+++ /dev/null
-<?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", &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>#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>
-
--- /dev/null
+============================================
+Unreliable Guide To Hacking The Linux Kernel
+============================================
+
+:Author: Rusty Russell
+
+Introduction
+============
+
+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.
+
+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.
+
+The Players
+===========
+
+At any time each of the CPUs in a system can be:
+
+- not associated with any process, serving a hardware interrupt;
+
+- not associated with any process, serving a softirq or tasklet;
+
+- running in kernel space, associated with a process (user context);
+
+- running a process in user space.
+
+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.
+
+We'll see a number of ways that the user context can block interrupts,
+to become truly non-preemptable.
+
+User Context
+------------
+
+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 :c:func:`schedule()`.
+
+.. note::
+
+ You are always in user context on module load and unload, and on
+ operations on the block device layer.
+
+In user context, the ``current`` pointer (indicating the task we are
+currently executing) is valid, and :c:func:`in_interrupt()`
+(``include/linux/interrupt.h``) is false.
+
+.. warning::
+
+ Beware that if you have preemption or softirqs disabled (see below),
+ :c:func:`in_interrupt()` will return a false positive.
+
+Hardware Interrupts (Hard IRQs)
+-------------------------------
+
+Timer ticks, network cards and keyboard 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.
+
+You can tell you are in a hardware interrupt, because
+:c:func:`in_irq()` returns true.
+
+.. warning::
+
+ Beware that this will return a false positive if interrupts are
+ disabled (see below).
+
+Software Interrupt Context: Softirqs and Tasklets
+-------------------------------------------------
+
+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 (``kernel/softirq.c``).
+
+Much of the real interrupt handling work is done here. Early in the
+transition to SMP, 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'.
+
+``include/linux/interrupt.h`` lists the different softirqs. A very
+important softirq is the timer softirq (``include/linux/timer.h``): you
+can register to have it call functions for you in a given length of
+time.
+
+Softirqs are often a pain to deal with, since the same softirq will run
+simultaneously on more than one CPU. For this reason, tasklets
+(``include/linux/interrupt.h``) 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.
+
+.. warning::
+
+ 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.
+
+You can tell you are in a softirq (or tasklet) using the
+:c:func:`in_softirq()` macro (``include/linux/interrupt.h``).
+
+.. warning::
+
+ Beware that this will return a false positive if a bh lock (see
+ below) is held.
+
+Some Basic Rules
+================
+
+No memory protection
+ 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?
+
+No floating point or MMX
+ The FPU context is not saved; even in user context the FPU state
+ probably won't correspond with the current process: you would mess
+ with some user process' FPU state. If you really want to do this,
+ you would have to explicitly save/restore the full FPU state (and
+ avoid context switches). It is generally a bad idea; use fixed point
+ arithmetic first.
+
+A rigid stack limit
+ 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).
+
+The Linux kernel is portable
+ 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.
+
+ioctls: Not writing a new system call
+=====================================
+
+A system call generally looks like this
+
+::
+
+ asmlinkage long sys_mycall(int arg)
+ {
+ return 0;
+ }
+
+
+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 ``include/asm/unistd.h`` and
+``arch/kernel/entry.S`` file, and is much more likely to be accepted by
+Linus.
+
+If all your routine does is read or write some parameter, consider
+implementing a :c:func:`sysfs()` interface instead.
+
+Inside the ioctl you're in user context to a process. When a error
+occurs you return a negated errno (see ``include/linux/errno.h``),
+otherwise you return 0.
+
+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
+``-ERESTARTSYS`` 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.
+
+::
+
+ if (signal_pending(current))
+ return -ERESTARTSYS;
+
+
+If you're doing longer computations: first think userspace. If you
+**really** 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:
+
+::
+
+ cond_resched(); /* Will sleep */
+
+
+A short note on interface design: the UNIX system call motto is "Provide
+mechanism not policy".
+
+Recipes for Deadlock
+====================
+
+You cannot call any routines which may sleep, unless:
+
+- You are in user context.
+
+- You do not own any spinlocks.
+
+- 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).
+
+Note that some functions may sleep implicitly: common ones are the user
+space access functions (\*_user) and memory allocation functions
+without ``GFP_ATOMIC``.
+
+You should always compile your kernel ``CONFIG_DEBUG_ATOMIC_SLEEP`` on,
+and it will warn you if you break these rules. If you **do** break the
+rules, you will eventually lock up your box.
+
+Really.
+
+Common Routines
+===============
+
+:c:func:`printk()` ``include/linux/kernel.h``
+---------------------------------------------
+
+:c:func:`printk()` 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:
+
+::
+
+ printk(KERN_INFO "i = %u\n", i);
+
+
+See ``include/linux/kernel.h``; for other ``KERN_`` values; these are
+interpreted by syslog as the level. Special case: for printing an IP
+address use
+
+::
+
+ __be32 ipaddress;
+ printk(KERN_INFO "my ip: %pI4\n", &ipaddress);
+
+
+:c:func:`printk()` internally uses a 1K buffer and does not catch
+overruns. Make sure that will be enough.
+
+.. note::
+
+ You will know when you are a real kernel hacker when you start
+ typoing printf as printk in your user programs :)
+
+.. note::
+
+ 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.
+
+:c:func:`copy_[to/from]_user()` / :c:func:`get_user()` / :c:func:`put_user()` ``include/linux/uaccess.h``
+---------------------------------------------------------------------------------------------------------
+
+**[SLEEPS]**
+
+:c:func:`put_user()` and :c:func:`get_user()` 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 ``-EFAULT`` or
+0.
+
+:c:func:`copy_to_user()` and :c:func:`copy_from_user()` are
+more general: they copy an arbitrary amount of data to and from
+userspace.
+
+.. warning::
+
+ Unlike :c:func:`put_user()` and :c:func:`get_user()`, they
+ return the amount of uncopied data (ie. 0 still means success).
+
+[Yes, this moronic interface makes me cringe. The flamewar comes up
+every year or so. --RR.]
+
+The functions may sleep implicitly. This should never be called outside
+user context (it makes no sense), with interrupts disabled, or a
+spinlock held.
+
+:c:func:`kmalloc()`/:c:func:`kfree()` ``include/linux/slab.h``
+--------------------------------------------------------------
+
+**[MAY SLEEP: SEE BELOW]**
+
+These routines are used to dynamically request pointer-aligned chunks of
+memory, like malloc and free do in userspace, but
+:c:func:`kmalloc()` takes an extra flag word. Important values:
+
+``GFP_KERNEL``
+ May sleep and swap to free memory. Only allowed in user context, but
+ is the most reliable way to allocate memory.
+
+``GFP_ATOMIC``
+ Don't sleep. Less reliable than ``GFP_KERNEL``, but may be called
+ from interrupt context. You should **really** have a good
+ out-of-memory error-handling strategy.
+
+``GFP_DMA``
+ Allocate ISA DMA lower than 16MB. If you don't know what that is you
+ don't need it. Very unreliable.
+
+If you see a sleeping function called from invalid context warning
+message, then maybe you called a sleeping allocation function from
+interrupt context without ``GFP_ATOMIC``. You should really fix that.
+Run, don't walk.
+
+If you are allocating at least ``PAGE_SIZE`` (``include/asm/page.h``)
+bytes, consider using :c:func:`__get_free_pages()`
+(``include/linux/mm.h``). 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.
+
+If you are allocating more than a page worth of bytes you can use
+:c:func:`vmalloc()`. It'll allocate virtual memory in the kernel
+map. This block is not contiguous in physical memory, but the MMU 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 :c:func:`alloc_bootmem()`
+routine.
+
+Before inventing your own cache of often-used objects consider using a
+slab cache in ``include/linux/slab.h``
+
+:c:func:`current()` ``include/asm/current.h``
+---------------------------------------------
+
+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 **not NULL** in interrupt context.
+
+:c:func:`mdelay()`/:c:func:`udelay()` ``include/asm/delay.h`` ``include/linux/delay.h``
+---------------------------------------------------------------------------------------
+
+The :c:func:`udelay()` and :c:func:`ndelay()` functions can be
+used for small pauses. Do not use large values with them as you risk
+overflow - the helper function :c:func:`mdelay()` is useful here, or
+consider :c:func:`msleep()`.
+
+:c:func:`cpu_to_be32()`/:c:func:`be32_to_cpu()`/:c:func:`cpu_to_le32()`/:c:func:`le32_to_cpu()` ``include/asm/byteorder.h``
+---------------------------------------------------------------------------------------------------------------------------
+
+The :c:func:`cpu_to_be32()` 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:
+:c:func:`be32_to_cpu()`, etc.
+
+There are two major variations of these functions: the pointer
+variation, such as :c:func:`cpu_to_be32p()`, which take a pointer
+to the given type, and return the converted value. The other variation
+is the "in-situ" family, such as :c:func:`cpu_to_be32s()`, which
+convert value referred to by the pointer, and return void.
+
+:c:func:`local_irq_save()`/:c:func:`local_irq_restore()` ``include/linux/irqflags.h``
+-------------------------------------------------------------------------------------
+
+These routines disable hard interrupts on the local CPU, and restore
+them. They are reentrant; saving the previous state in their one
+``unsigned long flags`` argument. If you know that interrupts are
+enabled, you can simply use :c:func:`local_irq_disable()` and
+:c:func:`local_irq_enable()`.
+
+:c:func:`local_bh_disable()`/:c:func:`local_bh_enable()` ``include/linux/interrupt.h``
+--------------------------------------------------------------------------------------
+
+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.
+
+:c:func:`smp_processor_id()`() ``include/asm/smp.h``
+----------------------------------------------------
+
+:c:func:`get_cpu()` disables preemption (so you won't suddenly get
+moved to another CPU) and returns the current processor number, between
+0 and ``NR_CPUS``. Note that the CPU numbers are not necessarily
+continuous. You return it again with :c:func:`put_cpu()` when you
+are done.
+
+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().
+
+``__init``/``__exit``/``__initdata`` ``include/linux/init.h``
+-------------------------------------------------------------
+
+After boot, the kernel frees up a special section; functions marked with
+``__init`` and data structures marked with ``__initdata`` are dropped
+after boot is complete: similarly modules discard this memory after
+initialization. ``__exit`` 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 ``__init`` to be exported to modules
+with :c:func:`EXPORT_SYMBOL()` - this will break.
+
+:c:func:`__initcall()`/:c:func:`module_init()` ``include/linux/init.h``
+-----------------------------------------------------------------------
+
+Many parts of the kernel are well served as a module
+(dynamically-loadable parts of the kernel). Using the
+:c:func:`module_init()` and :c:func:`module_exit()` macros it
+is easy to write code without #ifdefs which can operate both as a module
+or built into the kernel.
+
+The :c:func:`module_init()` 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
+:c:func:`module_init()` macro becomes equivalent to
+:c:func:`__initcall()`, which through linker magic ensures that
+the function is called on boot.
+
+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.
+
+:c:func:`module_exit()` ``include/linux/init.h``
+------------------------------------------------
+
+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.
+
+Note that this macro is optional: if it is not present, your module will
+not be removable (except for 'rmmod -f').
+
+:c:func:`try_module_get()`/:c:func:`module_put()` ``include/linux/module.h``
+----------------------------------------------------------------------------
+
+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
+:c:func:`try_module_get()` 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
+:c:func:`module_put()` when you're finished.
+
+Most registerable structures have an owner field, such as in the
+:c:type:`struct file_operations <file_operations>` structure.
+Set this field to the macro ``THIS_MODULE``.
+
+Wait Queues ``include/linux/wait.h``
+====================================
+
+**[SLEEPS]**
+
+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 ``wait_queue_head_t``, and then processes
+which want to wait for that condition declare a ``wait_queue_t``
+referring to themselves, and place that in the queue.
+
+Declaring
+---------
+
+You declare a ``wait_queue_head_t`` using the
+:c:func:`DECLARE_WAIT_QUEUE_HEAD()` macro, or using the
+:c:func:`init_waitqueue_head()` routine in your initialization
+code.
+
+Queuing
+-------
+
+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: :c:func:`wait_event_interruptible()`
+``include/linux/wait.h`` The first argument is the wait queue head, and
+the second is an expression which is evaluated; the macro returns 0 when
+this expression is true, or -ERESTARTSYS if a signal is received. The
+:c:func:`wait_event()` version ignores signals.
+
+Waking Up Queued Tasks
+----------------------
+
+Call :c:func:`wake_up()` ``include/linux/wait.h``;, which will wake
+up every process in the queue. The exception is if one has
+``TASK_EXCLUSIVE`` 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.
+
+Atomic Operations
+=================
+
+Certain operations are guaranteed atomic on all platforms. The first
+class of operations work on ``atomic_t`` ``include/asm/atomic.h``; this
+contains a signed integer (at least 32 bits long), and you must use
+these functions to manipulate or read atomic_t variables.
+:c:func:`atomic_read()` and :c:func:`atomic_set()` get and set
+the counter, :c:func:`atomic_add()`, :c:func:`atomic_sub()`,
+:c:func:`atomic_inc()`, :c:func:`atomic_dec()`, and
+:c:func:`atomic_dec_and_test()` (returns true if it was
+decremented to zero).
+
+Yes. It returns true (i.e. != 0) if the atomic variable is zero.
+
+Note that these functions are slower than normal arithmetic, and so
+should not be used unnecessarily.
+
+The second class of atomic operations is atomic bit operations on an
+``unsigned long``, defined in ``include/linux/bitops.h``. These
+operations generally take a pointer to the bit pattern, and a bit
+number: 0 is the least significant bit. :c:func:`set_bit()`,
+:c:func:`clear_bit()` and :c:func:`change_bit()` set, clear,
+and flip the given bit. :c:func:`test_and_set_bit()`,
+:c:func:`test_and_clear_bit()` and
+:c:func:`test_and_change_bit()` do the same thing, except return
+true if the bit was previously set; these are particularly useful for
+atomically setting flags.
+
+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.
+
+Symbols
+=======
+
+Within the kernel proper, the normal linking rules apply (ie. unless a
+symbol is declared to be file scope with the ``static`` 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.
+
+:c:func:`EXPORT_SYMBOL()` ``include/linux/export.h``
+----------------------------------------------------
+
+This is the classic method of exporting a symbol: dynamically loaded
+modules will be able to use the symbol as normal.
+
+:c:func:`EXPORT_SYMBOL_GPL()` ``include/linux/export.h``
+--------------------------------------------------------
+
+Similar to :c:func:`EXPORT_SYMBOL()` except that the symbols
+exported by :c:func:`EXPORT_SYMBOL_GPL()` can only be seen by
+modules with a :c:func:`MODULE_LICENSE()` 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.
+
+Routines and Conventions
+========================
+
+Double-linked lists ``include/linux/list.h``
+--------------------------------------------
+
+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.
+
+In particular, :c:func:`list_for_each_entry()` is useful.
+
+Return Conventions
+------------------
+
+For code called in user context, it's very common to defy C convention,
+and return 0 for success, and a negative error number (eg. -EFAULT) for
+failure. This can be unintuitive at first, but it's fairly widespread in
+the kernel.
+
+Using :c:func:`ERR_PTR()` ``include/linux/err.h``; to encode a
+negative error number into a pointer, and :c:func:`IS_ERR()` and
+:c:func:`PTR_ERR()` to get it back out again: avoids a separate
+pointer parameter for the error number. Icky, but in a good way.
+
+Breaking Compilation
+--------------------
+
+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 **worse**.
+
+Initializing structure members
+------------------------------
+
+The preferred method of initializing structures is to use designated
+initialisers, as defined by ISO C99, eg:
+
+::
+
+ static struct block_device_operations opt_fops = {
+ .open = opt_open,
+ .release = opt_release,
+ .ioctl = opt_ioctl,
+ .check_media_change = opt_media_change,
+ };
+
+
+This makes it easy to grep for, and makes it clear which structure
+fields are set. You should do this because it looks cool.
+
+GNU Extensions
+--------------
+
+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).
+
+- Inline functions
+
+- Statement expressions (ie. the ({ and }) constructs).
+
+- Declaring attributes of a function / variable / type
+ (__attribute__)
+
+- typeof
+
+- Zero length arrays
+
+- Macro varargs
+
+- Arithmetic on void pointers
+
+- Non-Constant initializers
+
+- Assembler Instructions (not outside arch/ and include/asm/)
+
+- Function names as strings (__func__).
+
+- __builtin_constant_p()
+
+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.
+
+C++
+---
+
+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.
+
+NUMif
+-----
+
+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.
+
+Putting Your Stuff in the Kernel
+================================
+
+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:
+
+- Figure out whose pond you've been pissing in. Look at the top of the
+ source files, inside the ``MAINTAINERS`` file, and last of all in the
+ ``CREDITS`` file. You should coordinate with this person to make sure
+ you're not duplicating effort, or trying something that's already
+ been rejected.
+
+ 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 **they** want to make a change.
+
+- Usually you want a configuration option for your kernel hack. Edit
+ ``Kconfig`` in the appropriate directory. The Config language is
+ simple to use by cut and paste, and there's complete documentation in
+ ``Documentation/kbuild/kconfig-language.txt``.
+
+ 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. **Definitely** end your
+ description with “if in doubt, say N” (or, occasionally, \`Y'); this
+ is for people who have no idea what you are talking about.
+
+- Edit the ``Makefile``: the CONFIG variables are exported here so you
+ can usually just add a "obj-$(CONFIG_xxx) += xxx.o" line. The syntax
+ is documented in ``Documentation/kbuild/makefiles.txt``.
+
+- Put yourself in ``CREDITS`` if you've done something noteworthy,
+ usually beyond a single file (your name should be at the top of the
+ source files anyway). ``MAINTAINERS`` 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.
+
+- Finally, don't forget to read
+ ``Documentation/process/submitting-patches.rst`` and possibly
+ ``Documentation/process/submitting-drivers.rst``.
+
+Kernel Cantrips
+===============
+
+Some favorites from browsing the source. Feel free to add to this list.
+
+``arch/x86/include/asm/delay.h:``
+
+::
+
+ #define ndelay(n) (__builtin_constant_p(n) ? \
+ ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \
+ __ndelay(n))
+
+
+``include/linux/fs.h``:
+
+::
+
+ /*
+ * 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))
+
+``arch/x86/include/asm/uaccess_32.h:``
+
+::
+
+ #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)))
+
+
+``arch/sparc/kernel/head.S:``
+
+::
+
+ /*
+ * 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"
+
+
+``arch/sparc/lib/checksum.S:``
+
+::
+
+ /* 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.
+ */
+
+
+Thanks
+======
+
+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 :c:func:`disable_irq()`, and Jes
+Sorensen and Andrea Arcangeli added caveats. Michael Elizabeth Chastain
+for checking and adding to the Configure section. Telsa Gwynne for
+teaching me DocBook.
projects / GitHub / LineageOS / android_kernel_motorola_exynos9610.git / commitdiff