Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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" []> |
| 4 | |
| 5 | <book id="lk-hacking-guide"> |
| 6 | <bookinfo> |
| 7 | <title>Unreliable Guide To Hacking The Linux Kernel</title> |
| 8 | |
| 9 | <authorgroup> |
| 10 | <author> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 11 | <firstname>Rusty</firstname> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 12 | <surname>Russell</surname> |
| 13 | <affiliation> |
| 14 | <address> |
| 15 | <email>rusty@rustcorp.com.au</email> |
| 16 | </address> |
| 17 | </affiliation> |
| 18 | </author> |
| 19 | </authorgroup> |
| 20 | |
| 21 | <copyright> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 22 | <year>2005</year> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 23 | <holder>Rusty Russell</holder> |
| 24 | </copyright> |
| 25 | |
| 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> |
| 34 | |
| 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> |
| 41 | |
| 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> |
| 48 | |
| 49 | <para> |
| 50 | For more details see the file COPYING in the source |
| 51 | distribution of Linux. |
| 52 | </para> |
| 53 | </legalnotice> |
| 54 | |
| 55 | <releaseinfo> |
| 56 | This is the first release of this document as part of the kernel tarball. |
| 57 | </releaseinfo> |
| 58 | |
| 59 | </bookinfo> |
| 60 | |
| 61 | <toc></toc> |
| 62 | |
| 63 | <chapter id="introduction"> |
| 64 | <title>Introduction</title> |
| 65 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 66 | Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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> |
| 81 | |
| 82 | <chapter id="basic-players"> |
| 83 | <title>The Players</title> |
| 84 | |
| 85 | <para> |
| 86 | At any time each of the CPUs in a system can be: |
| 87 | </para> |
| 88 | |
| 89 | <itemizedlist> |
| 90 | <listitem> |
| 91 | <para> |
| 92 | not associated with any process, serving a hardware interrupt; |
| 93 | </para> |
| 94 | </listitem> |
| 95 | |
| 96 | <listitem> |
| 97 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 98 | not associated with any process, serving a softirq or tasklet; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 99 | </para> |
| 100 | </listitem> |
| 101 | |
| 102 | <listitem> |
| 103 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 104 | running in kernel space, associated with a process (user context); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 105 | </para> |
| 106 | </listitem> |
| 107 | |
| 108 | <listitem> |
| 109 | <para> |
| 110 | running a process in user space. |
| 111 | </para> |
| 112 | </listitem> |
| 113 | </itemizedlist> |
| 114 | |
| 115 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 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. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 122 | </para> |
| 123 | |
| 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> |
| 128 | |
| 129 | <sect1 id="basics-usercontext"> |
| 130 | <title>User Context</title> |
| 131 | |
| 132 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 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>. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 137 | </para> |
| 138 | |
| 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> |
| 145 | |
| 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> |
| 153 | |
| 154 | <caution> |
| 155 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 156 | Beware that if you have preemption or softirqs disabled |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 157 | (see below), <function>in_interrupt()</function> will return a |
| 158 | false positive. |
| 159 | </para> |
| 160 | </caution> |
| 161 | </sect1> |
| 162 | |
| 163 | <sect1 id="basics-hardirqs"> |
| 164 | <title>Hardware Interrupts (Hard IRQs)</title> |
| 165 | |
| 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 |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 171 | guarantees that this handler is never re-entered: if the same |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 172 | interrupt arrives, it is queued (or dropped). Because it |
| 173 | disables interrupts, this handler has to be fast: frequently it |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 174 | simply acknowledges the interrupt, marks a 'software interrupt' |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 175 | for execution and exits. |
| 176 | </para> |
| 177 | |
| 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> |
| 189 | |
| 190 | <sect1 id="basics-softirqs"> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 191 | <title>Software Interrupt Context: Softirqs and Tasklets</title> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 192 | |
| 193 | <para> |
| 194 | Whenever a system call is about to return to userspace, or a |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 195 | hardware interrupt handler exits, any 'software interrupts' |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 196 | which are marked pending (usually by hardware interrupts) are |
| 197 | run (<filename>kernel/softirq.c</filename>). |
| 198 | </para> |
| 199 | |
| 200 | <para> |
| 201 | Much of the real interrupt handling work is done here. Early in |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 202 | the transition to <acronym>SMP</acronym>, there were only 'bottom |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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, |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 205 | we abandoned this limitation and switched to 'softirqs'. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 206 | </para> |
| 207 | |
| 208 | <para> |
| 209 | <filename class="headerfile">include/linux/interrupt.h</filename> lists the |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 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. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 215 | </para> |
| 216 | |
| 217 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 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. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 226 | </para> |
| 227 | <caution> |
| 228 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 229 | The name 'tasklet' is misleading: they have nothing to do with 'tasks', |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 230 | and probably more to do with some bad vodka Alexey Kuznetsov had at the |
| 231 | time. |
| 232 | </para> |
| 233 | </caution> |
| 234 | |
| 235 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 236 | You can tell you are in a softirq (or tasklet) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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> |
| 248 | |
| 249 | <chapter id="basic-rules"> |
| 250 | <title>Some Basic Rules</title> |
| 251 | |
| 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> |
| 263 | |
| 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> |
| 278 | |
| 279 | <varlistentry> |
| 280 | <term>A rigid stack limit</term> |
| 281 | <listitem> |
| 282 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 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). |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 287 | </para> |
| 288 | </listitem> |
| 289 | </varlistentry> |
| 290 | |
| 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> |
| 306 | |
| 307 | <chapter id="ioctls"> |
| 308 | <title>ioctls: Not writing a new system call</title> |
| 309 | |
| 310 | <para> |
| 311 | A system call generally looks like this |
| 312 | </para> |
| 313 | |
| 314 | <programlisting> |
| 315 | asmlinkage long sys_mycall(int arg) |
| 316 | { |
| 317 | return 0; |
| 318 | } |
| 319 | </programlisting> |
| 320 | |
| 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> |
| 330 | |
| 331 | <para> |
| 332 | If all your routine does is read or write some parameter, consider |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 333 | implementing a <function>sysfs</function> interface instead. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 334 | </para> |
| 335 | |
| 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> |
| 342 | |
| 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> |
| 353 | |
| 354 | <programlisting> |
Shawn Bohrer | 2770f18 | 2009-12-06 18:30:44 -0800 | [diff] [blame] | 355 | if (signal_pending(current)) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 356 | return -ERESTARTSYS; |
| 357 | </programlisting> |
| 358 | |
| 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> |
| 365 | |
| 366 | <programlisting> |
| 367 | cond_resched(); /* Will sleep */ |
| 368 | </programlisting> |
| 369 | |
| 370 | <para> |
| 371 | A short note on interface design: the UNIX system call motto is |
| 372 | "Provide mechanism not policy". |
| 373 | </para> |
| 374 | </chapter> |
| 375 | |
| 376 | <chapter id="deadlock-recipes"> |
| 377 | <title>Recipes for Deadlock</title> |
| 378 | |
| 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> |
| 388 | |
| 389 | <listitem> |
| 390 | <para> |
| 391 | You do not own any spinlocks. |
| 392 | </para> |
| 393 | </listitem> |
| 394 | |
| 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> |
| 403 | |
| 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> |
| 409 | |
| 410 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 411 | You should always compile your kernel |
Frederic Weisbecker | d902db1 | 2011-06-08 19:31:56 +0200 | [diff] [blame] | 412 | <symbol>CONFIG_DEBUG_ATOMIC_SLEEP</symbol> on, and it will warn |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 413 | you if you break these rules. If you <emphasis>do</emphasis> break |
| 414 | the rules, you will eventually lock up your box. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 415 | </para> |
| 416 | |
| 417 | <para> |
| 418 | Really. |
| 419 | </para> |
| 420 | </chapter> |
| 421 | |
| 422 | <chapter id="common-routines"> |
| 423 | <title>Common Routines</title> |
| 424 | |
| 425 | <sect1 id="routines-printk"> |
| 426 | <title> |
| 427 | <function>printk()</function> |
| 428 | <filename class="headerfile">include/linux/kernel.h</filename> |
| 429 | </title> |
| 430 | |
| 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> |
| 440 | |
| 441 | <programlisting> |
| 442 | printk(KERN_INFO "i = %u\n", i); |
| 443 | </programlisting> |
| 444 | |
| 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> |
| 450 | |
| 451 | <programlisting> |
Tobias Klauser | 2a8aaac | 2009-07-30 13:10:50 -0700 | [diff] [blame] | 452 | __be32 ipaddress; |
| 453 | printk(KERN_INFO "my ip: %pI4\n", &ipaddress); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 454 | </programlisting> |
| 455 | |
| 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> |
| 460 | |
| 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> |
| 467 | |
| 468 | <!--- From the Lions book reader department --> |
| 469 | |
| 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> |
| 478 | |
| 479 | <sect1 id="routines-copy"> |
| 480 | <title> |
| 481 | <function>copy_[to/from]_user()</function> |
| 482 | / |
| 483 | <function>get_user()</function> |
| 484 | / |
| 485 | <function>put_user()</function> |
| 486 | <filename class="headerfile">include/asm/uaccess.h</filename> |
| 487 | </title> |
| 488 | |
| 489 | <para> |
| 490 | <emphasis>[SLEEPS]</emphasis> |
| 491 | </para> |
| 492 | |
| 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> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 512 | [Yes, this moronic interface makes me cringe. The flamewar comes up every year or so. --RR.] |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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> |
| 520 | |
| 521 | <sect1 id="routines-kmalloc"> |
| 522 | <title><function>kmalloc()</function>/<function>kfree()</function> |
| 523 | <filename class="headerfile">include/linux/slab.h</filename></title> |
| 524 | |
| 525 | <para> |
| 526 | <emphasis>[MAY SLEEP: SEE BELOW]</emphasis> |
| 527 | </para> |
| 528 | |
| 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> |
| 535 | |
| 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> |
| 550 | |
| 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> |
| 566 | |
| 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> |
| 581 | |
| 582 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 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. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 588 | </para> |
| 589 | |
| 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> |
| 594 | |
| 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> |
| 600 | |
| 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> |
| 614 | |
| 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> |
| 621 | |
| 622 | <sect1 id="routines-current"> |
| 623 | <title><function>current</function> |
| 624 | <filename class="headerfile">include/asm/current.h</filename></title> |
| 625 | |
| 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> |
| 634 | |
| 635 | <sect1 id="routines-udelay"> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 636 | <title><function>mdelay()</function>/<function>udelay()</function> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 637 | <filename class="headerfile">include/asm/delay.h</filename> |
| 638 | <filename class="headerfile">include/linux/delay.h</filename> |
| 639 | </title> |
| 640 | |
| 641 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 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 |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 644 | overflow - the helper function <function>mdelay()</function> is useful |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 645 | here, or consider <function>msleep()</function>. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 646 | </para> |
| 647 | </sect1> |
| 648 | |
| 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> |
| 653 | |
| 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> |
| 661 | |
| 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> |
| 671 | |
| 672 | <sect1 id="routines-local-irqs"> |
| 673 | <title><function>local_irq_save()</function>/<function>local_irq_restore()</function> |
David Howells | 9566328 | 2014-04-07 15:39:21 -0700 | [diff] [blame] | 674 | <filename class="headerfile">include/linux/irqflags.h</filename> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 675 | </title> |
| 676 | |
| 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> |
| 686 | |
| 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> |
| 690 | |
| 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 |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 695 | of functions has been called. They prevent softirqs and tasklets |
| 696 | from running on the current CPU. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 697 | </para> |
| 698 | </sect1> |
| 699 | |
| 700 | <sect1 id="routines-processorids"> |
| 701 | <title><function>smp_processor_id</function>() |
| 702 | <filename class="headerfile">include/asm/smp.h</filename></title> |
| 703 | |
| 704 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 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(). |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 715 | </para> |
| 716 | </sect1> |
| 717 | |
| 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> |
| 721 | |
| 722 | <para> |
| 723 | After boot, the kernel frees up a special section; functions |
| 724 | marked with <type>__init</type> and data structures marked with |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 725 | <type>__initdata</type> are dropped after boot is complete: similarly |
| 726 | modules discard this memory after initialization. <type>__exit</type> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 733 | |
| 734 | </sect1> |
| 735 | |
| 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> |
| 747 | |
| 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> |
| 756 | |
| 757 | <para> |
| 758 | The function can return a negative error number to cause |
| 759 | module loading to fail (unfortunately, this has no effect if |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 760 | the module is compiled into the kernel). This function is |
| 761 | called in user context with interrupts enabled, so it can sleep. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 762 | </para> |
| 763 | </sect1> |
| 764 | |
| 765 | <sect1 id="routines-moduleexit"> |
| 766 | <title> <function>module_exit()</function> |
| 767 | <filename class="headerfile">include/linux/init.h</filename> </title> |
| 768 | |
| 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> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 776 | |
| 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> |
| 782 | |
| 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> |
| 786 | |
| 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> |
| 797 | |
| 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> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 804 | </sect1> |
| 805 | |
| 806 | <!-- add info on new-style module refcounting here --> |
| 807 | </chapter> |
| 808 | |
| 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> |
| 816 | |
| 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> |
| 825 | |
| 826 | <sect1 id="queue-declaring"> |
| 827 | <title>Declaring</title> |
| 828 | |
| 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> |
| 836 | |
| 837 | <sect1 id="queue-waitqueue"> |
| 838 | <title>Queuing</title> |
| 839 | |
| 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> |
| 845 | |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 846 | <filename class="headerfile">include/linux/wait.h</filename> The |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 853 | |
| 854 | </sect1> |
| 855 | |
| 856 | <sect1 id="queue-waking"> |
| 857 | <title>Waking Up Queued Tasks</title> |
| 858 | |
| 859 | <para> |
| 860 | Call <function>wake_up()</function> |
| 861 | |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 862 | <filename class="headerfile">include/linux/wait.h</filename>;, |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 863 | which will wake up every process in the queue. The exception is |
| 864 | if one has <constant>TASK_EXCLUSIVE</constant> set, in which case |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 865 | the remainder of the queue will not be woken. There are other variants |
| 866 | of this basic function available in the same header. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 867 | </para> |
| 868 | </sect1> |
| 869 | </chapter> |
| 870 | |
| 871 | <chapter id="atomic-ops"> |
| 872 | <title>Atomic Operations</title> |
| 873 | |
| 874 | <para> |
| 875 | Certain operations are guaranteed atomic on all platforms. The |
| 876 | first class of operations work on <type>atomic_t</type> |
| 877 | |
| 878 | <filename class="headerfile">include/asm/atomic.h</filename>; this |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 879 | contains a signed integer (at least 32 bits long), and you must use |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 880 | these functions to manipulate or read atomic_t variables. |
| 881 | <function>atomic_read()</function> and |
| 882 | <function>atomic_set()</function> get and set the counter, |
| 883 | <function>atomic_add()</function>, |
| 884 | <function>atomic_sub()</function>, |
| 885 | <function>atomic_inc()</function>, |
| 886 | <function>atomic_dec()</function>, and |
| 887 | <function>atomic_dec_and_test()</function> (returns |
| 888 | <returnvalue>true</returnvalue> if it was decremented to zero). |
| 889 | </para> |
| 890 | |
| 891 | <para> |
| 892 | Yes. It returns <returnvalue>true</returnvalue> (i.e. != 0) if the |
| 893 | atomic variable is zero. |
| 894 | </para> |
| 895 | |
| 896 | <para> |
| 897 | Note that these functions are slower than normal arithmetic, and |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 898 | so should not be used unnecessarily. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 899 | </para> |
| 900 | |
| 901 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 902 | The second class of atomic operations is atomic bit operations on an |
| 903 | <type>unsigned long</type>, defined in |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 904 | |
| 905 | <filename class="headerfile">include/linux/bitops.h</filename>. These |
| 906 | operations generally take a pointer to the bit pattern, and a bit |
| 907 | number: 0 is the least significant bit. |
| 908 | <function>set_bit()</function>, <function>clear_bit()</function> |
| 909 | and <function>change_bit()</function> set, clear, and flip the |
| 910 | given bit. <function>test_and_set_bit()</function>, |
| 911 | <function>test_and_clear_bit()</function> and |
| 912 | <function>test_and_change_bit()</function> do the same thing, |
| 913 | except return true if the bit was previously set; these are |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 914 | particularly useful for atomically setting flags. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 915 | </para> |
| 916 | |
| 917 | <para> |
| 918 | It is possible to call these operations with bit indices greater |
| 919 | than BITS_PER_LONG. The resulting behavior is strange on big-endian |
| 920 | platforms though so it is a good idea not to do this. |
| 921 | </para> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 922 | </chapter> |
| 923 | |
| 924 | <chapter id="symbols"> |
| 925 | <title>Symbols</title> |
| 926 | |
| 927 | <para> |
| 928 | Within the kernel proper, the normal linking rules apply |
| 929 | (ie. unless a symbol is declared to be file scope with the |
| 930 | <type>static</type> keyword, it can be used anywhere in the |
| 931 | kernel). However, for modules, a special exported symbol table is |
| 932 | kept which limits the entry points to the kernel proper. Modules |
| 933 | can also export symbols. |
| 934 | </para> |
| 935 | |
| 936 | <sect1 id="sym-exportsymbols"> |
| 937 | <title><function>EXPORT_SYMBOL()</function> |
Paul Gortmaker | df63447 | 2013-02-14 15:32:04 -0500 | [diff] [blame] | 938 | <filename class="headerfile">include/linux/export.h</filename></title> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 939 | |
| 940 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 941 | This is the classic method of exporting a symbol: dynamically |
| 942 | loaded modules will be able to use the symbol as normal. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 943 | </para> |
| 944 | </sect1> |
| 945 | |
| 946 | <sect1 id="sym-exportsymbols-gpl"> |
| 947 | <title><function>EXPORT_SYMBOL_GPL()</function> |
Paul Gortmaker | df63447 | 2013-02-14 15:32:04 -0500 | [diff] [blame] | 948 | <filename class="headerfile">include/linux/export.h</filename></title> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 949 | |
| 950 | <para> |
| 951 | Similar to <function>EXPORT_SYMBOL()</function> except that the |
| 952 | symbols exported by <function>EXPORT_SYMBOL_GPL()</function> can |
| 953 | only be seen by modules with a |
| 954 | <function>MODULE_LICENSE()</function> that specifies a GPL |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 955 | compatible license. It implies that the function is considered |
| 956 | an internal implementation issue, and not really an interface. |
Luis R. Rodriguez | 582ed8d | 2015-05-28 11:56:01 -0700 | [diff] [blame] | 957 | Some maintainers and developers may however |
| 958 | require EXPORT_SYMBOL_GPL() when adding any new APIs or functionality. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 959 | </para> |
| 960 | </sect1> |
| 961 | </chapter> |
| 962 | |
| 963 | <chapter id="conventions"> |
| 964 | <title>Routines and Conventions</title> |
| 965 | |
| 966 | <sect1 id="conventions-doublelinkedlist"> |
| 967 | <title>Double-linked lists |
| 968 | <filename class="headerfile">include/linux/list.h</filename></title> |
| 969 | |
| 970 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 971 | There used to be three sets of linked-list routines in the kernel |
| 972 | headers, but this one is the winner. If you don't have some |
| 973 | particular pressing need for a single list, it's a good choice. |
| 974 | </para> |
| 975 | |
| 976 | <para> |
| 977 | In particular, <function>list_for_each_entry</function> is useful. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 978 | </para> |
| 979 | </sect1> |
| 980 | |
| 981 | <sect1 id="convention-returns"> |
| 982 | <title>Return Conventions</title> |
| 983 | |
| 984 | <para> |
| 985 | For code called in user context, it's very common to defy C |
| 986 | convention, and return <returnvalue>0</returnvalue> for success, |
| 987 | and a negative error number |
| 988 | (eg. <returnvalue>-EFAULT</returnvalue>) for failure. This can be |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 989 | unintuitive at first, but it's fairly widespread in the kernel. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 990 | </para> |
| 991 | |
| 992 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 993 | Using <function>ERR_PTR()</function> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 994 | |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 995 | <filename class="headerfile">include/linux/err.h</filename>; to |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 996 | encode a negative error number into a pointer, and |
| 997 | <function>IS_ERR()</function> and <function>PTR_ERR()</function> |
| 998 | to get it back out again: avoids a separate pointer parameter for |
| 999 | the error number. Icky, but in a good way. |
| 1000 | </para> |
| 1001 | </sect1> |
| 1002 | |
| 1003 | <sect1 id="conventions-borkedcompile"> |
| 1004 | <title>Breaking Compilation</title> |
| 1005 | |
| 1006 | <para> |
| 1007 | Linus and the other developers sometimes change function or |
| 1008 | structure names in development kernels; this is not done just to |
| 1009 | keep everyone on their toes: it reflects a fundamental change |
| 1010 | (eg. can no longer be called with interrupts on, or does extra |
| 1011 | checks, or doesn't do checks which were caught before). Usually |
| 1012 | this is accompanied by a fairly complete note to the linux-kernel |
| 1013 | mailing list; search the archive. Simply doing a global replace |
| 1014 | on the file usually makes things <emphasis>worse</emphasis>. |
| 1015 | </para> |
| 1016 | </sect1> |
| 1017 | |
| 1018 | <sect1 id="conventions-initialising"> |
| 1019 | <title>Initializing structure members</title> |
| 1020 | |
| 1021 | <para> |
| 1022 | The preferred method of initializing structures is to use |
| 1023 | designated initialisers, as defined by ISO C99, eg: |
| 1024 | </para> |
| 1025 | <programlisting> |
| 1026 | static struct block_device_operations opt_fops = { |
| 1027 | .open = opt_open, |
| 1028 | .release = opt_release, |
| 1029 | .ioctl = opt_ioctl, |
| 1030 | .check_media_change = opt_media_change, |
| 1031 | }; |
| 1032 | </programlisting> |
| 1033 | <para> |
| 1034 | This makes it easy to grep for, and makes it clear which |
| 1035 | structure fields are set. You should do this because it looks |
| 1036 | cool. |
| 1037 | </para> |
| 1038 | </sect1> |
| 1039 | |
| 1040 | <sect1 id="conventions-gnu-extns"> |
| 1041 | <title>GNU Extensions</title> |
| 1042 | |
| 1043 | <para> |
| 1044 | GNU Extensions are explicitly allowed in the Linux kernel. |
| 1045 | Note that some of the more complex ones are not very well |
| 1046 | supported, due to lack of general use, but the following are |
| 1047 | considered standard (see the GCC info page section "C |
| 1048 | Extensions" for more details - Yes, really the info page, the |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 1049 | man page is only a short summary of the stuff in info). |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1050 | </para> |
| 1051 | <itemizedlist> |
| 1052 | <listitem> |
| 1053 | <para> |
| 1054 | Inline functions |
| 1055 | </para> |
| 1056 | </listitem> |
| 1057 | <listitem> |
| 1058 | <para> |
| 1059 | Statement expressions (ie. the ({ and }) constructs). |
| 1060 | </para> |
| 1061 | </listitem> |
| 1062 | <listitem> |
| 1063 | <para> |
| 1064 | Declaring attributes of a function / variable / type |
| 1065 | (__attribute__) |
| 1066 | </para> |
| 1067 | </listitem> |
| 1068 | <listitem> |
| 1069 | <para> |
| 1070 | typeof |
| 1071 | </para> |
| 1072 | </listitem> |
| 1073 | <listitem> |
| 1074 | <para> |
| 1075 | Zero length arrays |
| 1076 | </para> |
| 1077 | </listitem> |
| 1078 | <listitem> |
| 1079 | <para> |
| 1080 | Macro varargs |
| 1081 | </para> |
| 1082 | </listitem> |
| 1083 | <listitem> |
| 1084 | <para> |
| 1085 | Arithmetic on void pointers |
| 1086 | </para> |
| 1087 | </listitem> |
| 1088 | <listitem> |
| 1089 | <para> |
| 1090 | Non-Constant initializers |
| 1091 | </para> |
| 1092 | </listitem> |
| 1093 | <listitem> |
| 1094 | <para> |
| 1095 | Assembler Instructions (not outside arch/ and include/asm/) |
| 1096 | </para> |
| 1097 | </listitem> |
| 1098 | <listitem> |
| 1099 | <para> |
Harvey Harrison | 653c031 | 2008-10-20 16:00:08 -0700 | [diff] [blame] | 1100 | Function names as strings (__func__). |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1101 | </para> |
| 1102 | </listitem> |
| 1103 | <listitem> |
| 1104 | <para> |
| 1105 | __builtin_constant_p() |
| 1106 | </para> |
| 1107 | </listitem> |
| 1108 | </itemizedlist> |
| 1109 | |
| 1110 | <para> |
| 1111 | Be wary when using long long in the kernel, the code gcc generates for |
| 1112 | it is horrible and worse: division and multiplication does not work |
| 1113 | on i386 because the GCC runtime functions for it are missing from |
| 1114 | the kernel environment. |
| 1115 | </para> |
| 1116 | |
| 1117 | <!-- FIXME: add a note about ANSI aliasing cleanness --> |
| 1118 | </sect1> |
| 1119 | |
| 1120 | <sect1 id="conventions-cplusplus"> |
| 1121 | <title>C++</title> |
| 1122 | |
| 1123 | <para> |
| 1124 | Using C++ in the kernel is usually a bad idea, because the |
| 1125 | kernel does not provide the necessary runtime environment |
| 1126 | and the include files are not tested for it. It is still |
| 1127 | possible, but not recommended. If you really want to do |
| 1128 | this, forget about exceptions at least. |
| 1129 | </para> |
| 1130 | </sect1> |
| 1131 | |
| 1132 | <sect1 id="conventions-ifdef"> |
| 1133 | <title>#if</title> |
| 1134 | |
| 1135 | <para> |
| 1136 | It is generally considered cleaner to use macros in header files |
| 1137 | (or at the top of .c files) to abstract away functions rather than |
| 1138 | using `#if' pre-processor statements throughout the source code. |
| 1139 | </para> |
| 1140 | </sect1> |
| 1141 | </chapter> |
| 1142 | |
| 1143 | <chapter id="submitting"> |
| 1144 | <title>Putting Your Stuff in the Kernel</title> |
| 1145 | |
| 1146 | <para> |
| 1147 | In order to get your stuff into shape for official inclusion, or |
| 1148 | even to make a neat patch, there's administrative work to be |
| 1149 | done: |
| 1150 | </para> |
| 1151 | <itemizedlist> |
| 1152 | <listitem> |
| 1153 | <para> |
| 1154 | Figure out whose pond you've been pissing in. Look at the top of |
| 1155 | the source files, inside the <filename>MAINTAINERS</filename> |
| 1156 | file, and last of all in the <filename>CREDITS</filename> file. |
| 1157 | You should coordinate with this person to make sure you're not |
| 1158 | duplicating effort, or trying something that's already been |
| 1159 | rejected. |
| 1160 | </para> |
| 1161 | |
| 1162 | <para> |
| 1163 | Make sure you put your name and EMail address at the top of |
| 1164 | any files you create or mangle significantly. This is the |
| 1165 | first place people will look when they find a bug, or when |
| 1166 | <emphasis>they</emphasis> want to make a change. |
| 1167 | </para> |
| 1168 | </listitem> |
| 1169 | |
| 1170 | <listitem> |
| 1171 | <para> |
| 1172 | Usually you want a configuration option for your kernel hack. |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 1173 | Edit <filename>Kconfig</filename> in the appropriate directory. |
| 1174 | The Config language is simple to use by cut and paste, and there's |
| 1175 | complete documentation in |
| 1176 | <filename>Documentation/kbuild/kconfig-language.txt</filename>. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1177 | </para> |
| 1178 | |
| 1179 | <para> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 1180 | In your description of the option, make sure you address both the |
| 1181 | expert user and the user who knows nothing about your feature. Mention |
| 1182 | incompatibilities and issues here. <emphasis> Definitely |
| 1183 | </emphasis> end your description with <quote> if in doubt, say N |
| 1184 | </quote> (or, occasionally, `Y'); this is for people who have no |
| 1185 | idea what you are talking about. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1186 | </para> |
| 1187 | </listitem> |
| 1188 | |
| 1189 | <listitem> |
| 1190 | <para> |
| 1191 | Edit the <filename>Makefile</filename>: the CONFIG variables are |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 1192 | exported here so you can usually just add a "obj-$(CONFIG_xxx) += |
| 1193 | xxx.o" line. The syntax is documented in |
| 1194 | <filename>Documentation/kbuild/makefiles.txt</filename>. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1195 | </para> |
| 1196 | </listitem> |
| 1197 | |
| 1198 | <listitem> |
| 1199 | <para> |
| 1200 | Put yourself in <filename>CREDITS</filename> if you've done |
| 1201 | something noteworthy, usually beyond a single file (your name |
| 1202 | should be at the top of the source files anyway). |
| 1203 | <filename>MAINTAINERS</filename> means you want to be consulted |
| 1204 | when changes are made to a subsystem, and hear about bugs; it |
| 1205 | implies a more-than-passing commitment to some part of the code. |
| 1206 | </para> |
| 1207 | </listitem> |
| 1208 | |
| 1209 | <listitem> |
| 1210 | <para> |
| 1211 | Finally, don't forget to read <filename>Documentation/SubmittingPatches</filename> |
| 1212 | and possibly <filename>Documentation/SubmittingDrivers</filename>. |
| 1213 | </para> |
| 1214 | </listitem> |
| 1215 | </itemizedlist> |
| 1216 | </chapter> |
| 1217 | |
| 1218 | <chapter id="cantrips"> |
| 1219 | <title>Kernel Cantrips</title> |
| 1220 | |
| 1221 | <para> |
| 1222 | Some favorites from browsing the source. Feel free to add to this |
| 1223 | list. |
| 1224 | </para> |
| 1225 | |
| 1226 | <para> |
Randy Dunlap | a1a739c | 2008-10-25 17:24:36 -0700 | [diff] [blame] | 1227 | <filename>arch/x86/include/asm/delay.h:</filename> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1228 | </para> |
| 1229 | <programlisting> |
Rusty Russell | b6c17ea | 2005-09-09 13:10:11 -0700 | [diff] [blame] | 1230 | #define ndelay(n) (__builtin_constant_p(n) ? \ |
| 1231 | ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \ |
| 1232 | __ndelay(n)) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1233 | </programlisting> |
| 1234 | |
| 1235 | <para> |
| 1236 | <filename>include/linux/fs.h</filename>: |
| 1237 | </para> |
| 1238 | <programlisting> |
| 1239 | /* |
| 1240 | * Kernel pointers have redundant information, so we can use a |
| 1241 | * scheme where we can return either an error code or a dentry |
| 1242 | * pointer with the same return value. |
| 1243 | * |
| 1244 | * This should be a per-architecture thing, to allow different |
| 1245 | * error and pointer decisions. |
| 1246 | */ |
| 1247 | #define ERR_PTR(err) ((void *)((long)(err))) |
| 1248 | #define PTR_ERR(ptr) ((long)(ptr)) |
| 1249 | #define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000)) |
| 1250 | </programlisting> |
| 1251 | |
| 1252 | <para> |
Randy Dunlap | a1a739c | 2008-10-25 17:24:36 -0700 | [diff] [blame] | 1253 | <filename>arch/x86/include/asm/uaccess_32.h:</filename> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1254 | </para> |
| 1255 | |
| 1256 | <programlisting> |
| 1257 | #define copy_to_user(to,from,n) \ |
| 1258 | (__builtin_constant_p(n) ? \ |
| 1259 | __constant_copy_to_user((to),(from),(n)) : \ |
| 1260 | __generic_copy_to_user((to),(from),(n))) |
| 1261 | </programlisting> |
| 1262 | |
| 1263 | <para> |
| 1264 | <filename>arch/sparc/kernel/head.S:</filename> |
| 1265 | </para> |
| 1266 | |
| 1267 | <programlisting> |
| 1268 | /* |
| 1269 | * Sun people can't spell worth damn. "compatability" indeed. |
| 1270 | * At least we *know* we can't spell, and use a spell-checker. |
| 1271 | */ |
| 1272 | |
| 1273 | /* Uh, actually Linus it is I who cannot spell. Too much murky |
| 1274 | * Sparc assembly will do this to ya. |
| 1275 | */ |
| 1276 | C_LABEL(cputypvar): |
Masanari Iida | c94bed8e | 2012-04-10 00:22:13 +0900 | [diff] [blame] | 1277 | .asciz "compatibility" |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1278 | |
| 1279 | /* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */ |
| 1280 | .align 4 |
| 1281 | C_LABEL(cputypvar_sun4m): |
| 1282 | .asciz "compatible" |
| 1283 | </programlisting> |
| 1284 | |
| 1285 | <para> |
| 1286 | <filename>arch/sparc/lib/checksum.S:</filename> |
| 1287 | </para> |
| 1288 | |
| 1289 | <programlisting> |
| 1290 | /* Sun, you just can't beat me, you just can't. Stop trying, |
| 1291 | * give up. I'm serious, I am going to kick the living shit |
| 1292 | * out of you, game over, lights out. |
| 1293 | */ |
| 1294 | </programlisting> |
| 1295 | </chapter> |
| 1296 | |
| 1297 | <chapter id="credits"> |
| 1298 | <title>Thanks</title> |
| 1299 | |
| 1300 | <para> |
| 1301 | Thanks to Andi Kleen for the idea, answering my questions, fixing |
| 1302 | my mistakes, filling content, etc. Philipp Rumpf for more spelling |
| 1303 | and clarity fixes, and some excellent non-obvious points. Werner |
| 1304 | Almesberger for giving me a great summary of |
| 1305 | <function>disable_irq()</function>, and Jes Sorensen and Andrea |
| 1306 | Arcangeli added caveats. Michael Elizabeth Chastain for checking |
| 1307 | and adding to the Configure section. <!-- Rusty insisted on this |
| 1308 | bit; I didn't do it! --> Telsa Gwynne for teaching me DocBook. |
| 1309 | </para> |
| 1310 | </chapter> |
| 1311 | </book> |
| 1312 | |