helper function <function>mdelay()</function> is useful
-    here, or consider <function>msleep()</function>.
-   </para> 
-  </sect1>
- 
-  <sect1 id="routines-endian">
-   <title><function>cpu_to_be32()</function>/<function>be32_to_cpu()</function>/<function>cpu_to_le32()</function>/<function>le32_to_cpu()</function>
-     <filename class="headerfile">include/asm/byteorder.h</filename>
-   </title>
-
-   <para>
-    The <function>cpu_to_be32()</function> family (where the "32" can
-    be replaced by 64 or 16, and the "be" can be replaced by "le") are
-    the general way to do endian conversions in the kernel: they
-    return the converted value.  All variations supply the reverse as
-    well: <function>be32_to_cpu()</function>, etc.
-   </para>
-
-   <para>
-    There are two major variations of these functions: the pointer
-    variation, such as <function>cpu_to_be32p()</function>, which take
-    a pointer to the given type, and return the converted value.  The
-    other variation is the "in-situ" family, such as
-    <function>cpu_to_be32s()</function>, which convert value referred
-    to by the pointer, and return void.
-   </para> 
-  </sect1>
-
-  <sect1 id="routines-local-irqs">
-   <title><function>local_irq_save()</function>/<function>local_irq_restore()</function>
-    <filename class="headerfile">include/linux/irqflags.h</filename>
-   </title>
-
-   <para>
-    These routines disable hard interrupts on the local CPU, and
-    restore them.  They are reentrant; saving the previous state in
-    their one <varname>unsigned long flags</varname> argument.  If you
-    know that interrupts are enabled, you can simply use
-    <function>local_irq_disable()</function> and
-    <function>local_irq_enable()</function>.
-   </para>
-  </sect1>
-
-  <sect1 id="routines-softirqs">
-   <title><function>local_bh_disable()</function>/<function>local_bh_enable()</function>
-    <filename class="headerfile">include/linux/interrupt.h</filename></title>
-
-   <para>
-    These routines disable soft interrupts on the local CPU, and
-    restore them.  They are reentrant; if soft interrupts were
-    disabled before, they will still be disabled after this pair
-    of functions has been called.  They prevent softirqs and tasklets
-    from running on the current CPU.
-   </para>
-  </sect1>
-
-  <sect1 id="routines-processorids">
-   <title><function>smp_processor_id</function>()
-    <filename class="headerfile">include/asm/smp.h</filename></title>
-   
-   <para>
-    <function>get_cpu()</function> disables preemption (so you won't
-    suddenly get moved to another CPU) and returns the current
-    processor number, between 0 and <symbol>NR_CPUS</symbol>.  Note
-    that the CPU numbers are not necessarily continuous.  You return
-    it again with <function>put_cpu()</function> when you are done.
-   </para>
-   <para>
-    If you know you cannot be preempted by another task (ie. you are
-    in interrupt context, or have preemption disabled) you can use
-    smp_processor_id().
-   </para>
-  </sect1>
-
-  <sect1 id="routines-init">
-   <title><type>__init</type>/<type>__exit</type>/<type>__initdata</type>
-    <filename class="headerfile">include/linux/init.h</filename></title>
-
-   <para>
-    After boot, the kernel frees up a special section; functions
-    marked with <type>__init</type> and data structures marked with
-    <type>__initdata</type> are dropped after boot is complete: similarly
-    modules discard this memory after initialization.  <type>__exit</type>
-    is used to declare a function which is only required on exit: the
-    function will be dropped if this file is not compiled as a module.
-    See the header file for use. Note that it makes no sense for a function
-    marked with <type>__init</type> to be exported to modules with 
-    <function>EXPORT_SYMBOL()</function> - this will break.
-   </para>
-
-  </sect1>
-
-  <sect1 id="routines-init-again">
-   <title><function>__initcall()</function>/<function>module_init()</function>
-    <filename class="headerfile">include/linux/init.h</filename></title>
-   <para>
-    Many parts of the kernel are well served as a module
-    (dynamically-loadable parts of the kernel).  Using the
-    <function>module_init()</function> and
-    <function>module_exit()</function> macros it is easy to write code
-    without #ifdefs which can operate both as a module or built into
-    the kernel.
-   </para>
-
-   <para>
-    The <function>module_init()</function> macro defines which
-    function is to be called at module insertion time (if the file is
-    compiled as a module), or at boot time: if the file is not
-    compiled as a module the <function>module_init()</function> macro
-    becomes equivalent to <function>__initcall()</function>, which
-    through linker magic ensures that the function is called on boot.
-   </para>
-
-   <para>
-    The function can return a negative error number to cause
-    module loading to fail (unfortunately, this has no effect if
-    the module is compiled into the kernel).  This function is
-    called in user context with interrupts enabled, so it can sleep.
-   </para>
-  </sect1>
-  
-  <sect1 id="routines-moduleexit">
-   <title> <function>module_exit()</function>
-    <filename class="headerfile">include/linux/init.h</filename> </title>
-
-   <para>
-    This macro defines the function to be called at module removal
-    time (or never, in the case of the file compiled into the
-    kernel).  It will only be called if the module usage count has
-    reached zero.  This function can also sleep, but cannot fail:
-    everything must be cleaned up by the time it returns.
-   </para>
-
-   <para>
-    Note that this macro is optional: if it is not present, your
-    module will not be removable (except for 'rmmod -f').
-   </para>
-  </sect1>
-
-  <sect1 id="routines-module-use-counters">
-   <title> <function>try_module_get()</function>/<function>module_put()</function>
-    <filename class="headerfile">include/linux/module.h</filename></title>
-
-   <para>
-    These manipulate the module usage count, to protect against
-    removal (a module also can't be removed if another module uses one
-    of its exported symbols: see below).  Before calling into module
-    code, you should call <function>try_module_get()</function> on
-    that module: if it fails, then the module is being removed and you
-    should act as if it wasn't there.  Otherwise, you can safely enter
-    the module, and call <function>module_put()</function> when you're
-    finished.
-   </para>
-
-   <para>
-   Most registerable structures have an
-   <structfield>owner</structfield> field, such as in the
-   <structname>file_operations</structname> structure. Set this field
-   to the macro <symbol>THIS_MODULE</symbol>.
-   </para>
-  </sect1>
-
- <!-- add info on new-style module refcounting here -->
- </chapter>
-
- <chapter id="queues">
-  <title>Wait Queues
-   <filename class="headerfile">include/linux/wait.h</filename>
-  </title>
-  <para>
-   <emphasis>[SLEEPS]</emphasis>
-  </para>
-
-  <para>
-   A wait queue is used to wait for someone to wake you up when a
-   certain condition is true.  They must be used carefully to ensure
-   there is no race condition.  You declare a
-   <type>wait_queue_head_t</type>, and then processes which want to
-   wait for that condition declare a <type>wait_queue_t</type>
-   referring to themselves, and place that in the queue.
-  </para>
-
-  <sect1 id="queue-declaring">
-   <title>Declaring</title>
-   
-   <para>
-    You declare a <type>wait_queue_head_t</type> using the
-    <function>DECLARE_WAIT_QUEUE_HEAD()</function> macro, or using the
-    <function>init_waitqueue_head()</function> routine in your
-    initialization code.
-   </para>
-  </sect1>
-  
-  <sect1 id="queue-waitqueue">
-   <title>Queuing</title>
-   
-   <para>
-    Placing yourself in the waitqueue is fairly complex, because you
-    must put yourself in the queue before checking the condition.
-    There is a macro to do this:
-    <function>wait_event_interruptible()</function>
-
-    <filename class="headerfile">include/linux/wait.h</filename> The
-    first argument is the wait queue head, and the second is an
-    expression which is evaluated; the macro returns
-    <returnvalue>0</returnvalue> when this expression is true, or
-    <returnvalue>-ERESTARTSYS</returnvalue> if a signal is received.
-    The <function>wait_event()</function> version ignores signals.
-   </para>
- 
-  </sect1>
-
-  <sect1 id="queue-waking">
-   <title>Waking Up Queued Tasks</title>
-   
-   <para>
-    Call <function>wake_up()</function>
-
-    <filename class="headerfile">include/linux/wait.h</filename>;,
-    which will wake up every process in the queue.  The exception is
-    if one has <constant>TASK_EXCLUSIVE</constant> set, in which case
-    the remainder of the queue will not be woken.  There are other variants
-    of this basic function available in the same header.
-   </para>
-  </sect1>
- </chapter>
-
- <chapter id="atomic-ops">
-  <title>Atomic Operations</title>
-
-  <para>
-   Certain operations are guaranteed atomic on all platforms.  The
-   first class of operations work on <type>atomic_t</type>
-
-   <filename class="headerfile">include/asm/atomic.h</filename>; this
-   contains a signed integer (at least 32 bits long), and you must use
-   these functions to manipulate or read atomic_t variables.
-   <function>atomic_read()</function> and
-   <function>atomic_set()</function> get and set the counter,
-   <function>atomic_add()</function>,
-   <function>atomic_sub()</function>,
-   <function>atomic_inc()</function>,
-   <function>atomic_dec()</function>, and
-   <function>atomic_dec_and_test()</function> (returns
-   <returnvalue>true</returnvalue> if it was decremented to zero).
-  </para>
-
-  <para>
-   Yes.  It returns <returnvalue>true</returnvalue> (i.e. != 0) if the
-   atomic variable is zero.
-  </para>
-
-  <para>
-   Note that these functions are slower than normal arithmetic, and
-   so should not be used unnecessarily.
-  </para>
-
-  <para>
-   The second class of atomic operations is atomic bit operations on an
-   <type>unsigned long</type>, defined in
-
-   <filename class="headerfile">include/linux/bitops.h</filename>.  These
-   operations generally take a pointer to the bit pattern, and a bit
-   number: 0 is the least significant bit.
-   <function>set_bit()</function>, <function>clear_bit()</function>
-   and <function>change_bit()</function> set, clear, and flip the
-   given bit.  <function>test_and_set_bit()</function>,
-   <function>test_and_clear_bit()</function> and
-   <function>test_and_change_bit()</function> do the same thing,
-   except return true if the bit was previously set; these are
-   particularly useful for atomically setting flags.
-  </para>
-  
-  <para>
-   It is possible to call these operations with bit indices greater
-   than BITS_PER_LONG.  The resulting behavior is strange on big-endian
-   platforms though so it is a good idea not to do this.
-  </para>
- </chapter>
-
- <chapter id="symbols">
-  <title>Symbols</title>
-
-  <para>
-   Within the kernel proper, the normal linking rules apply
-   (ie. unless a symbol is declared to be file scope with the
-   <type>static</type> keyword, it can be used anywhere in the
-   kernel).  However, for modules, a special exported symbol table is
-   kept which limits the entry points to the kernel proper.  Modules
-   can also export symbols.
-  </para>
-
-  <sect1 id="sym-exportsymbols">
-   <title><function>EXPORT_SYMBOL()</function>
-    <filename class="headerfile">include/linux/export.h</filename></title>
-
-   <para>
-    This is the classic method of exporting a symbol: dynamically
-    loaded modules will be able to use the symbol as normal.
-   </para>
-  </sect1>
-
-  <sect1 id="sym-exportsymbols-gpl">
-   <title><function>EXPORT_SYMBOL_GPL()</function>
-    <filename class="headerfile">include/linux/export.h</filename></title>
-
-   <para>
-    Similar to <function>EXPORT_SYMBOL()</function> except that the
-    symbols exported by <function>EXPORT_SYMBOL_GPL()</function> can
-    only be seen by modules with a
-    <function>MODULE_LICENSE()</function> that specifies a GPL
-    compatible license.  It implies that the function is considered
-    an internal implementation issue, and not really an interface.
-    Some maintainers and developers may however
-    require EXPORT_SYMBOL_GPL() when adding any new APIs or functionality.
-   </para>
-  </sect1>
- </chapter>
-
- <chapter id="conventions">
-  <title>Routines and Conventions</title>
-
-  <sect1 id="conventions-doublelinkedlist">
-   <title>Double-linked lists
-    <filename class="headerfile">include/linux/list.h</filename></title>
-
-   <para>
-    There used to be three sets of linked-list routines in the kernel
-    headers, but this one is the winner.  If you don't have some
-    particular pressing need for a single list, it's a good choice.
-   </para>
-
-   <para>
-    In particular, <function>list_for_each_entry</function> is useful.
-   </para>
-  </sect1>
-
-  <sect1 id="convention-returns">
-   <title>Return Conventions</title>
-
-   <para>
-    For code called in user context, it's very common to defy C
-    convention, and return <returnvalue>0</returnvalue> for success,
-    and a negative error number
-    (eg. <returnvalue>-EFAULT</returnvalue>) for failure.  This can be
-    unintuitive at first, but it's fairly widespread in the kernel.
-   </para>
-
-   <para>
-    Using <function>ERR_PTR()</function>
-
-    <filename class="headerfile">include/linux/err.h</filename>; to
-    encode a negative error number into a pointer, and
-    <function>IS_ERR()</function> and <function>PTR_ERR()</function>
-    to get it back out again: avoids a separate pointer parameter for
-    the error number.  Icky, but in a good way.
-   </para>
-  </sect1>
-
-  <sect1 id="conventions-borkedcompile">
-   <title>Breaking Compilation</title>
-
-   <para>
-    Linus and the other developers sometimes change function or
-    structure names in development kernels; this is not done just to
-    keep everyone on their toes: it reflects a fundamental change
-    (eg. can no longer be called with interrupts on, or does extra
-    checks, or doesn't do checks which were caught before).  Usually
-    this is accompanied by a fairly complete note to the linux-kernel
-    mailing list; search the archive.  Simply doing a global replace
-    on the file usually makes things <emphasis>worse</emphasis>.
-   </para>
-  </sect1>
-
-  <sect1 id="conventions-initialising">
-   <title>Initializing structure members</title>
-
-   <para>
-    The preferred method of initializing structures is to use
-    designated initialisers, as defined by ISO C99, eg:
-   </para>
-   <programlisting>
-static struct block_device_operations opt_fops = {
-        .open               = opt_open,
-        .release            = opt_release,
-        .ioctl              = opt_ioctl,
-        .check_media_change = opt_media_change,
-};
-   </programlisting>
-   <para>
-    This makes it easy to grep for, and makes it clear which
-    structure fields are set.  You should do this because it looks
-    cool.
-   </para>
-  </sect1>
-
-  <sect1 id="conventions-gnu-extns">
-   <title>GNU Extensions</title>
-
-   <para>
-    GNU Extensions are explicitly allowed in the Linux kernel.
-    Note that some of the more complex ones are not very well
-    supported, due to lack of general use, but the following are
-    considered standard (see the GCC info page section "C
-    Extensions" for more details - Yes, really the info page, the
-    man page is only a short summary of the stuff in info).
-   </para>
-   <itemizedlist>
-    <listitem>
-     <para>
-      Inline functions
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      Statement expressions (ie. the ({ and }) constructs).
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      Declaring attributes of a function / variable / type
-      (__attribute__)
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      typeof
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      Zero length arrays
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      Macro varargs
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      Arithmetic on void pointers
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      Non-Constant initializers
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      Assembler Instructions (not outside arch/ and include/asm/)
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      Function names as strings (__func__).
-     </para>
-    </listitem>
-    <listitem>
-     <para>
-      __builtin_constant_p()
-     </para>
-    </listitem>
-   </itemizedlist>
-
-   <para>
-    Be wary when using long long in the kernel, the code gcc generates for
-    it is horrible and worse: division and multiplication does not work
-    on i386 because the GCC runtime functions for it are missing from
-    the kernel environment.
-   </para>
-
-    <!-- FIXME: add a note about ANSI aliasing cleanness -->
-  </sect1>
-
-  <sect1 id="conventions-cplusplus">
-   <title>C++</title>
-   
-   <para>
-    Using C++ in the kernel is usually a bad idea, because the
-    kernel does not provide the necessary runtime environment
-    and the include files are not tested for it.  It is still
-    possible, but not recommended.  If you really want to do
-    this, forget about exceptions at least.
-   </para>
-  </sect1>
-
-  <sect1 id="conventions-ifdef">
-   <title>&num;if</title>
-   
-   <para>
-    It is generally considered cleaner to use macros in header files
-    (or at the top of .c files) to abstract away functions rather than
-    using `#if' pre-processor statements throughout the source code.
-   </para>
-  </sect1>
- </chapter>
-
- <chapter id="submitting">
-  <title>Putting Your Stuff in the Kernel</title>
-
-  <para>
-   In order to get your stuff into shape for official inclusion, or
-   even to make a neat patch, there's administrative work to be
-   done:
-  </para>
-  <itemizedlist>
-   <listitem>
-    <para>
-     Figure out whose pond you've been pissing in.  Look at the top of
-     the source files, inside the <filename>MAINTAINERS</filename>
-     file, and last of all in the <filename>CREDITS</filename> file.
-     You should coordinate with this person to make sure you're not
-     duplicating effort, or trying something that's already been
-     rejected.
-    </para>
-
-    <para>
-     Make sure you put your name and EMail address at the top of
-     any files you create or mangle significantly.  This is the
-     first place people will look when they find a bug, or when
-     <emphasis>they</emphasis> want to make a change.
-    </para>
-   </listitem>
-
-   <listitem>
-    <para>
-     Usually you want a configuration option for your kernel hack.
-     Edit <filename>Kconfig</filename> in the appropriate directory.
-     The Config language is simple to use by cut and paste, and there's
-     complete documentation in
-     <filename>Documentation/kbuild/kconfig-language.txt</filename>.
-    </para>
-
-    <para>
-     In your description of the option, make sure you address both the
-     expert user and the user who knows nothing about your feature.  Mention
-     incompatibilities and issues here.  <emphasis> Definitely
-     </emphasis> end your description with <quote> if in doubt, say N
-     </quote> (or, occasionally, `Y'); this is for people who have no
-     idea what you are talking about.
-    </para>
-   </listitem>
-
-   <listitem>
-    <para>
-     Edit the <filename>Makefile</filename>: the CONFIG variables are
-     exported here so you can usually just add a "obj-$(CONFIG_xxx) +=
-     xxx.o" line.  The syntax is documented in
-     <filename>Documentation/kbuild/makefiles.txt</filename>.
-    </para>
-   </listitem>
-
-   <listitem>
-    <para>
-     Put yourself in <filename>CREDITS</filename> if you've done
-     something noteworthy, usually beyond a single file (your name
-     should be at the top of the source files anyway).
-     <filename>MAINTAINERS</filename> means you want to be consulted
-     when changes are made to a subsystem, and hear about bugs; it
-     implies a more-than-passing commitment to some part of the code.
-    </para>
-   </listitem>
-   
-   <listitem>
-    <para>
-     Finally, don't forget to read <filename>Documentation/process/submitting-patches.rst</filename>
-     and possibly <filename>Documentation/process/submitting-drivers.rst</filename>.
-    </para>
-   </listitem>
-  </itemizedlist>
- </chapter>
-
- <chapter id="cantrips">
-  <title>Kernel Cantrips</title>
-
-  <para>
-   Some favorites from browsing the source.  Feel free to add to this
-   list.
-  </para>
-
-  <para>
-   <filename>arch/x86/include/asm/delay.h:</filename>
-  </para>
-  <programlisting>
-#define ndelay(n) (__builtin_constant_p(n) ? \
-        ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \
-        __ndelay(n))
-  </programlisting>
-
-  <para>
-   <filename>include/linux/fs.h</filename>:
-  </para>
-  <programlisting>
-/*
- * Kernel pointers have redundant information, so we can use a
- * scheme where we can return either an error code or a dentry
- * pointer with the same return value.
- *
- * This should be a per-architecture thing, to allow different
- * error and pointer decisions.
- */
- #define ERR_PTR(err)    ((void *)((long)(err)))
- #define PTR_ERR(ptr)    ((long)(ptr))
- #define IS_ERR(ptr)     ((unsigned long)(ptr) > (unsigned long)(-1000))
-</programlisting>
-
-  <para>
-   <filename>arch/x86/include/asm/uaccess_32.h:</filename>
-  </para>
-
-  <programlisting>
-#define copy_to_user(to,from,n)                         \
-        (__builtin_constant_p(n) ?                      \
-         __constant_copy_to_user((to),(from),(n)) :     \
-         __generic_copy_to_user((to),(from),(n)))
-  </programlisting>
-
-  <para>
-   <filename>arch/sparc/kernel/head.S:</filename>
-  </para>
-
-  <programlisting>
-/*
- * Sun people can't spell worth damn. "compatability" indeed.
- * At least we *know* we can't spell, and use a spell-checker.
- */
-
-/* Uh, actually Linus it is I who cannot spell. Too much murky
- * Sparc assembly will do this to ya.
- */
-C_LABEL(cputypvar):
-        .asciz "compatibility"
-
-/* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */
-        .align 4
-C_LABEL(cputypvar_sun4m):
-        .asciz "compatible"
-  </programlisting>
-
-  <para>
-   <filename>arch/sparc/lib/checksum.S:</filename>
-  </para>
-
-  <programlisting>
-        /* Sun, you just can't beat me, you just can't.  Stop trying,
-         * give up.  I'm serious, I am going to kick the living shit
-         * out of you, game over, lights out.
-         */
-  </programlisting>
- </chapter>
-
- <chapter id="credits">
-  <title>Thanks</title>
-
-  <para>
-   Thanks to Andi Kleen for the idea, answering my questions, fixing
-   my mistakes, filling content, etc.  Philipp Rumpf for more spelling
-   and clarity fixes, and some excellent non-obvious points.  Werner
-   Almesberger for giving me a great summary of
-   <function>disable_irq()</function>, and Jes Sorensen and Andrea
-   Arcangeli added caveats. Michael Elizabeth Chastain for checking
-   and adding to the Configure section. <!-- Rusty insisted on this
-   bit; I didn't do it! --> Telsa Gwynne for teaching me DocBook. 
-  </para>
- </chapter>
-</book>
-