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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>Paul</firstname>
+ <othername>Rusty</othername>
+ <surname>Russell</surname>
+ <affiliation>
+ <address>
+ <email>rusty@rustcorp.com.au</email>
+ </address>
+ </affiliation>
+ </author>
+ </authorgroup>
+
+ <copyright>
+ <year>2001</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 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, tasklet or bh;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ running in kernel space, associated with a process;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ running a process in user space.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ There is a strict ordering between these: other than the last
+ category (userspace) each can only be pre-empted by those above.
+ For example, while a softirq is running on a CPU, no other
+ softirq will pre-empt 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: you can sleep, and you own the CPU (except for
+ interrupts) until you call <function>schedule()</function>.
+ In other words, user context (unlike userspace) is not pre-emptable.
+ </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 interrupts or bottom halves 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 another
+ 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: Bottom Halves, Tasklets, softirqs</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.
+ </para>
+
+ <para>
+ <filename class="headerfile">include/linux/interrupt.h</filename> lists the
+ different BH's. No matter how many CPUs you have, no two BHs will run at
+ the same time. This made the transition to SMP simpler, but sucks hard for
+ scalable performance. A very important bottom half is the timer
+ BH (<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>
+ 2.3.43 introduced softirqs, and re-implemented the (now
+ deprecated) BHs underneath them. Softirqs are fully-SMP
+ versions of BHs: they can run on as many CPUs at once as
+ required. This means they need to deal with any races in shared
+ data using their own locks. A bitmask is used to keep track of
+ which are enabled, so the 32 available softirqs should not be
+ used up lightly. (<emphasis>Yes</emphasis>, people will
+ notice).
+ </para>
+
+ <para>
+ tasklets (<filename class="headerfile">include/linux/interrupt.h</filename>)
+ are like softirqs, except 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 (unlike different BHs).
+ </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 bottom half, 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>
+ The kernel stack is about 6K in 2.2 (for most
+ architectures: it's about 14K on the Alpha), and 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>sysctl</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())
+ 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 will eventually lock up your box if you break these rules.
+ </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>
+__u32 ipaddress;
+printk(KERN_INFO "my ip: %d.%d.%d.%d\n", NIPQUAD(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/asm/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. Please submit a
+ patch and become my hero --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>kmem_grow: Called nonatomically from int
+ </errorname> warning message you called a memory 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>udelay()</function>/<function>mdelay()</function>
+ <filename class="headerfile">include/asm/delay.h</filename>
+ <filename class="headerfile">include/linux/delay.h</filename>
+ </title>
+
+ <para>
+ The <function>udelay()</function> function can be used for small pauses.
+ Do not use large values with <function>udelay()</function> as you risk
+ overflow - the helper function <function>mdelay()</function> is useful
+ here, or even consider <function>schedule_timeout()</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/asm/system.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, tasklets
+ and bottom halves 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>smp_processor_id()</function> returns the current
+ processor number, between 0 and <symbol>NR_CPUS</symbol> (the
+ maximum number of CPUs supported by Linux, currently 32). These
+ values are not necessarily continuous.
+ </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 (within
+ modules this directive is currently ignored). <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>
+ <para>
+ Static data structures marked as <type>__initdata</type> must be initialised
+ (as opposed to ordinary static data which is zeroed BSS) and cannot be
+ <type>const</type>.
+ </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). For modules, this is
+ called in user context, with interrupts enabled, and the
+ kernel lock held, 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>
+ </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/sched.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>
+ <para>
+ Do not use the <function>sleep_on()</function> function family -
+ it is very easy to accidentally introduce races; almost certainly
+ one of the <function>wait_event()</function> family will do, or a
+ loop around <function>schedule_timeout()</function>. If you choose
+ to loop around <function>schedule_timeout()</function> remember
+ you must set the task state (with
+ <function>set_current_state()</function>) on each iteration to avoid
+ busy-looping.
+ </para>
+
+ </sect1>
+
+ <sect1 id="queue-waking">
+ <title>Waking Up Queued Tasks</title>
+
+ <para>
+ Call <function>wake_up()</function>
+
+ <filename class="headerfile">include/linux/sched.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.
+ </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 24 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. On some platforms they
+ are much slower, like 32-bit Sparc where they use a spinlock.
+ </para>
+
+ <para>
+ The second class of atomic operations is atomic bit operations on a
+ <type>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 very simple locking.
+ </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>
+
+ <para>
+ Note that the order of bits depends on the architecture, and in
+ particular, the bitfield passed to these operations must be at
+ least as large as a <type>long</type>.
+ </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/module.h</filename></title>
+
+ <para>
+ This is the classic method of exporting a symbol, and it works
+ for both modules and non-modules. In the kernel all these
+ declarations are often bundled into a single file to help
+ genksyms (which searches source files for these declarations).
+ See the comment on genksyms and Makefiles below.
+ </para>
+ </sect1>
+
+ <sect1 id="sym-exportsymbols-gpl">
+ <title><function>EXPORT_SYMBOL_GPL()</function>
+ <filename class="headerfile">include/linux/module.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.
+ </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 are three sets of linked-list routines in the kernel
+ headers, but this one seems to be winning out (and Linus has
+ used it). If you don't have some particular pressing need for
+ a single list, it's a good choice. In fact, I don't care
+ whether it's a good choice or not, just use it so we can get
+ rid of the others.
+ </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 networking
+ code, for example.
+ </para>
+
+ <para>
+ The filesystem code uses <function>ERR_PTR()</function>
+
+ <filename class="headerfile">include/linux/fs.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 (__FUNCTION__)
+ </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>Config.in</filename> in the appropriate directory
+ (but under <filename>arch/</filename> it's called
+ <filename>config.in</filename>). The Config Language used is not
+ bash, even though it looks like bash; the safe way is to use only
+ the constructs that you already see in
+ <filename>Config.in</filename> files (see
+ <filename>Documentation/kbuild/kconfig-language.txt</filename>).
+ It's good to run "make xconfig" at least once to test (because
+ it's the only one with a static parser).
+ </para>
+
+ <para>
+ Variables which can be Y or N use <type>bool</type> followed by a
+ tagline and the config define name (which must start with
+ CONFIG_). The <type>tristate</type> function is the same, but
+ allows the answer M (which defines
+ <symbol>CONFIG_foo_MODULE</symbol> in your source, instead of
+ <symbol>CONFIG_FOO</symbol>) if <symbol>CONFIG_MODULES</symbol>
+ is enabled.
+ </para>
+
+ <para>
+ You may well want to make your CONFIG option only visible if
+ <symbol>CONFIG_EXPERIMENTAL</symbol> is enabled: this serves as a
+ warning to users. There many other fancy things you can do: see
+ the various <filename>Config.in</filename> files for ideas.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Edit the <filename>Makefile</filename>: the CONFIG variables are
+ exported here so you can conditionalize compilation with `ifeq'.
+ If your file exports symbols then add the names to
+ <varname>export-objs</varname> so that genksyms will find them.
+ <caution>
+ <para>
+ There is a restriction on the kernel build system that objects
+ which export symbols must have globally unique names.
+ If your object does not have a globally unique name then the
+ standard fix is to move the
+ <function>EXPORT_SYMBOL()</function> statements to their own
+ object with a unique name.
+ This is why several systems have separate exporting objects,
+ usually suffixed with ksyms.
+ </para>
+ </caution>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Document your option in Documentation/Configure.help. 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>
+ 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/SubmittingPatches</filename>
+ and possibly <filename>Documentation/SubmittingDrivers</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>include/linux/brlock.h:</filename>
+ </para>
+ <programlisting>
+extern inline void br_read_lock (enum brlock_indices idx)
+{
+ /*
+ * This causes a link-time bug message if an
+ * invalid index is used:
+ */
+ if (idx >= __BR_END)
+ __br_lock_usage_bug();
+
+ read_lock(&amp;__brlock_array[smp_processor_id()][idx]);
+}
+ </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>include/asm-i386/uaccess.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 "compatability"
+
+/* 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>
+