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1 <?xml version="1.0" encoding="UTF-8"?>
2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
5 <book id="lk-hacking-guide">
6 <bookinfo>
7 <title>Unreliable Guide To Hacking The Linux Kernel</title>
9 <authorgroup>
10 <author>
11 <firstname>Rusty</firstname>
12 <surname>Russell</surname>
13 <affiliation>
14 <address>
15 <email>rusty@rustcorp.com.au</email>
16 </address>
17 </affiliation>
18 </author>
19 </authorgroup>
21 <copyright>
22 <year>2005</year>
23 <holder>Rusty Russell</holder>
24 </copyright>
26 <legalnotice>
27 <para>
28 This documentation is free software; you can redistribute
29 it and/or modify it under the terms of the GNU General Public
30 License as published by the Free Software Foundation; either
31 version 2 of the License, or (at your option) any later
32 version.
33 </para>
35 <para>
36 This program is distributed in the hope that it will be
37 useful, but WITHOUT ANY WARRANTY; without even the implied
38 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
39 See the GNU General Public License for more details.
40 </para>
42 <para>
43 You should have received a copy of the GNU General Public
44 License along with this program; if not, write to the Free
45 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
46 MA 02111-1307 USA
47 </para>
49 <para>
50 For more details see the file COPYING in the source
51 distribution of Linux.
52 </para>
53 </legalnotice>
55 <releaseinfo>
56 This is the first release of this document as part of the kernel tarball.
57 </releaseinfo>
59 </bookinfo>
61 <toc></toc>
63 <chapter id="introduction">
64 <title>Introduction</title>
65 <para>
66 Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux
67 Kernel Hacking. This document describes the common routines and
68 general requirements for kernel code: its goal is to serve as a
69 primer for Linux kernel development for experienced C
70 programmers. I avoid implementation details: that's what the
71 code is for, and I ignore whole tracts of useful routines.
72 </para>
73 <para>
74 Before you read this, please understand that I never wanted to
75 write this document, being grossly under-qualified, but I always
76 wanted to read it, and this was the only way. I hope it will
77 grow into a compendium of best practice, common starting points
78 and random information.
79 </para>
80 </chapter>
82 <chapter id="basic-players">
83 <title>The Players</title>
85 <para>
86 At any time each of the CPUs in a system can be:
87 </para>
89 <itemizedlist>
90 <listitem>
91 <para>
92 not associated with any process, serving a hardware interrupt;
93 </para>
94 </listitem>
96 <listitem>
97 <para>
98 not associated with any process, serving a softirq or tasklet;
99 </para>
100 </listitem>
102 <listitem>
103 <para>
104 running in kernel space, associated with a process (user context);
105 </para>
106 </listitem>
108 <listitem>
109 <para>
110 running a process in user space.
111 </para>
112 </listitem>
113 </itemizedlist>
115 <para>
116 There is an ordering between these. The bottom two can preempt
117 each other, but above that is a strict hierarchy: each can only be
118 preempted by the ones above it. For example, while a softirq is
119 running on a CPU, no other softirq will preempt it, but a hardware
120 interrupt can. However, any other CPUs in the system execute
121 independently.
122 </para>
124 <para>
125 We'll see a number of ways that the user context can block
126 interrupts, to become truly non-preemptable.
127 </para>
129 <sect1 id="basics-usercontext">
130 <title>User Context</title>
132 <para>
133 User context is when you are coming in from a system call or other
134 trap: like userspace, you can be preempted by more important tasks
135 and by interrupts. You can sleep, by calling
136 <function>schedule()</function>.
137 </para>
139 <note>
140 <para>
141 You are always in user context on module load and unload,
142 and on operations on the block device layer.
143 </para>
144 </note>
146 <para>
147 In user context, the <varname>current</varname> pointer (indicating
148 the task we are currently executing) is valid, and
149 <function>in_interrupt()</function>
150 (<filename>include/linux/interrupt.h</filename>) is <returnvalue>false
151 </returnvalue>.
152 </para>
154 <caution>
155 <para>
156 Beware that if you have preemption or softirqs disabled
157 (see below), <function>in_interrupt()</function> will return a
158 false positive.
159 </para>
160 </caution>
161 </sect1>
163 <sect1 id="basics-hardirqs">
164 <title>Hardware Interrupts (Hard IRQs)</title>
166 <para>
167 Timer ticks, <hardware>network cards</hardware> and
168 <hardware>keyboard</hardware> are examples of real
169 hardware which produce interrupts at any time. The kernel runs
170 interrupt handlers, which services the hardware. The kernel
171 guarantees that this handler is never re-entered: if the same
172 interrupt arrives, it is queued (or dropped). Because it
173 disables interrupts, this handler has to be fast: frequently it
174 simply acknowledges the interrupt, marks a 'software interrupt'
175 for execution and exits.
176 </para>
178 <para>
179 You can tell you are in a hardware interrupt, because
180 <function>in_irq()</function> returns <returnvalue>true</returnvalue>.
181 </para>
182 <caution>
183 <para>
184 Beware that this will return a false positive if interrupts are disabled
185 (see below).
186 </para>
187 </caution>
188 </sect1>
190 <sect1 id="basics-softirqs">
191 <title>Software Interrupt Context: Softirqs and Tasklets</title>
193 <para>
194 Whenever a system call is about to return to userspace, or a
195 hardware interrupt handler exits, any 'software interrupts'
196 which are marked pending (usually by hardware interrupts) are
197 run (<filename>kernel/softirq.c</filename>).
198 </para>
200 <para>
201 Much of the real interrupt handling work is done here. Early in
202 the transition to <acronym>SMP</acronym>, there were only 'bottom
203 halves' (BHs), which didn't take advantage of multiple CPUs. Shortly
204 after we switched from wind-up computers made of match-sticks and snot,
205 we abandoned this limitation and switched to 'softirqs'.
206 </para>
208 <para>
209 <filename class="headerfile">include/linux/interrupt.h</filename> lists the
210 different softirqs. A very important softirq is the
211 timer softirq (<filename
212 class="headerfile">include/linux/timer.h</filename>): you can
213 register to have it call functions for you in a given length of
214 time.
215 </para>
217 <para>
218 Softirqs are often a pain to deal with, since the same softirq
219 will run simultaneously on more than one CPU. For this reason,
220 tasklets (<filename
221 class="headerfile">include/linux/interrupt.h</filename>) are more
222 often used: they are dynamically-registrable (meaning you can have
223 as many as you want), and they also guarantee that any tasklet
224 will only run on one CPU at any time, although different tasklets
225 can run simultaneously.
226 </para>
227 <caution>
228 <para>
229 The name 'tasklet' is misleading: they have nothing to do with 'tasks',
230 and probably more to do with some bad vodka Alexey Kuznetsov had at the
231 time.
232 </para>
233 </caution>
235 <para>
236 You can tell you are in a softirq (or tasklet)
237 using the <function>in_softirq()</function> macro
238 (<filename class="headerfile">include/linux/interrupt.h</filename>).
239 </para>
240 <caution>
241 <para>
242 Beware that this will return a false positive if a bh lock (see below)
243 is held.
244 </para>
245 </caution>
246 </sect1>
247 </chapter>
249 <chapter id="basic-rules">
250 <title>Some Basic Rules</title>
252 <variablelist>
253 <varlistentry>
254 <term>No memory protection</term>
255 <listitem>
256 <para>
257 If you corrupt memory, whether in user context or
258 interrupt context, the whole machine will crash. Are you
259 sure you can't do what you want in userspace?
260 </para>
261 </listitem>
262 </varlistentry>
264 <varlistentry>
265 <term>No floating point or <acronym>MMX</acronym></term>
266 <listitem>
267 <para>
268 The <acronym>FPU</acronym> context is not saved; even in user
269 context the <acronym>FPU</acronym> state probably won't
270 correspond with the current process: you would mess with some
271 user process' <acronym>FPU</acronym> state. If you really want
272 to do this, you would have to explicitly save/restore the full
273 <acronym>FPU</acronym> state (and avoid context switches). It
274 is generally a bad idea; use fixed point arithmetic first.
275 </para>
276 </listitem>
277 </varlistentry>
279 <varlistentry>
280 <term>A rigid stack limit</term>
281 <listitem>
282 <para>
283 Depending on configuration options the kernel stack is about 3K to 6K for most 32-bit architectures: it's
284 about 14K on most 64-bit archs, and often shared with interrupts
285 so you can't use it all. Avoid deep recursion and huge local
286 arrays on the stack (allocate them dynamically instead).
287 </para>
288 </listitem>
289 </varlistentry>
291 <varlistentry>
292 <term>The Linux kernel is portable</term>
293 <listitem>
294 <para>
295 Let's keep it that way. Your code should be 64-bit clean,
296 and endian-independent. You should also minimize CPU
297 specific stuff, e.g. inline assembly should be cleanly
298 encapsulated and minimized to ease porting. Generally it
299 should be restricted to the architecture-dependent part of
300 the kernel tree.
301 </para>
302 </listitem>
303 </varlistentry>
304 </variablelist>
305 </chapter>
307 <chapter id="ioctls">
308 <title>ioctls: Not writing a new system call</title>
310 <para>
311 A system call generally looks like this
312 </para>
314 <programlisting>
315 asmlinkage long sys_mycall(int arg)
317 return 0;
319 </programlisting>
321 <para>
322 First, in most cases you don't want to create a new system call.
323 You create a character device and implement an appropriate ioctl
324 for it. This is much more flexible than system calls, doesn't have
325 to be entered in every architecture's
326 <filename class="headerfile">include/asm/unistd.h</filename> and
327 <filename>arch/kernel/entry.S</filename> file, and is much more
328 likely to be accepted by Linus.
329 </para>
331 <para>
332 If all your routine does is read or write some parameter, consider
333 implementing a <function>sysfs</function> interface instead.
334 </para>
336 <para>
337 Inside the ioctl you're in user context to a process. When a
338 error occurs you return a negated errno (see
339 <filename class="headerfile">include/linux/errno.h</filename>),
340 otherwise you return <returnvalue>0</returnvalue>.
341 </para>
343 <para>
344 After you slept you should check if a signal occurred: the
345 Unix/Linux way of handling signals is to temporarily exit the
346 system call with the <constant>-ERESTARTSYS</constant> error. The
347 system call entry code will switch back to user context, process
348 the signal handler and then your system call will be restarted
349 (unless the user disabled that). So you should be prepared to
350 process the restart, e.g. if you're in the middle of manipulating
351 some data structure.
352 </para>
354 <programlisting>
355 if (signal_pending())
356 return -ERESTARTSYS;
357 </programlisting>
359 <para>
360 If you're doing longer computations: first think userspace. If you
361 <emphasis>really</emphasis> want to do it in kernel you should
362 regularly check if you need to give up the CPU (remember there is
363 cooperative multitasking per CPU). Idiom:
364 </para>
366 <programlisting>
367 cond_resched(); /* Will sleep */
368 </programlisting>
370 <para>
371 A short note on interface design: the UNIX system call motto is
372 "Provide mechanism not policy".
373 </para>
374 </chapter>
376 <chapter id="deadlock-recipes">
377 <title>Recipes for Deadlock</title>
379 <para>
380 You cannot call any routines which may sleep, unless:
381 </para>
382 <itemizedlist>
383 <listitem>
384 <para>
385 You are in user context.
386 </para>
387 </listitem>
389 <listitem>
390 <para>
391 You do not own any spinlocks.
392 </para>
393 </listitem>
395 <listitem>
396 <para>
397 You have interrupts enabled (actually, Andi Kleen says
398 that the scheduling code will enable them for you, but
399 that's probably not what you wanted).
400 </para>
401 </listitem>
402 </itemizedlist>
404 <para>
405 Note that some functions may sleep implicitly: common ones are
406 the user space access functions (*_user) and memory allocation
407 functions without <symbol>GFP_ATOMIC</symbol>.
408 </para>
410 <para>
411 You should always compile your kernel
412 <symbol>CONFIG_DEBUG_SPINLOCK_SLEEP</symbol> on, and it will warn
413 you if you break these rules. If you <emphasis>do</emphasis> break
414 the rules, you will eventually lock up your box.
415 </para>
417 <para>
418 Really.
419 </para>
420 </chapter>
422 <chapter id="common-routines">
423 <title>Common Routines</title>
425 <sect1 id="routines-printk">
426 <title>
427 <function>printk()</function>
428 <filename class="headerfile">include/linux/kernel.h</filename>
429 </title>
431 <para>
432 <function>printk()</function> feeds kernel messages to the
433 console, dmesg, and the syslog daemon. It is useful for debugging
434 and reporting errors, and can be used inside interrupt context,
435 but use with caution: a machine which has its console flooded with
436 printk messages is unusable. It uses a format string mostly
437 compatible with ANSI C printf, and C string concatenation to give
438 it a first "priority" argument:
439 </para>
441 <programlisting>
442 printk(KERN_INFO "i = %u\n", i);
443 </programlisting>
445 <para>
446 See <filename class="headerfile">include/linux/kernel.h</filename>;
447 for other KERN_ values; these are interpreted by syslog as the
448 level. Special case: for printing an IP address use
449 </para>
451 <programlisting>
452 __u32 ipaddress;
453 printk(KERN_INFO "my ip: %d.%d.%d.%d\n", NIPQUAD(ipaddress));
454 </programlisting>
456 <para>
457 <function>printk()</function> internally uses a 1K buffer and does
458 not catch overruns. Make sure that will be enough.
459 </para>
461 <note>
462 <para>
463 You will know when you are a real kernel hacker
464 when you start typoing printf as printk in your user programs :)
465 </para>
466 </note>
468 <!--- From the Lions book reader department -->
470 <note>
471 <para>
472 Another sidenote: the original Unix Version 6 sources had a
473 comment on top of its printf function: "Printf should not be
474 used for chit-chat". You should follow that advice.
475 </para>
476 </note>
477 </sect1>
479 <sect1 id="routines-copy">
480 <title>
481 <function>copy_[to/from]_user()</function>
483 <function>get_user()</function>
485 <function>put_user()</function>
486 <filename class="headerfile">include/asm/uaccess.h</filename>
487 </title>
489 <para>
490 <emphasis>[SLEEPS]</emphasis>
491 </para>
493 <para>
494 <function>put_user()</function> and <function>get_user()</function>
495 are used to get and put single values (such as an int, char, or
496 long) from and to userspace. A pointer into userspace should
497 never be simply dereferenced: data should be copied using these
498 routines. Both return <constant>-EFAULT</constant> or 0.
499 </para>
500 <para>
501 <function>copy_to_user()</function> and
502 <function>copy_from_user()</function> are more general: they copy
503 an arbitrary amount of data to and from userspace.
504 <caution>
505 <para>
506 Unlike <function>put_user()</function> and
507 <function>get_user()</function>, they return the amount of
508 uncopied data (ie. <returnvalue>0</returnvalue> still means
509 success).
510 </para>
511 </caution>
512 [Yes, this moronic interface makes me cringe. The flamewar comes up every year or so. --RR.]
513 </para>
514 <para>
515 The functions may sleep implicitly. This should never be called
516 outside user context (it makes no sense), with interrupts
517 disabled, or a spinlock held.
518 </para>
519 </sect1>
521 <sect1 id="routines-kmalloc">
522 <title><function>kmalloc()</function>/<function>kfree()</function>
523 <filename class="headerfile">include/linux/slab.h</filename></title>
525 <para>
526 <emphasis>[MAY SLEEP: SEE BELOW]</emphasis>
527 </para>
529 <para>
530 These routines are used to dynamically request pointer-aligned
531 chunks of memory, like malloc and free do in userspace, but
532 <function>kmalloc()</function> takes an extra flag word.
533 Important values:
534 </para>
536 <variablelist>
537 <varlistentry>
538 <term>
539 <constant>
540 GFP_KERNEL
541 </constant>
542 </term>
543 <listitem>
544 <para>
545 May sleep and swap to free memory. Only allowed in user
546 context, but is the most reliable way to allocate memory.
547 </para>
548 </listitem>
549 </varlistentry>
551 <varlistentry>
552 <term>
553 <constant>
554 GFP_ATOMIC
555 </constant>
556 </term>
557 <listitem>
558 <para>
559 Don't sleep. Less reliable than <constant>GFP_KERNEL</constant>,
560 but may be called from interrupt context. You should
561 <emphasis>really</emphasis> have a good out-of-memory
562 error-handling strategy.
563 </para>
564 </listitem>
565 </varlistentry>
567 <varlistentry>
568 <term>
569 <constant>
570 GFP_DMA
571 </constant>
572 </term>
573 <listitem>
574 <para>
575 Allocate ISA DMA lower than 16MB. If you don't know what that
576 is you don't need it. Very unreliable.
577 </para>
578 </listitem>
579 </varlistentry>
580 </variablelist>
582 <para>
583 If you see a <errorname>sleeping function called from invalid
584 context</errorname> warning message, then maybe you called a
585 sleeping allocation function from interrupt context without
586 <constant>GFP_ATOMIC</constant>. You should really fix that.
587 Run, don't walk.
588 </para>
590 <para>
591 If you are allocating at least <constant>PAGE_SIZE</constant>
592 (<filename class="headerfile">include/asm/page.h</filename>) bytes,
593 consider using <function>__get_free_pages()</function>
595 (<filename class="headerfile">include/linux/mm.h</filename>). It
596 takes an order argument (0 for page sized, 1 for double page, 2
597 for four pages etc.) and the same memory priority flag word as
598 above.
599 </para>
601 <para>
602 If you are allocating more than a page worth of bytes you can use
603 <function>vmalloc()</function>. It'll allocate virtual memory in
604 the kernel map. This block is not contiguous in physical memory,
605 but the <acronym>MMU</acronym> makes it look like it is for you
606 (so it'll only look contiguous to the CPUs, not to external device
607 drivers). If you really need large physically contiguous memory
608 for some weird device, you have a problem: it is poorly supported
609 in Linux because after some time memory fragmentation in a running
610 kernel makes it hard. The best way is to allocate the block early
611 in the boot process via the <function>alloc_bootmem()</function>
612 routine.
613 </para>
615 <para>
616 Before inventing your own cache of often-used objects consider
617 using a slab cache in
618 <filename class="headerfile">include/linux/slab.h</filename>
619 </para>
620 </sect1>
622 <sect1 id="routines-current">
623 <title><function>current</function>
624 <filename class="headerfile">include/asm/current.h</filename></title>
626 <para>
627 This global variable (really a macro) contains a pointer to
628 the current task structure, so is only valid in user context.
629 For example, when a process makes a system call, this will
630 point to the task structure of the calling process. It is
631 <emphasis>not NULL</emphasis> in interrupt context.
632 </para>
633 </sect1>
635 <sect1 id="routines-udelay">
636 <title><function>mdelay()</function>/<function>udelay()</function>
637 <filename class="headerfile">include/asm/delay.h</filename>
638 <filename class="headerfile">include/linux/delay.h</filename>
639 </title>
641 <para>
642 The <function>udelay()</function> and <function>ndelay()</function> functions can be used for small pauses.
643 Do not use large values with them as you risk
644 overflow - the helper function <function>mdelay()</function> is useful
645 here, or consider <function>msleep()</function>.
646 </para>
647 </sect1>
649 <sect1 id="routines-endian">
650 <title><function>cpu_to_be32()</function>/<function>be32_to_cpu()</function>/<function>cpu_to_le32()</function>/<function>le32_to_cpu()</function>
651 <filename class="headerfile">include/asm/byteorder.h</filename>
652 </title>
654 <para>
655 The <function>cpu_to_be32()</function> family (where the "32" can
656 be replaced by 64 or 16, and the "be" can be replaced by "le") are
657 the general way to do endian conversions in the kernel: they
658 return the converted value. All variations supply the reverse as
659 well: <function>be32_to_cpu()</function>, etc.
660 </para>
662 <para>
663 There are two major variations of these functions: the pointer
664 variation, such as <function>cpu_to_be32p()</function>, which take
665 a pointer to the given type, and return the converted value. The
666 other variation is the "in-situ" family, such as
667 <function>cpu_to_be32s()</function>, which convert value referred
668 to by the pointer, and return void.
669 </para>
670 </sect1>
672 <sect1 id="routines-local-irqs">
673 <title><function>local_irq_save()</function>/<function>local_irq_restore()</function>
674 <filename class="headerfile">include/asm/system.h</filename>
675 </title>
677 <para>
678 These routines disable hard interrupts on the local CPU, and
679 restore them. They are reentrant; saving the previous state in
680 their one <varname>unsigned long flags</varname> argument. If you
681 know that interrupts are enabled, you can simply use
682 <function>local_irq_disable()</function> and
683 <function>local_irq_enable()</function>.
684 </para>
685 </sect1>
687 <sect1 id="routines-softirqs">
688 <title><function>local_bh_disable()</function>/<function>local_bh_enable()</function>
689 <filename class="headerfile">include/linux/interrupt.h</filename></title>
691 <para>
692 These routines disable soft interrupts on the local CPU, and
693 restore them. They are reentrant; if soft interrupts were
694 disabled before, they will still be disabled after this pair
695 of functions has been called. They prevent softirqs and tasklets
696 from running on the current CPU.
697 </para>
698 </sect1>
700 <sect1 id="routines-processorids">
701 <title><function>smp_processor_id</function>()
702 <filename class="headerfile">include/asm/smp.h</filename></title>
704 <para>
705 <function>get_cpu()</function> disables preemption (so you won't
706 suddenly get moved to another CPU) and returns the current
707 processor number, between 0 and <symbol>NR_CPUS</symbol>. Note
708 that the CPU numbers are not necessarily continuous. You return
709 it again with <function>put_cpu()</function> when you are done.
710 </para>
711 <para>
712 If you know you cannot be preempted by another task (ie. you are
713 in interrupt context, or have preemption disabled) you can use
714 smp_processor_id().
715 </para>
716 </sect1>
718 <sect1 id="routines-init">
719 <title><type>__init</type>/<type>__exit</type>/<type>__initdata</type>
720 <filename class="headerfile">include/linux/init.h</filename></title>
722 <para>
723 After boot, the kernel frees up a special section; functions
724 marked with <type>__init</type> and data structures marked with
725 <type>__initdata</type> are dropped after boot is complete: similarly
726 modules discard this memory after initialization. <type>__exit</type>
727 is used to declare a function which is only required on exit: the
728 function will be dropped if this file is not compiled as a module.
729 See the header file for use. Note that it makes no sense for a function
730 marked with <type>__init</type> to be exported to modules with
731 <function>EXPORT_SYMBOL()</function> - this will break.
732 </para>
734 </sect1>
736 <sect1 id="routines-init-again">
737 <title><function>__initcall()</function>/<function>module_init()</function>
738 <filename class="headerfile">include/linux/init.h</filename></title>
739 <para>
740 Many parts of the kernel are well served as a module
741 (dynamically-loadable parts of the kernel). Using the
742 <function>module_init()</function> and
743 <function>module_exit()</function> macros it is easy to write code
744 without #ifdefs which can operate both as a module or built into
745 the kernel.
746 </para>
748 <para>
749 The <function>module_init()</function> macro defines which
750 function is to be called at module insertion time (if the file is
751 compiled as a module), or at boot time: if the file is not
752 compiled as a module the <function>module_init()</function> macro
753 becomes equivalent to <function>__initcall()</function>, which
754 through linker magic ensures that the function is called on boot.
755 </para>
757 <para>
758 The function can return a negative error number to cause
759 module loading to fail (unfortunately, this has no effect if
760 the module is compiled into the kernel). This function is
761 called in user context with interrupts enabled, so it can sleep.
762 </para>
763 </sect1>
765 <sect1 id="routines-moduleexit">
766 <title> <function>module_exit()</function>
767 <filename class="headerfile">include/linux/init.h</filename> </title>
769 <para>
770 This macro defines the function to be called at module removal
771 time (or never, in the case of the file compiled into the
772 kernel). It will only be called if the module usage count has
773 reached zero. This function can also sleep, but cannot fail:
774 everything must be cleaned up by the time it returns.
775 </para>
777 <para>
778 Note that this macro is optional: if it is not present, your
779 module will not be removable (except for 'rmmod -f').
780 </para>
781 </sect1>
783 <sect1 id="routines-module-use-counters">
784 <title> <function>try_module_get()</function>/<function>module_put()</function>
785 <filename class="headerfile">include/linux/module.h</filename></title>
787 <para>
788 These manipulate the module usage count, to protect against
789 removal (a module also can't be removed if another module uses one
790 of its exported symbols: see below). Before calling into module
791 code, you should call <function>try_module_get()</function> on
792 that module: if it fails, then the module is being removed and you
793 should act as if it wasn't there. Otherwise, you can safely enter
794 the module, and call <function>module_put()</function> when you're
795 finished.
796 </para>
798 <para>
799 Most registerable structures have an
800 <structfield>owner</structfield> field, such as in the
801 <structname>file_operations</structname> structure. Set this field
802 to the macro <symbol>THIS_MODULE</symbol>.
803 </para>
804 </sect1>
806 <!-- add info on new-style module refcounting here -->
807 </chapter>
809 <chapter id="queues">
810 <title>Wait Queues
811 <filename class="headerfile">include/linux/wait.h</filename>
812 </title>
813 <para>
814 <emphasis>[SLEEPS]</emphasis>
815 </para>
817 <para>
818 A wait queue is used to wait for someone to wake you up when a
819 certain condition is true. They must be used carefully to ensure
820 there is no race condition. You declare a
821 <type>wait_queue_head_t</type>, and then processes which want to
822 wait for that condition declare a <type>wait_queue_t</type>
823 referring to themselves, and place that in the queue.
824 </para>
826 <sect1 id="queue-declaring">
827 <title>Declaring</title>
829 <para>
830 You declare a <type>wait_queue_head_t</type> using the
831 <function>DECLARE_WAIT_QUEUE_HEAD()</function> macro, or using the
832 <function>init_waitqueue_head()</function> routine in your
833 initialization code.
834 </para>
835 </sect1>
837 <sect1 id="queue-waitqueue">
838 <title>Queuing</title>
840 <para>
841 Placing yourself in the waitqueue is fairly complex, because you
842 must put yourself in the queue before checking the condition.
843 There is a macro to do this:
844 <function>wait_event_interruptible()</function>
846 <filename class="headerfile">include/linux/wait.h</filename> The
847 first argument is the wait queue head, and the second is an
848 expression which is evaluated; the macro returns
849 <returnvalue>0</returnvalue> when this expression is true, or
850 <returnvalue>-ERESTARTSYS</returnvalue> if a signal is received.
851 The <function>wait_event()</function> version ignores signals.
852 </para>
853 <para>
854 Do not use the <function>sleep_on()</function> function family -
855 it is very easy to accidentally introduce races; almost certainly
856 one of the <function>wait_event()</function> family will do, or a
857 loop around <function>schedule_timeout()</function>. If you choose
858 to loop around <function>schedule_timeout()</function> remember
859 you must set the task state (with
860 <function>set_current_state()</function>) on each iteration to avoid
861 busy-looping.
862 </para>
864 </sect1>
866 <sect1 id="queue-waking">
867 <title>Waking Up Queued Tasks</title>
869 <para>
870 Call <function>wake_up()</function>
872 <filename class="headerfile">include/linux/wait.h</filename>;,
873 which will wake up every process in the queue. The exception is
874 if one has <constant>TASK_EXCLUSIVE</constant> set, in which case
875 the remainder of the queue will not be woken. There are other variants
876 of this basic function available in the same header.
877 </para>
878 </sect1>
879 </chapter>
881 <chapter id="atomic-ops">
882 <title>Atomic Operations</title>
884 <para>
885 Certain operations are guaranteed atomic on all platforms. The
886 first class of operations work on <type>atomic_t</type>
888 <filename class="headerfile">include/asm/atomic.h</filename>; this
889 contains a signed integer (at least 32 bits long), and you must use
890 these functions to manipulate or read atomic_t variables.
891 <function>atomic_read()</function> and
892 <function>atomic_set()</function> get and set the counter,
893 <function>atomic_add()</function>,
894 <function>atomic_sub()</function>,
895 <function>atomic_inc()</function>,
896 <function>atomic_dec()</function>, and
897 <function>atomic_dec_and_test()</function> (returns
898 <returnvalue>true</returnvalue> if it was decremented to zero).
899 </para>
901 <para>
902 Yes. It returns <returnvalue>true</returnvalue> (i.e. != 0) if the
903 atomic variable is zero.
904 </para>
906 <para>
907 Note that these functions are slower than normal arithmetic, and
908 so should not be used unnecessarily.
909 </para>
911 <para>
912 The second class of atomic operations is atomic bit operations on an
913 <type>unsigned long</type>, defined in
915 <filename class="headerfile">include/linux/bitops.h</filename>. These
916 operations generally take a pointer to the bit pattern, and a bit
917 number: 0 is the least significant bit.
918 <function>set_bit()</function>, <function>clear_bit()</function>
919 and <function>change_bit()</function> set, clear, and flip the
920 given bit. <function>test_and_set_bit()</function>,
921 <function>test_and_clear_bit()</function> and
922 <function>test_and_change_bit()</function> do the same thing,
923 except return true if the bit was previously set; these are
924 particularly useful for atomically setting flags.
925 </para>
927 <para>
928 It is possible to call these operations with bit indices greater
929 than BITS_PER_LONG. The resulting behavior is strange on big-endian
930 platforms though so it is a good idea not to do this.
931 </para>
932 </chapter>
934 <chapter id="symbols">
935 <title>Symbols</title>
937 <para>
938 Within the kernel proper, the normal linking rules apply
939 (ie. unless a symbol is declared to be file scope with the
940 <type>static</type> keyword, it can be used anywhere in the
941 kernel). However, for modules, a special exported symbol table is
942 kept which limits the entry points to the kernel proper. Modules
943 can also export symbols.
944 </para>
946 <sect1 id="sym-exportsymbols">
947 <title><function>EXPORT_SYMBOL()</function>
948 <filename class="headerfile">include/linux/module.h</filename></title>
950 <para>
951 This is the classic method of exporting a symbol: dynamically
952 loaded modules will be able to use the symbol as normal.
953 </para>
954 </sect1>
956 <sect1 id="sym-exportsymbols-gpl">
957 <title><function>EXPORT_SYMBOL_GPL()</function>
958 <filename class="headerfile">include/linux/module.h</filename></title>
960 <para>
961 Similar to <function>EXPORT_SYMBOL()</function> except that the
962 symbols exported by <function>EXPORT_SYMBOL_GPL()</function> can
963 only be seen by modules with a
964 <function>MODULE_LICENSE()</function> that specifies a GPL
965 compatible license. It implies that the function is considered
966 an internal implementation issue, and not really an interface.
967 </para>
968 </sect1>
969 </chapter>
971 <chapter id="conventions">
972 <title>Routines and Conventions</title>
974 <sect1 id="conventions-doublelinkedlist">
975 <title>Double-linked lists
976 <filename class="headerfile">include/linux/list.h</filename></title>
978 <para>
979 There used to be three sets of linked-list routines in the kernel
980 headers, but this one is the winner. If you don't have some
981 particular pressing need for a single list, it's a good choice.
982 </para>
984 <para>
985 In particular, <function>list_for_each_entry</function> is useful.
986 </para>
987 </sect1>
989 <sect1 id="convention-returns">
990 <title>Return Conventions</title>
992 <para>
993 For code called in user context, it's very common to defy C
994 convention, and return <returnvalue>0</returnvalue> for success,
995 and a negative error number
996 (eg. <returnvalue>-EFAULT</returnvalue>) for failure. This can be
997 unintuitive at first, but it's fairly widespread in the kernel.
998 </para>
1000 <para>
1001 Using <function>ERR_PTR()</function>
1003 <filename class="headerfile">include/linux/err.h</filename>; to
1004 encode a negative error number into a pointer, and
1005 <function>IS_ERR()</function> and <function>PTR_ERR()</function>
1006 to get it back out again: avoids a separate pointer parameter for
1007 the error number. Icky, but in a good way.
1008 </para>
1009 </sect1>
1011 <sect1 id="conventions-borkedcompile">
1012 <title>Breaking Compilation</title>
1014 <para>
1015 Linus and the other developers sometimes change function or
1016 structure names in development kernels; this is not done just to
1017 keep everyone on their toes: it reflects a fundamental change
1018 (eg. can no longer be called with interrupts on, or does extra
1019 checks, or doesn't do checks which were caught before). Usually
1020 this is accompanied by a fairly complete note to the linux-kernel
1021 mailing list; search the archive. Simply doing a global replace
1022 on the file usually makes things <emphasis>worse</emphasis>.
1023 </para>
1024 </sect1>
1026 <sect1 id="conventions-initialising">
1027 <title>Initializing structure members</title>
1029 <para>
1030 The preferred method of initializing structures is to use
1031 designated initialisers, as defined by ISO C99, eg:
1032 </para>
1033 <programlisting>
1034 static struct block_device_operations opt_fops = {
1035 .open = opt_open,
1036 .release = opt_release,
1037 .ioctl = opt_ioctl,
1038 .check_media_change = opt_media_change,
1040 </programlisting>
1041 <para>
1042 This makes it easy to grep for, and makes it clear which
1043 structure fields are set. You should do this because it looks
1044 cool.
1045 </para>
1046 </sect1>
1048 <sect1 id="conventions-gnu-extns">
1049 <title>GNU Extensions</title>
1051 <para>
1052 GNU Extensions are explicitly allowed in the Linux kernel.
1053 Note that some of the more complex ones are not very well
1054 supported, due to lack of general use, but the following are
1055 considered standard (see the GCC info page section "C
1056 Extensions" for more details - Yes, really the info page, the
1057 man page is only a short summary of the stuff in info).
1058 </para>
1059 <itemizedlist>
1060 <listitem>
1061 <para>
1062 Inline functions
1063 </para>
1064 </listitem>
1065 <listitem>
1066 <para>
1067 Statement expressions (ie. the ({ and }) constructs).
1068 </para>
1069 </listitem>
1070 <listitem>
1071 <para>
1072 Declaring attributes of a function / variable / type
1073 (__attribute__)
1074 </para>
1075 </listitem>
1076 <listitem>
1077 <para>
1078 typeof
1079 </para>
1080 </listitem>
1081 <listitem>
1082 <para>
1083 Zero length arrays
1084 </para>
1085 </listitem>
1086 <listitem>
1087 <para>
1088 Macro varargs
1089 </para>
1090 </listitem>
1091 <listitem>
1092 <para>
1093 Arithmetic on void pointers
1094 </para>
1095 </listitem>
1096 <listitem>
1097 <para>
1098 Non-Constant initializers
1099 </para>
1100 </listitem>
1101 <listitem>
1102 <para>
1103 Assembler Instructions (not outside arch/ and include/asm/)
1104 </para>
1105 </listitem>
1106 <listitem>
1107 <para>
1108 Function names as strings (__func__).
1109 </para>
1110 </listitem>
1111 <listitem>
1112 <para>
1113 __builtin_constant_p()
1114 </para>
1115 </listitem>
1116 </itemizedlist>
1118 <para>
1119 Be wary when using long long in the kernel, the code gcc generates for
1120 it is horrible and worse: division and multiplication does not work
1121 on i386 because the GCC runtime functions for it are missing from
1122 the kernel environment.
1123 </para>
1125 <!-- FIXME: add a note about ANSI aliasing cleanness -->
1126 </sect1>
1128 <sect1 id="conventions-cplusplus">
1129 <title>C++</title>
1131 <para>
1132 Using C++ in the kernel is usually a bad idea, because the
1133 kernel does not provide the necessary runtime environment
1134 and the include files are not tested for it. It is still
1135 possible, but not recommended. If you really want to do
1136 this, forget about exceptions at least.
1137 </para>
1138 </sect1>
1140 <sect1 id="conventions-ifdef">
1141 <title>&num;if</title>
1143 <para>
1144 It is generally considered cleaner to use macros in header files
1145 (or at the top of .c files) to abstract away functions rather than
1146 using `#if' pre-processor statements throughout the source code.
1147 </para>
1148 </sect1>
1149 </chapter>
1151 <chapter id="submitting">
1152 <title>Putting Your Stuff in the Kernel</title>
1154 <para>
1155 In order to get your stuff into shape for official inclusion, or
1156 even to make a neat patch, there's administrative work to be
1157 done:
1158 </para>
1159 <itemizedlist>
1160 <listitem>
1161 <para>
1162 Figure out whose pond you've been pissing in. Look at the top of
1163 the source files, inside the <filename>MAINTAINERS</filename>
1164 file, and last of all in the <filename>CREDITS</filename> file.
1165 You should coordinate with this person to make sure you're not
1166 duplicating effort, or trying something that's already been
1167 rejected.
1168 </para>
1170 <para>
1171 Make sure you put your name and EMail address at the top of
1172 any files you create or mangle significantly. This is the
1173 first place people will look when they find a bug, or when
1174 <emphasis>they</emphasis> want to make a change.
1175 </para>
1176 </listitem>
1178 <listitem>
1179 <para>
1180 Usually you want a configuration option for your kernel hack.
1181 Edit <filename>Kconfig</filename> in the appropriate directory.
1182 The Config language is simple to use by cut and paste, and there's
1183 complete documentation in
1184 <filename>Documentation/kbuild/kconfig-language.txt</filename>.
1185 </para>
1187 <para>
1188 You may well want to make your CONFIG option only visible if
1189 <symbol>CONFIG_EXPERIMENTAL</symbol> is enabled: this serves as a
1190 warning to users. There many other fancy things you can do: see
1191 the various <filename>Kconfig</filename> files for ideas.
1192 </para>
1194 <para>
1195 In your description of the option, make sure you address both the
1196 expert user and the user who knows nothing about your feature. Mention
1197 incompatibilities and issues here. <emphasis> Definitely
1198 </emphasis> end your description with <quote> if in doubt, say N
1199 </quote> (or, occasionally, `Y'); this is for people who have no
1200 idea what you are talking about.
1201 </para>
1202 </listitem>
1204 <listitem>
1205 <para>
1206 Edit the <filename>Makefile</filename>: the CONFIG variables are
1207 exported here so you can usually just add a "obj-$(CONFIG_xxx) +=
1208 xxx.o" line. The syntax is documented in
1209 <filename>Documentation/kbuild/makefiles.txt</filename>.
1210 </para>
1211 </listitem>
1213 <listitem>
1214 <para>
1215 Put yourself in <filename>CREDITS</filename> if you've done
1216 something noteworthy, usually beyond a single file (your name
1217 should be at the top of the source files anyway).
1218 <filename>MAINTAINERS</filename> means you want to be consulted
1219 when changes are made to a subsystem, and hear about bugs; it
1220 implies a more-than-passing commitment to some part of the code.
1221 </para>
1222 </listitem>
1224 <listitem>
1225 <para>
1226 Finally, don't forget to read <filename>Documentation/SubmittingPatches</filename>
1227 and possibly <filename>Documentation/SubmittingDrivers</filename>.
1228 </para>
1229 </listitem>
1230 </itemizedlist>
1231 </chapter>
1233 <chapter id="cantrips">
1234 <title>Kernel Cantrips</title>
1236 <para>
1237 Some favorites from browsing the source. Feel free to add to this
1238 list.
1239 </para>
1241 <para>
1242 <filename>arch/x86/include/asm/delay.h:</filename>
1243 </para>
1244 <programlisting>
1245 #define ndelay(n) (__builtin_constant_p(n) ? \
1246 ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \
1247 __ndelay(n))
1248 </programlisting>
1250 <para>
1251 <filename>include/linux/fs.h</filename>:
1252 </para>
1253 <programlisting>
1255 * Kernel pointers have redundant information, so we can use a
1256 * scheme where we can return either an error code or a dentry
1257 * pointer with the same return value.
1259 * This should be a per-architecture thing, to allow different
1260 * error and pointer decisions.
1262 #define ERR_PTR(err) ((void *)((long)(err)))
1263 #define PTR_ERR(ptr) ((long)(ptr))
1264 #define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000))
1265 </programlisting>
1267 <para>
1268 <filename>arch/x86/include/asm/uaccess_32.h:</filename>
1269 </para>
1271 <programlisting>
1272 #define copy_to_user(to,from,n) \
1273 (__builtin_constant_p(n) ? \
1274 __constant_copy_to_user((to),(from),(n)) : \
1275 __generic_copy_to_user((to),(from),(n)))
1276 </programlisting>
1278 <para>
1279 <filename>arch/sparc/kernel/head.S:</filename>
1280 </para>
1282 <programlisting>
1284 * Sun people can't spell worth damn. "compatability" indeed.
1285 * At least we *know* we can't spell, and use a spell-checker.
1288 /* Uh, actually Linus it is I who cannot spell. Too much murky
1289 * Sparc assembly will do this to ya.
1291 C_LABEL(cputypvar):
1292 .asciz "compatability"
1294 /* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */
1295 .align 4
1296 C_LABEL(cputypvar_sun4m):
1297 .asciz "compatible"
1298 </programlisting>
1300 <para>
1301 <filename>arch/sparc/lib/checksum.S:</filename>
1302 </para>
1304 <programlisting>
1305 /* Sun, you just can't beat me, you just can't. Stop trying,
1306 * give up. I'm serious, I am going to kick the living shit
1307 * out of you, game over, lights out.
1309 </programlisting>
1310 </chapter>
1312 <chapter id="credits">
1313 <title>Thanks</title>
1315 <para>
1316 Thanks to Andi Kleen for the idea, answering my questions, fixing
1317 my mistakes, filling content, etc. Philipp Rumpf for more spelling
1318 and clarity fixes, and some excellent non-obvious points. Werner
1319 Almesberger for giving me a great summary of
1320 <function>disable_irq()</function>, and Jes Sorensen and Andrea
1321 Arcangeli added caveats. Michael Elizabeth Chastain for checking
1322 and adding to the Configure section. <!-- Rusty insisted on this
1323 bit; I didn't do it! --> Telsa Gwynne for teaching me DocBook.
1324 </para>
1325 </chapter>
1326 </book>