Based on kernel version 4.10.8. Page generated on 2017-04-01 14:43 EST.
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> 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> 20 21 <copyright> 22 <year>2005</year> 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> 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> 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> 98 not associated with any process, serving a softirq or tasklet; 99 </para> 100 </listitem> 101 102 <listitem> 103 <para> 104 running in kernel space, associated with a process (user context); 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> 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> 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> 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> 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> 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> 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 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> 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"> 191 <title>Software Interrupt Context: Softirqs and Tasklets</title> 192 193 <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> 199 200 <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> 207 208 <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> 216 217 <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> 234 235 <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> 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> 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> 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 333 implementing a <function>sysfs</function> interface instead. 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> 355 if (signal_pending(current)) 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> 411 You should always compile your kernel 412 <symbol>CONFIG_DEBUG_ATOMIC_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> 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> 452 __be32 ipaddress; 453 printk(KERN_INFO "my ip: %pI4\n", &ipaddress); 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/linux/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> 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> 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> 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> 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"> 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> 640 641 <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> 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> 674 <filename class="headerfile">include/linux/irqflags.h</filename> 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 695 of functions has been called. They prevent softirqs and tasklets 696 from running on the current CPU. 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> 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> 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 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> 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 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> 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> 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> 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> 845 846 <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> 853 854 </sect1> 855 856 <sect1 id="queue-waking"> 857 <title>Waking Up Queued Tasks</title> 858 859 <para> 860 Call <function>wake_up()</function> 861 862 <filename class="headerfile">include/linux/wait.h</filename>;, 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 865 the remainder of the queue will not be woken. There are other variants 866 of this basic function available in the same header. 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 879 contains a signed integer (at least 32 bits long), and you must use 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 898 so should not be used unnecessarily. 899 </para> 900 901 <para> 902 The second class of atomic operations is atomic bit operations on an 903 <type>unsigned long</type>, defined in 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 914 particularly useful for atomically setting flags. 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> 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> 938 <filename class="headerfile">include/linux/export.h</filename></title> 939 940 <para> 941 This is the classic method of exporting a symbol: dynamically 942 loaded modules will be able to use the symbol as normal. 943 </para> 944 </sect1> 945 946 <sect1 id="sym-exportsymbols-gpl"> 947 <title><function>EXPORT_SYMBOL_GPL()</function> 948 <filename class="headerfile">include/linux/export.h</filename></title> 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 955 compatible license. It implies that the function is considered 956 an internal implementation issue, and not really an interface. 957 Some maintainers and developers may however 958 require EXPORT_SYMBOL_GPL() when adding any new APIs or functionality. 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> 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. 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 989 unintuitive at first, but it's fairly widespread in the kernel. 990 </para> 991 992 <para> 993 Using <function>ERR_PTR()</function> 994 995 <filename class="headerfile">include/linux/err.h</filename>; to 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 1049 man page is only a short summary of the stuff in info). 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> 1100 Function names as strings (__func__). 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. 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>. 1177 </para> 1178 1179 <para> 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. 1186 </para> 1187 </listitem> 1188 1189 <listitem> 1190 <para> 1191 Edit the <filename>Makefile</filename>: the CONFIG variables are 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>. 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/process/submitting-patches.rst</filename> 1212 and possibly <filename>Documentation/process/submitting-drivers.rst</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> 1227 <filename>arch/x86/include/asm/delay.h:</filename> 1228 </para> 1229 <programlisting> 1230 #define ndelay(n) (__builtin_constant_p(n) ? \ 1231 ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \ 1232 __ndelay(n)) 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> 1253 <filename>arch/x86/include/asm/uaccess_32.h:</filename> 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): 1277 .asciz "compatibility" 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>