1328 строки
43 KiB
XML
1328 строки
43 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<book id="lk-hacking-guide">
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<bookinfo>
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<title>Unreliable Guide To Hacking The Linux Kernel</title>
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<authorgroup>
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<author>
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<firstname>Rusty</firstname>
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<surname>Russell</surname>
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<affiliation>
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<address>
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<email>rusty@rustcorp.com.au</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<copyright>
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<year>2005</year>
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<holder>Rusty Russell</holder>
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</copyright>
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<legalnotice>
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<para>
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This documentation is free software; you can redistribute
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it and/or modify it under the terms of the GNU General Public
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License as published by the Free Software Foundation; either
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version 2 of the License, or (at your option) any later
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version.
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</para>
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<para>
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This program is distributed in the hope that it will be
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useful, but WITHOUT ANY WARRANTY; without even the implied
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warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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See the GNU General Public License for more details.
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</para>
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<para>
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You should have received a copy of the GNU General Public
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License along with this program; if not, write to the Free
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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MA 02111-1307 USA
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</para>
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<para>
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For more details see the file COPYING in the source
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distribution of Linux.
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</para>
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</legalnotice>
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<releaseinfo>
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This is the first release of this document as part of the kernel tarball.
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</releaseinfo>
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</bookinfo>
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<toc></toc>
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<chapter id="introduction">
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<title>Introduction</title>
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<para>
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Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux
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Kernel Hacking. This document describes the common routines and
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general requirements for kernel code: its goal is to serve as a
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primer for Linux kernel development for experienced C
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programmers. I avoid implementation details: that's what the
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code is for, and I ignore whole tracts of useful routines.
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</para>
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<para>
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Before you read this, please understand that I never wanted to
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write this document, being grossly under-qualified, but I always
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wanted to read it, and this was the only way. I hope it will
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grow into a compendium of best practice, common starting points
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and random information.
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</para>
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</chapter>
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<chapter id="basic-players">
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<title>The Players</title>
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<para>
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At any time each of the CPUs in a system can be:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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not associated with any process, serving a hardware interrupt;
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</para>
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</listitem>
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<listitem>
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<para>
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not associated with any process, serving a softirq or tasklet;
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</para>
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</listitem>
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<listitem>
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<para>
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running in kernel space, associated with a process (user context);
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</para>
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</listitem>
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<listitem>
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<para>
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running a process in user space.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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There is an ordering between these. The bottom two can preempt
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each other, but above that is a strict hierarchy: each can only be
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preempted by the ones above it. For example, while a softirq is
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running on a CPU, no other softirq will preempt it, but a hardware
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interrupt can. However, any other CPUs in the system execute
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independently.
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</para>
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<para>
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We'll see a number of ways that the user context can block
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interrupts, to become truly non-preemptable.
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</para>
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<sect1 id="basics-usercontext">
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<title>User Context</title>
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<para>
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User context is when you are coming in from a system call or other
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trap: like userspace, you can be preempted by more important tasks
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and by interrupts. You can sleep, by calling
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<function>schedule()</function>.
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</para>
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<note>
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<para>
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You are always in user context on module load and unload,
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and on operations on the block device layer.
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</para>
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</note>
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<para>
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In user context, the <varname>current</varname> pointer (indicating
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the task we are currently executing) is valid, and
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<function>in_interrupt()</function>
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(<filename>include/linux/interrupt.h</filename>) is <returnvalue>false
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</returnvalue>.
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</para>
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<caution>
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<para>
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Beware that if you have preemption or softirqs disabled
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(see below), <function>in_interrupt()</function> will return a
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false positive.
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</para>
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</caution>
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</sect1>
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<sect1 id="basics-hardirqs">
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<title>Hardware Interrupts (Hard IRQs)</title>
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<para>
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Timer ticks, <hardware>network cards</hardware> and
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<hardware>keyboard</hardware> are examples of real
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hardware which produce interrupts at any time. The kernel runs
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interrupt handlers, which services the hardware. The kernel
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guarantees that this handler is never re-entered: if the same
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interrupt arrives, it is queued (or dropped). Because it
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disables interrupts, this handler has to be fast: frequently it
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simply acknowledges the interrupt, marks a 'software interrupt'
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for execution and exits.
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</para>
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<para>
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You can tell you are in a hardware interrupt, because
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<function>in_irq()</function> returns <returnvalue>true</returnvalue>.
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</para>
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<caution>
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<para>
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Beware that this will return a false positive if interrupts are disabled
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(see below).
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</para>
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</caution>
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</sect1>
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<sect1 id="basics-softirqs">
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<title>Software Interrupt Context: Softirqs and Tasklets</title>
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<para>
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Whenever a system call is about to return to userspace, or a
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hardware interrupt handler exits, any 'software interrupts'
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which are marked pending (usually by hardware interrupts) are
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run (<filename>kernel/softirq.c</filename>).
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</para>
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<para>
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Much of the real interrupt handling work is done here. Early in
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the transition to <acronym>SMP</acronym>, there were only 'bottom
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halves' (BHs), which didn't take advantage of multiple CPUs. Shortly
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after we switched from wind-up computers made of match-sticks and snot,
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we abandoned this limitation and switched to 'softirqs'.
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</para>
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<para>
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<filename class="headerfile">include/linux/interrupt.h</filename> lists the
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different softirqs. A very important softirq is the
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timer softirq (<filename
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class="headerfile">include/linux/timer.h</filename>): you can
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register to have it call functions for you in a given length of
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time.
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</para>
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<para>
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Softirqs are often a pain to deal with, since the same softirq
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will run simultaneously on more than one CPU. For this reason,
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tasklets (<filename
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class="headerfile">include/linux/interrupt.h</filename>) are more
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often used: they are dynamically-registrable (meaning you can have
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as many as you want), and they also guarantee that any tasklet
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will only run on one CPU at any time, although different tasklets
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can run simultaneously.
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</para>
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<caution>
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<para>
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The name 'tasklet' is misleading: they have nothing to do with 'tasks',
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and probably more to do with some bad vodka Alexey Kuznetsov had at the
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time.
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</para>
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</caution>
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<para>
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You can tell you are in a softirq (or tasklet)
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using the <function>in_softirq()</function> macro
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(<filename class="headerfile">include/linux/interrupt.h</filename>).
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</para>
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<caution>
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<para>
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Beware that this will return a false positive if a bh lock (see below)
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is held.
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</para>
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</caution>
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</sect1>
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</chapter>
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<chapter id="basic-rules">
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<title>Some Basic Rules</title>
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<variablelist>
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<varlistentry>
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<term>No memory protection</term>
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<listitem>
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<para>
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If you corrupt memory, whether in user context or
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interrupt context, the whole machine will crash. Are you
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sure you can't do what you want in userspace?
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>No floating point or <acronym>MMX</acronym></term>
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<listitem>
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<para>
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The <acronym>FPU</acronym> context is not saved; even in user
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context the <acronym>FPU</acronym> state probably won't
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correspond with the current process: you would mess with some
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user process' <acronym>FPU</acronym> state. If you really want
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to do this, you would have to explicitly save/restore the full
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<acronym>FPU</acronym> state (and avoid context switches). It
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is generally a bad idea; use fixed point arithmetic first.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>A rigid stack limit</term>
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<listitem>
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<para>
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Depending on configuration options the kernel stack is about 3K to 6K for most 32-bit architectures: it's
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about 14K on most 64-bit archs, and often shared with interrupts
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so you can't use it all. Avoid deep recursion and huge local
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arrays on the stack (allocate them dynamically instead).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>The Linux kernel is portable</term>
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<listitem>
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<para>
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Let's keep it that way. Your code should be 64-bit clean,
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and endian-independent. You should also minimize CPU
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specific stuff, e.g. inline assembly should be cleanly
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encapsulated and minimized to ease porting. Generally it
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should be restricted to the architecture-dependent part of
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the kernel tree.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</chapter>
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<chapter id="ioctls">
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<title>ioctls: Not writing a new system call</title>
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<para>
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A system call generally looks like this
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</para>
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<programlisting>
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asmlinkage long sys_mycall(int arg)
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{
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return 0;
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}
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</programlisting>
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<para>
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First, in most cases you don't want to create a new system call.
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You create a character device and implement an appropriate ioctl
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for it. This is much more flexible than system calls, doesn't have
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to be entered in every architecture's
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<filename class="headerfile">include/asm/unistd.h</filename> and
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<filename>arch/kernel/entry.S</filename> file, and is much more
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likely to be accepted by Linus.
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</para>
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<para>
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If all your routine does is read or write some parameter, consider
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implementing a <function>sysfs</function> interface instead.
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</para>
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<para>
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Inside the ioctl you're in user context to a process. When a
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error occurs you return a negated errno (see
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<filename class="headerfile">include/linux/errno.h</filename>),
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otherwise you return <returnvalue>0</returnvalue>.
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</para>
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<para>
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After you slept you should check if a signal occurred: the
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Unix/Linux way of handling signals is to temporarily exit the
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system call with the <constant>-ERESTARTSYS</constant> error. The
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system call entry code will switch back to user context, process
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the signal handler and then your system call will be restarted
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(unless the user disabled that). So you should be prepared to
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process the restart, e.g. if you're in the middle of manipulating
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some data structure.
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</para>
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<programlisting>
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if (signal_pending())
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return -ERESTARTSYS;
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</programlisting>
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<para>
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If you're doing longer computations: first think userspace. If you
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<emphasis>really</emphasis> want to do it in kernel you should
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regularly check if you need to give up the CPU (remember there is
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cooperative multitasking per CPU). Idiom:
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</para>
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<programlisting>
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cond_resched(); /* Will sleep */
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</programlisting>
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<para>
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A short note on interface design: the UNIX system call motto is
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"Provide mechanism not policy".
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</para>
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</chapter>
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<chapter id="deadlock-recipes">
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<title>Recipes for Deadlock</title>
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<para>
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You cannot call any routines which may sleep, unless:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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You are in user context.
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</para>
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</listitem>
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<listitem>
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<para>
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You do not own any spinlocks.
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</para>
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</listitem>
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<listitem>
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<para>
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You have interrupts enabled (actually, Andi Kleen says
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that the scheduling code will enable them for you, but
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that's probably not what you wanted).
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</para>
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</listitem>
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</itemizedlist>
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<para>
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Note that some functions may sleep implicitly: common ones are
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the user space access functions (*_user) and memory allocation
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functions without <symbol>GFP_ATOMIC</symbol>.
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</para>
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<para>
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You should always compile your kernel
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<symbol>CONFIG_DEBUG_SPINLOCK_SLEEP</symbol> on, and it will warn
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you if you break these rules. If you <emphasis>do</emphasis> break
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the rules, you will eventually lock up your box.
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</para>
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<para>
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Really.
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</para>
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</chapter>
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<chapter id="common-routines">
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<title>Common Routines</title>
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<sect1 id="routines-printk">
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<title>
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<function>printk()</function>
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<filename class="headerfile">include/linux/kernel.h</filename>
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</title>
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<para>
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<function>printk()</function> feeds kernel messages to the
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console, dmesg, and the syslog daemon. It is useful for debugging
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and reporting errors, and can be used inside interrupt context,
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but use with caution: a machine which has its console flooded with
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printk messages is unusable. It uses a format string mostly
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compatible with ANSI C printf, and C string concatenation to give
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it a first "priority" argument:
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</para>
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<programlisting>
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printk(KERN_INFO "i = %u\n", i);
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</programlisting>
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<para>
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See <filename class="headerfile">include/linux/kernel.h</filename>;
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for other KERN_ values; these are interpreted by syslog as the
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level. Special case: for printing an IP address use
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</para>
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<programlisting>
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__u32 ipaddress;
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printk(KERN_INFO "my ip: %d.%d.%d.%d\n", NIPQUAD(ipaddress));
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</programlisting>
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<para>
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<function>printk()</function> internally uses a 1K buffer and does
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not catch overruns. Make sure that will be enough.
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</para>
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|
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<note>
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<para>
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You will know when you are a real kernel hacker
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when you start typoing printf as printk in your user programs :)
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</para>
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</note>
|
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|
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<!--- From the Lions book reader department -->
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<note>
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<para>
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Another sidenote: the original Unix Version 6 sources had a
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comment on top of its printf function: "Printf should not be
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used for chit-chat". You should follow that advice.
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</para>
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</note>
|
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</sect1>
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|
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<sect1 id="routines-copy">
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<title>
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<function>copy_[to/from]_user()</function>
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/
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<function>get_user()</function>
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/
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<function>put_user()</function>
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<filename class="headerfile">include/asm/uaccess.h</filename>
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</title>
|
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|
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<para>
|
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<emphasis>[SLEEPS]</emphasis>
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</para>
|
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|
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<para>
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<function>put_user()</function> and <function>get_user()</function>
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are used to get and put single values (such as an int, char, or
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long) from and to userspace. A pointer into userspace should
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never be simply dereferenced: data should be copied using these
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routines. Both return <constant>-EFAULT</constant> or 0.
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</para>
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<para>
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<function>copy_to_user()</function> and
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<function>copy_from_user()</function> are more general: they copy
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an arbitrary amount of data to and from userspace.
|
|
<caution>
|
|
<para>
|
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Unlike <function>put_user()</function> and
|
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<function>get_user()</function>, they return the amount of
|
|
uncopied data (ie. <returnvalue>0</returnvalue> still means
|
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success).
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|
</para>
|
|
</caution>
|
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[Yes, this moronic interface makes me cringe. The flamewar comes up every year or so. --RR.]
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</para>
|
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<para>
|
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The functions may sleep implicitly. This should never be called
|
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outside user context (it makes no sense), with interrupts
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disabled, or a spinlock held.
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</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>
|
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|
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<para>
|
|
These routines are used to dynamically request pointer-aligned
|
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chunks of memory, like malloc and free do in userspace, but
|
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<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>
|
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|
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<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/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 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>
|
|
<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/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/module.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/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. It implies that the function is considered
|
|
an internal implementation issue, and not really an interface.
|
|
</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">
|
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<title>GNU Extensions</title>
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<para>
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GNU Extensions are explicitly allowed in the Linux kernel.
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Note that some of the more complex ones are not very well
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supported, due to lack of general use, but the following are
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considered standard (see the GCC info page section "C
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Extensions" for more details - Yes, really the info page, the
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man page is only a short summary of the stuff in info).
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</para>
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<itemizedlist>
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<listitem>
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<para>
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Inline functions
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</para>
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</listitem>
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<listitem>
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<para>
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Statement expressions (ie. the ({ and }) constructs).
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</para>
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</listitem>
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<listitem>
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<para>
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Declaring attributes of a function / variable / type
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(__attribute__)
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</para>
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</listitem>
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<listitem>
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<para>
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typeof
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</para>
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</listitem>
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<listitem>
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<para>
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Zero length arrays
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</para>
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</listitem>
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<listitem>
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<para>
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Macro varargs
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</para>
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</listitem>
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<listitem>
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<para>
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Arithmetic on void pointers
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</para>
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</listitem>
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<listitem>
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<para>
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Non-Constant initializers
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</para>
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</listitem>
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<listitem>
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<para>
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Assembler Instructions (not outside arch/ and include/asm/)
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</para>
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</listitem>
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<listitem>
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<para>
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Function names as strings (__func__).
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</para>
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</listitem>
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<listitem>
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<para>
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__builtin_constant_p()
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</para>
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</listitem>
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</itemizedlist>
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<para>
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Be wary when using long long in the kernel, the code gcc generates for
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it is horrible and worse: division and multiplication does not work
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on i386 because the GCC runtime functions for it are missing from
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the kernel environment.
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</para>
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<!-- FIXME: add a note about ANSI aliasing cleanness -->
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</sect1>
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<sect1 id="conventions-cplusplus">
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<title>C++</title>
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<para>
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Using C++ in the kernel is usually a bad idea, because the
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kernel does not provide the necessary runtime environment
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and the include files are not tested for it. It is still
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possible, but not recommended. If you really want to do
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this, forget about exceptions at least.
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</para>
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</sect1>
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<sect1 id="conventions-ifdef">
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<title>#if</title>
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<para>
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It is generally considered cleaner to use macros in header files
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(or at the top of .c files) to abstract away functions rather than
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using `#if' pre-processor statements throughout the source code.
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</para>
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</sect1>
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</chapter>
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<chapter id="submitting">
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<title>Putting Your Stuff in the Kernel</title>
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<para>
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In order to get your stuff into shape for official inclusion, or
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even to make a neat patch, there's administrative work to be
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done:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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Figure out whose pond you've been pissing in. Look at the top of
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the source files, inside the <filename>MAINTAINERS</filename>
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file, and last of all in the <filename>CREDITS</filename> file.
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You should coordinate with this person to make sure you're not
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duplicating effort, or trying something that's already been
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rejected.
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</para>
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<para>
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Make sure you put your name and EMail address at the top of
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any files you create or mangle significantly. This is the
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first place people will look when they find a bug, or when
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<emphasis>they</emphasis> want to make a change.
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</para>
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</listitem>
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<listitem>
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<para>
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Usually you want a configuration option for your kernel hack.
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Edit <filename>Kconfig</filename> in the appropriate directory.
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The Config language is simple to use by cut and paste, and there's
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complete documentation in
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<filename>Documentation/kbuild/kconfig-language.txt</filename>.
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</para>
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<para>
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You may well want to make your CONFIG option only visible if
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<symbol>CONFIG_EXPERIMENTAL</symbol> is enabled: this serves as a
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warning to users. There many other fancy things you can do: see
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the various <filename>Kconfig</filename> files for ideas.
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</para>
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<para>
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In your description of the option, make sure you address both the
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expert user and the user who knows nothing about your feature. Mention
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incompatibilities and issues here. <emphasis> Definitely
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</emphasis> end your description with <quote> if in doubt, say N
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</quote> (or, occasionally, `Y'); this is for people who have no
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idea what you are talking about.
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</para>
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</listitem>
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<listitem>
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<para>
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Edit the <filename>Makefile</filename>: the CONFIG variables are
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exported here so you can usually just add a "obj-$(CONFIG_xxx) +=
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xxx.o" line. The syntax is documented in
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<filename>Documentation/kbuild/makefiles.txt</filename>.
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</para>
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</listitem>
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|
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<listitem>
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<para>
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Put yourself in <filename>CREDITS</filename> if you've done
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something noteworthy, usually beyond a single file (your name
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should be at the top of the source files anyway).
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<filename>MAINTAINERS</filename> means you want to be consulted
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when changes are made to a subsystem, and hear about bugs; it
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implies a more-than-passing commitment to some part of the code.
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</para>
|
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</listitem>
|
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|
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<listitem>
|
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<para>
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Finally, don't forget to read <filename>Documentation/SubmittingPatches</filename>
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and possibly <filename>Documentation/SubmittingDrivers</filename>.
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</para>
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</listitem>
|
|
</itemizedlist>
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</chapter>
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<chapter id="cantrips">
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<title>Kernel Cantrips</title>
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<para>
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Some favorites from browsing the source. Feel free to add to this
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list.
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</para>
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|
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<para>
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<filename>arch/x86/include/asm/delay.h:</filename>
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</para>
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<programlisting>
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#define ndelay(n) (__builtin_constant_p(n) ? \
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((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \
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__ndelay(n))
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</programlisting>
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|
|
<para>
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<filename>include/linux/fs.h</filename>:
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</para>
|
|
<programlisting>
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/*
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* Kernel pointers have redundant information, so we can use a
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* scheme where we can return either an error code or a dentry
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* pointer with the same return value.
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*
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* This should be a per-architecture thing, to allow different
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* error and pointer decisions.
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*/
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#define ERR_PTR(err) ((void *)((long)(err)))
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#define PTR_ERR(ptr) ((long)(ptr))
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#define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000))
|
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</programlisting>
|
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|
|
<para>
|
|
<filename>arch/x86/include/asm/uaccess_32.h:</filename>
|
|
</para>
|
|
|
|
<programlisting>
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|
#define copy_to_user(to,from,n) \
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(__builtin_constant_p(n) ? \
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__constant_copy_to_user((to),(from),(n)) : \
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__generic_copy_to_user((to),(from),(n)))
|
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</programlisting>
|
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|
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<para>
|
|
<filename>arch/sparc/kernel/head.S:</filename>
|
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</para>
|
|
|
|
<programlisting>
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/*
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* Sun people can't spell worth damn. "compatability" indeed.
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* At least we *know* we can't spell, and use a spell-checker.
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*/
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/* Uh, actually Linus it is I who cannot spell. Too much murky
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* Sparc assembly will do this to ya.
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*/
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C_LABEL(cputypvar):
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.asciz "compatability"
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/* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */
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.align 4
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C_LABEL(cputypvar_sun4m):
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.asciz "compatible"
|
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</programlisting>
|
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|
|
<para>
|
|
<filename>arch/sparc/lib/checksum.S:</filename>
|
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</para>
|
|
|
|
<programlisting>
|
|
/* Sun, you just can't beat me, you just can't. Stop trying,
|
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* give up. I'm serious, I am going to kick the living shit
|
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* out of you, game over, lights out.
|
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*/
|
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</programlisting>
|
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</chapter>
|
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|
|
<chapter id="credits">
|
|
<title>Thanks</title>
|
|
|
|
<para>
|
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Thanks to Andi Kleen for the idea, answering my questions, fixing
|
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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>
|
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|