Blame view
Documentation/DocBook/kernel-hacking.tmpl
42.7 KB
1da177e4c Linux-2.6.12-rc2 |
1 2 3 4 5 6 7 8 9 10 |
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []> <book id="lk-hacking-guide"> <bookinfo> <title>Unreliable Guide To Hacking The Linux Kernel</title> <authorgroup> <author> |
b6c17ea4e [PATCH] Update Do... |
11 |
<firstname>Rusty</firstname> |
1da177e4c Linux-2.6.12-rc2 |
12 13 14 15 16 17 18 19 20 21 |
<surname>Russell</surname> <affiliation> <address> <email>rusty@rustcorp.com.au</email> </address> </affiliation> </author> </authorgroup> <copyright> |
b6c17ea4e [PATCH] Update Do... |
22 |
<year>2005</year> |
1da177e4c Linux-2.6.12-rc2 |
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 |
<holder>Rusty Russell</holder> </copyright> <legalnotice> <para> This documentation is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. </para> <para> This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. </para> <para> You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA </para> <para> For more details see the file COPYING in the source distribution of Linux. </para> </legalnotice> <releaseinfo> This is the first release of this document as part of the kernel tarball. </releaseinfo> </bookinfo> <toc></toc> <chapter id="introduction"> <title>Introduction</title> <para> |
b6c17ea4e [PATCH] Update Do... |
66 |
Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux |
1da177e4c Linux-2.6.12-rc2 |
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 |
Kernel Hacking. This document describes the common routines and general requirements for kernel code: its goal is to serve as a primer for Linux kernel development for experienced C programmers. I avoid implementation details: that's what the code is for, and I ignore whole tracts of useful routines. </para> <para> Before you read this, please understand that I never wanted to write this document, being grossly under-qualified, but I always wanted to read it, and this was the only way. I hope it will grow into a compendium of best practice, common starting points and random information. </para> </chapter> <chapter id="basic-players"> <title>The Players</title> <para> At any time each of the CPUs in a system can be: </para> <itemizedlist> <listitem> <para> not associated with any process, serving a hardware interrupt; </para> </listitem> <listitem> <para> |
b6c17ea4e [PATCH] Update Do... |
98 |
not associated with any process, serving a softirq or tasklet; |
1da177e4c Linux-2.6.12-rc2 |
99 100 101 102 103 |
</para> </listitem> <listitem> <para> |
b6c17ea4e [PATCH] Update Do... |
104 |
running in kernel space, associated with a process (user context); |
1da177e4c Linux-2.6.12-rc2 |
105 106 107 108 109 110 111 112 113 114 115 |
</para> </listitem> <listitem> <para> running a process in user space. </para> </listitem> </itemizedlist> <para> |
b6c17ea4e [PATCH] Update Do... |
116 117 118 119 120 121 |
There is an ordering between these. The bottom two can preempt each other, but above that is a strict hierarchy: each can only be preempted by the ones above it. For example, while a softirq is running on a CPU, no other softirq will preempt it, but a hardware interrupt can. However, any other CPUs in the system execute independently. |
1da177e4c Linux-2.6.12-rc2 |
122 123 124 125 126 127 128 129 130 131 132 |
</para> <para> We'll see a number of ways that the user context can block interrupts, to become truly non-preemptable. </para> <sect1 id="basics-usercontext"> <title>User Context</title> <para> |
b6c17ea4e [PATCH] Update Do... |
133 134 135 136 |
User context is when you are coming in from a system call or other trap: like userspace, you can be preempted by more important tasks and by interrupts. You can sleep, by calling <function>schedule()</function>. |
1da177e4c Linux-2.6.12-rc2 |
137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 |
</para> <note> <para> You are always in user context on module load and unload, and on operations on the block device layer. </para> </note> <para> In user context, the <varname>current</varname> pointer (indicating the task we are currently executing) is valid, and <function>in_interrupt()</function> (<filename>include/linux/interrupt.h</filename>) is <returnvalue>false </returnvalue>. </para> <caution> <para> |
b6c17ea4e [PATCH] Update Do... |
156 |
Beware that if you have preemption or softirqs disabled |
1da177e4c Linux-2.6.12-rc2 |
157 158 159 160 161 162 163 164 165 166 167 168 169 170 |
(see below), <function>in_interrupt()</function> will return a false positive. </para> </caution> </sect1> <sect1 id="basics-hardirqs"> <title>Hardware Interrupts (Hard IRQs)</title> <para> Timer ticks, <hardware>network cards</hardware> and <hardware>keyboard</hardware> are examples of real hardware which produce interrupts at any time. The kernel runs interrupt handlers, which services the hardware. The kernel |
b6c17ea4e [PATCH] Update Do... |
171 |
guarantees that this handler is never re-entered: if the same |
1da177e4c Linux-2.6.12-rc2 |
172 173 |
interrupt arrives, it is queued (or dropped). Because it disables interrupts, this handler has to be fast: frequently it |
b6c17ea4e [PATCH] Update Do... |
174 |
simply acknowledges the interrupt, marks a 'software interrupt' |
1da177e4c Linux-2.6.12-rc2 |
175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 |
for execution and exits. </para> <para> You can tell you are in a hardware interrupt, because <function>in_irq()</function> returns <returnvalue>true</returnvalue>. </para> <caution> <para> Beware that this will return a false positive if interrupts are disabled (see below). </para> </caution> </sect1> <sect1 id="basics-softirqs"> |
b6c17ea4e [PATCH] Update Do... |
191 |
<title>Software Interrupt Context: Softirqs and Tasklets</title> |
1da177e4c Linux-2.6.12-rc2 |
192 193 194 |
<para> Whenever a system call is about to return to userspace, or a |
b6c17ea4e [PATCH] Update Do... |
195 |
hardware interrupt handler exits, any 'software interrupts' |
1da177e4c Linux-2.6.12-rc2 |
196 197 198 199 200 201 |
which are marked pending (usually by hardware interrupts) are run (<filename>kernel/softirq.c</filename>). </para> <para> Much of the real interrupt handling work is done here. Early in |
b6c17ea4e [PATCH] Update Do... |
202 |
the transition to <acronym>SMP</acronym>, there were only 'bottom |
1da177e4c Linux-2.6.12-rc2 |
203 204 |
halves' (BHs), which didn't take advantage of multiple CPUs. Shortly after we switched from wind-up computers made of match-sticks and snot, |
b6c17ea4e [PATCH] Update Do... |
205 |
we abandoned this limitation and switched to 'softirqs'. |
1da177e4c Linux-2.6.12-rc2 |
206 207 208 209 |
</para> <para> <filename class="headerfile">include/linux/interrupt.h</filename> lists the |
b6c17ea4e [PATCH] Update Do... |
210 211 212 213 214 |
different softirqs. A very important softirq is the timer softirq (<filename class="headerfile">include/linux/timer.h</filename>): you can register to have it call functions for you in a given length of time. |
1da177e4c Linux-2.6.12-rc2 |
215 216 217 |
</para> <para> |
b6c17ea4e [PATCH] Update Do... |
218 219 220 221 222 223 224 225 |
Softirqs are often a pain to deal with, since the same softirq will run simultaneously on more than one CPU. For this reason, tasklets (<filename class="headerfile">include/linux/interrupt.h</filename>) are more often used: they are dynamically-registrable (meaning you can have as many as you want), and they also guarantee that any tasklet will only run on one CPU at any time, although different tasklets can run simultaneously. |
1da177e4c Linux-2.6.12-rc2 |
226 227 228 |
</para> <caution> <para> |
b6c17ea4e [PATCH] Update Do... |
229 |
The name 'tasklet' is misleading: they have nothing to do with 'tasks', |
1da177e4c Linux-2.6.12-rc2 |
230 231 232 233 234 235 |
and probably more to do with some bad vodka Alexey Kuznetsov had at the time. </para> </caution> <para> |
b6c17ea4e [PATCH] Update Do... |
236 |
You can tell you are in a softirq (or tasklet) |
1da177e4c Linux-2.6.12-rc2 |
237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 |
using the <function>in_softirq()</function> macro (<filename class="headerfile">include/linux/interrupt.h</filename>). </para> <caution> <para> Beware that this will return a false positive if a bh lock (see below) is held. </para> </caution> </sect1> </chapter> <chapter id="basic-rules"> <title>Some Basic Rules</title> <variablelist> <varlistentry> <term>No memory protection</term> <listitem> <para> If you corrupt memory, whether in user context or interrupt context, the whole machine will crash. Are you sure you can't do what you want in userspace? </para> </listitem> </varlistentry> <varlistentry> <term>No floating point or <acronym>MMX</acronym></term> <listitem> <para> The <acronym>FPU</acronym> context is not saved; even in user context the <acronym>FPU</acronym> state probably won't correspond with the current process: you would mess with some user process' <acronym>FPU</acronym> state. If you really want to do this, you would have to explicitly save/restore the full <acronym>FPU</acronym> state (and avoid context switches). It is generally a bad idea; use fixed point arithmetic first. </para> </listitem> </varlistentry> <varlistentry> <term>A rigid stack limit</term> <listitem> <para> |
b6c17ea4e [PATCH] Update Do... |
283 284 285 286 |
Depending on configuration options the kernel stack is about 3K to 6K for most 32-bit architectures: it's about 14K on most 64-bit archs, and often shared with interrupts so you can't use it all. Avoid deep recursion and huge local arrays on the stack (allocate them dynamically instead). |
1da177e4c Linux-2.6.12-rc2 |
287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 |
</para> </listitem> </varlistentry> <varlistentry> <term>The Linux kernel is portable</term> <listitem> <para> Let's keep it that way. Your code should be 64-bit clean, and endian-independent. You should also minimize CPU specific stuff, e.g. inline assembly should be cleanly encapsulated and minimized to ease porting. Generally it should be restricted to the architecture-dependent part of the kernel tree. </para> </listitem> </varlistentry> </variablelist> </chapter> <chapter id="ioctls"> <title>ioctls: Not writing a new system call</title> <para> A system call generally looks like this </para> <programlisting> asmlinkage long sys_mycall(int arg) { return 0; } </programlisting> <para> First, in most cases you don't want to create a new system call. You create a character device and implement an appropriate ioctl for it. This is much more flexible than system calls, doesn't have to be entered in every architecture's <filename class="headerfile">include/asm/unistd.h</filename> and <filename>arch/kernel/entry.S</filename> file, and is much more likely to be accepted by Linus. </para> <para> If all your routine does is read or write some parameter, consider |
b6c17ea4e [PATCH] Update Do... |
333 |
implementing a <function>sysfs</function> interface instead. |
1da177e4c Linux-2.6.12-rc2 |
334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 |
</para> <para> Inside the ioctl you're in user context to a process. When a error occurs you return a negated errno (see <filename class="headerfile">include/linux/errno.h</filename>), otherwise you return <returnvalue>0</returnvalue>. </para> <para> After you slept you should check if a signal occurred: the Unix/Linux way of handling signals is to temporarily exit the system call with the <constant>-ERESTARTSYS</constant> error. The system call entry code will switch back to user context, process the signal handler and then your system call will be restarted (unless the user disabled that). So you should be prepared to process the restart, e.g. if you're in the middle of manipulating some data structure. </para> <programlisting> |
2770f189b docbook: fix sign... |
355 |
if (signal_pending(current)) |
1da177e4c Linux-2.6.12-rc2 |
356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 |
return -ERESTARTSYS; </programlisting> <para> If you're doing longer computations: first think userspace. If you <emphasis>really</emphasis> want to do it in kernel you should regularly check if you need to give up the CPU (remember there is cooperative multitasking per CPU). Idiom: </para> <programlisting> cond_resched(); /* Will sleep */ </programlisting> <para> A short note on interface design: the UNIX system call motto is "Provide mechanism not policy". </para> </chapter> <chapter id="deadlock-recipes"> <title>Recipes for Deadlock</title> <para> You cannot call any routines which may sleep, unless: </para> <itemizedlist> <listitem> <para> You are in user context. </para> </listitem> <listitem> <para> You do not own any spinlocks. </para> </listitem> <listitem> <para> You have interrupts enabled (actually, Andi Kleen says that the scheduling code will enable them for you, but that's probably not what you wanted). </para> </listitem> </itemizedlist> <para> Note that some functions may sleep implicitly: common ones are the user space access functions (*_user) and memory allocation functions without <symbol>GFP_ATOMIC</symbol>. </para> <para> |
b6c17ea4e [PATCH] Update Do... |
411 |
You should always compile your kernel |
d902db1eb sched: Generalize... |
412 |
<symbol>CONFIG_DEBUG_ATOMIC_SLEEP</symbol> on, and it will warn |
b6c17ea4e [PATCH] Update Do... |
413 414 |
you if you break these rules. If you <emphasis>do</emphasis> break the rules, you will eventually lock up your box. |
1da177e4c Linux-2.6.12-rc2 |
415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 |
</para> <para> Really. </para> </chapter> <chapter id="common-routines"> <title>Common Routines</title> <sect1 id="routines-printk"> <title> <function>printk()</function> <filename class="headerfile">include/linux/kernel.h</filename> </title> <para> <function>printk()</function> feeds kernel messages to the console, dmesg, and the syslog daemon. It is useful for debugging and reporting errors, and can be used inside interrupt context, but use with caution: a machine which has its console flooded with printk messages is unusable. It uses a format string mostly compatible with ANSI C printf, and C string concatenation to give it a first "priority" argument: </para> <programlisting> printk(KERN_INFO "i = %u ", i); </programlisting> <para> See <filename class="headerfile">include/linux/kernel.h</filename>; for other KERN_ values; these are interpreted by syslog as the level. Special case: for printing an IP address use </para> <programlisting> |
2a8aaacda docbook: fix prin... |
453 454 455 |
__be32 ipaddress; printk(KERN_INFO "my ip: %pI4 ", &ipaddress); |
1da177e4c Linux-2.6.12-rc2 |
456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 |
</programlisting> <para> <function>printk()</function> internally uses a 1K buffer and does not catch overruns. Make sure that will be enough. </para> <note> <para> You will know when you are a real kernel hacker when you start typoing printf as printk in your user programs :) </para> </note> <!--- From the Lions book reader department --> <note> <para> Another sidenote: the original Unix Version 6 sources had a comment on top of its printf function: "Printf should not be used for chit-chat". You should follow that advice. </para> </note> </sect1> <sect1 id="routines-copy"> <title> <function>copy_[to/from]_user()</function> / <function>get_user()</function> / <function>put_user()</function> <filename class="headerfile">include/asm/uaccess.h</filename> </title> <para> <emphasis>[SLEEPS]</emphasis> </para> <para> <function>put_user()</function> and <function>get_user()</function> are used to get and put single values (such as an int, char, or long) from and to userspace. A pointer into userspace should never be simply dereferenced: data should be copied using these routines. Both return <constant>-EFAULT</constant> or 0. </para> <para> <function>copy_to_user()</function> and <function>copy_from_user()</function> are more general: they copy an arbitrary amount of data to and from userspace. <caution> <para> Unlike <function>put_user()</function> and <function>get_user()</function>, they return the amount of uncopied data (ie. <returnvalue>0</returnvalue> still means success). </para> </caution> |
b6c17ea4e [PATCH] Update Do... |
514 |
[Yes, this moronic interface makes me cringe. The flamewar comes up every year or so. --RR.] |
1da177e4c Linux-2.6.12-rc2 |
515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 |
</para> <para> The functions may sleep implicitly. This should never be called outside user context (it makes no sense), with interrupts disabled, or a spinlock held. </para> </sect1> <sect1 id="routines-kmalloc"> <title><function>kmalloc()</function>/<function>kfree()</function> <filename class="headerfile">include/linux/slab.h</filename></title> <para> <emphasis>[MAY SLEEP: SEE BELOW]</emphasis> </para> <para> These routines are used to dynamically request pointer-aligned chunks of memory, like malloc and free do in userspace, but <function>kmalloc()</function> takes an extra flag word. Important values: </para> <variablelist> <varlistentry> <term> <constant> GFP_KERNEL </constant> </term> <listitem> <para> May sleep and swap to free memory. Only allowed in user context, but is the most reliable way to allocate memory. </para> </listitem> </varlistentry> <varlistentry> <term> <constant> GFP_ATOMIC </constant> </term> <listitem> <para> Don't sleep. Less reliable than <constant>GFP_KERNEL</constant>, but may be called from interrupt context. You should <emphasis>really</emphasis> have a good out-of-memory error-handling strategy. </para> </listitem> </varlistentry> <varlistentry> <term> <constant> GFP_DMA </constant> </term> <listitem> <para> Allocate ISA DMA lower than 16MB. If you don't know what that is you don't need it. Very unreliable. </para> </listitem> </varlistentry> </variablelist> <para> |
b6c17ea4e [PATCH] Update Do... |
585 586 587 588 589 |
If you see a <errorname>sleeping function called from invalid context</errorname> warning message, then maybe you called a sleeping allocation function from interrupt context without <constant>GFP_ATOMIC</constant>. You should really fix that. Run, don't walk. |
1da177e4c Linux-2.6.12-rc2 |
590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 |
</para> <para> If you are allocating at least <constant>PAGE_SIZE</constant> (<filename class="headerfile">include/asm/page.h</filename>) bytes, consider using <function>__get_free_pages()</function> (<filename class="headerfile">include/linux/mm.h</filename>). It takes an order argument (0 for page sized, 1 for double page, 2 for four pages etc.) and the same memory priority flag word as above. </para> <para> If you are allocating more than a page worth of bytes you can use <function>vmalloc()</function>. It'll allocate virtual memory in the kernel map. This block is not contiguous in physical memory, but the <acronym>MMU</acronym> makes it look like it is for you (so it'll only look contiguous to the CPUs, not to external device drivers). If you really need large physically contiguous memory for some weird device, you have a problem: it is poorly supported in Linux because after some time memory fragmentation in a running kernel makes it hard. The best way is to allocate the block early in the boot process via the <function>alloc_bootmem()</function> routine. </para> <para> Before inventing your own cache of often-used objects consider using a slab cache in <filename class="headerfile">include/linux/slab.h</filename> </para> </sect1> <sect1 id="routines-current"> <title><function>current</function> <filename class="headerfile">include/asm/current.h</filename></title> <para> This global variable (really a macro) contains a pointer to the current task structure, so is only valid in user context. For example, when a process makes a system call, this will point to the task structure of the calling process. It is <emphasis>not NULL</emphasis> in interrupt context. </para> </sect1> <sect1 id="routines-udelay"> |
b6c17ea4e [PATCH] Update Do... |
638 |
<title><function>mdelay()</function>/<function>udelay()</function> |
1da177e4c Linux-2.6.12-rc2 |
639 640 641 642 643 |
<filename class="headerfile">include/asm/delay.h</filename> <filename class="headerfile">include/linux/delay.h</filename> </title> <para> |
b6c17ea4e [PATCH] Update Do... |
644 645 |
The <function>udelay()</function> and <function>ndelay()</function> functions can be used for small pauses. Do not use large values with them as you risk |
1da177e4c Linux-2.6.12-rc2 |
646 |
overflow - the helper function <function>mdelay()</function> is useful |
b6c17ea4e [PATCH] Update Do... |
647 |
here, or consider <function>msleep()</function>. |
1da177e4c Linux-2.6.12-rc2 |
648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 |
</para> </sect1> <sect1 id="routines-endian"> <title><function>cpu_to_be32()</function>/<function>be32_to_cpu()</function>/<function>cpu_to_le32()</function>/<function>le32_to_cpu()</function> <filename class="headerfile">include/asm/byteorder.h</filename> </title> <para> The <function>cpu_to_be32()</function> family (where the "32" can be replaced by 64 or 16, and the "be" can be replaced by "le") are the general way to do endian conversions in the kernel: they return the converted value. All variations supply the reverse as well: <function>be32_to_cpu()</function>, etc. </para> <para> There are two major variations of these functions: the pointer variation, such as <function>cpu_to_be32p()</function>, which take a pointer to the given type, and return the converted value. The other variation is the "in-situ" family, such as <function>cpu_to_be32s()</function>, which convert value referred to by the pointer, and return void. </para> </sect1> <sect1 id="routines-local-irqs"> <title><function>local_irq_save()</function>/<function>local_irq_restore()</function> <filename class="headerfile">include/asm/system.h</filename> </title> <para> These routines disable hard interrupts on the local CPU, and restore them. They are reentrant; saving the previous state in their one <varname>unsigned long flags</varname> argument. If you know that interrupts are enabled, you can simply use <function>local_irq_disable()</function> and <function>local_irq_enable()</function>. </para> </sect1> <sect1 id="routines-softirqs"> <title><function>local_bh_disable()</function>/<function>local_bh_enable()</function> <filename class="headerfile">include/linux/interrupt.h</filename></title> <para> These routines disable soft interrupts on the local CPU, and restore them. They are reentrant; if soft interrupts were disabled before, they will still be disabled after this pair |
b6c17ea4e [PATCH] Update Do... |
697 698 |
of functions has been called. They prevent softirqs and tasklets from running on the current CPU. |
1da177e4c Linux-2.6.12-rc2 |
699 700 701 702 703 704 705 706 |
</para> </sect1> <sect1 id="routines-processorids"> <title><function>smp_processor_id</function>() <filename class="headerfile">include/asm/smp.h</filename></title> <para> |
b6c17ea4e [PATCH] Update Do... |
707 708 709 710 711 712 713 714 715 716 |
<function>get_cpu()</function> disables preemption (so you won't suddenly get moved to another CPU) and returns the current processor number, between 0 and <symbol>NR_CPUS</symbol>. Note that the CPU numbers are not necessarily continuous. You return it again with <function>put_cpu()</function> when you are done. </para> <para> If you know you cannot be preempted by another task (ie. you are in interrupt context, or have preemption disabled) you can use smp_processor_id(). |
1da177e4c Linux-2.6.12-rc2 |
717 718 719 720 721 722 723 724 725 726 |
</para> </sect1> <sect1 id="routines-init"> <title><type>__init</type>/<type>__exit</type>/<type>__initdata</type> <filename class="headerfile">include/linux/init.h</filename></title> <para> After boot, the kernel frees up a special section; functions marked with <type>__init</type> and data structures marked with |
b6c17ea4e [PATCH] Update Do... |
727 728 |
<type>__initdata</type> are dropped after boot is complete: similarly modules discard this memory after initialization. <type>__exit</type> |
1da177e4c Linux-2.6.12-rc2 |
729 730 731 732 733 734 |
is used to declare a function which is only required on exit: the function will be dropped if this file is not compiled as a module. See the header file for use. Note that it makes no sense for a function marked with <type>__init</type> to be exported to modules with <function>EXPORT_SYMBOL()</function> - this will break. </para> |
1da177e4c Linux-2.6.12-rc2 |
735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 |
</sect1> <sect1 id="routines-init-again"> <title><function>__initcall()</function>/<function>module_init()</function> <filename class="headerfile">include/linux/init.h</filename></title> <para> Many parts of the kernel are well served as a module (dynamically-loadable parts of the kernel). Using the <function>module_init()</function> and <function>module_exit()</function> macros it is easy to write code without #ifdefs which can operate both as a module or built into the kernel. </para> <para> The <function>module_init()</function> macro defines which function is to be called at module insertion time (if the file is compiled as a module), or at boot time: if the file is not compiled as a module the <function>module_init()</function> macro becomes equivalent to <function>__initcall()</function>, which through linker magic ensures that the function is called on boot. </para> <para> The function can return a negative error number to cause module loading to fail (unfortunately, this has no effect if |
b6c17ea4e [PATCH] Update Do... |
762 763 |
the module is compiled into the kernel). This function is called in user context with interrupts enabled, so it can sleep. |
1da177e4c Linux-2.6.12-rc2 |
764 765 766 767 768 769 770 771 772 773 774 775 776 777 |
</para> </sect1> <sect1 id="routines-moduleexit"> <title> <function>module_exit()</function> <filename class="headerfile">include/linux/init.h</filename> </title> <para> This macro defines the function to be called at module removal time (or never, in the case of the file compiled into the kernel). It will only be called if the module usage count has reached zero. This function can also sleep, but cannot fail: everything must be cleaned up by the time it returns. </para> |
b6c17ea4e [PATCH] Update Do... |
778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 |
<para> Note that this macro is optional: if it is not present, your module will not be removable (except for 'rmmod -f'). </para> </sect1> <sect1 id="routines-module-use-counters"> <title> <function>try_module_get()</function>/<function>module_put()</function> <filename class="headerfile">include/linux/module.h</filename></title> <para> These manipulate the module usage count, to protect against removal (a module also can't be removed if another module uses one of its exported symbols: see below). Before calling into module code, you should call <function>try_module_get()</function> on that module: if it fails, then the module is being removed and you should act as if it wasn't there. Otherwise, you can safely enter the module, and call <function>module_put()</function> when you're finished. </para> <para> Most registerable structures have an <structfield>owner</structfield> field, such as in the <structname>file_operations</structname> structure. Set this field to the macro <symbol>THIS_MODULE</symbol>. </para> |
1da177e4c Linux-2.6.12-rc2 |
806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 |
</sect1> <!-- add info on new-style module refcounting here --> </chapter> <chapter id="queues"> <title>Wait Queues <filename class="headerfile">include/linux/wait.h</filename> </title> <para> <emphasis>[SLEEPS]</emphasis> </para> <para> A wait queue is used to wait for someone to wake you up when a certain condition is true. They must be used carefully to ensure there is no race condition. You declare a <type>wait_queue_head_t</type>, and then processes which want to wait for that condition declare a <type>wait_queue_t</type> referring to themselves, and place that in the queue. </para> <sect1 id="queue-declaring"> <title>Declaring</title> <para> You declare a <type>wait_queue_head_t</type> using the <function>DECLARE_WAIT_QUEUE_HEAD()</function> macro, or using the <function>init_waitqueue_head()</function> routine in your initialization code. </para> </sect1> <sect1 id="queue-waitqueue"> <title>Queuing</title> <para> Placing yourself in the waitqueue is fairly complex, because you must put yourself in the queue before checking the condition. There is a macro to do this: <function>wait_event_interruptible()</function> |
b6c17ea4e [PATCH] Update Do... |
847 |
<filename class="headerfile">include/linux/wait.h</filename> The |
1da177e4c Linux-2.6.12-rc2 |
848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 |
first argument is the wait queue head, and the second is an expression which is evaluated; the macro returns <returnvalue>0</returnvalue> when this expression is true, or <returnvalue>-ERESTARTSYS</returnvalue> if a signal is received. The <function>wait_event()</function> version ignores signals. </para> <para> Do not use the <function>sleep_on()</function> function family - it is very easy to accidentally introduce races; almost certainly one of the <function>wait_event()</function> family will do, or a loop around <function>schedule_timeout()</function>. If you choose to loop around <function>schedule_timeout()</function> remember you must set the task state (with <function>set_current_state()</function>) on each iteration to avoid busy-looping. </para> </sect1> <sect1 id="queue-waking"> <title>Waking Up Queued Tasks</title> <para> Call <function>wake_up()</function> |
b6c17ea4e [PATCH] Update Do... |
872 |
<filename class="headerfile">include/linux/wait.h</filename>;, |
1da177e4c Linux-2.6.12-rc2 |
873 874 |
which will wake up every process in the queue. The exception is if one has <constant>TASK_EXCLUSIVE</constant> set, in which case |
b6c17ea4e [PATCH] Update Do... |
875 876 |
the remainder of the queue will not be woken. There are other variants of this basic function available in the same header. |
1da177e4c Linux-2.6.12-rc2 |
877 878 879 880 881 882 883 884 885 886 887 888 |
</para> </sect1> </chapter> <chapter id="atomic-ops"> <title>Atomic Operations</title> <para> Certain operations are guaranteed atomic on all platforms. The first class of operations work on <type>atomic_t</type> <filename class="headerfile">include/asm/atomic.h</filename>; this |
b6c17ea4e [PATCH] Update Do... |
889 |
contains a signed integer (at least 32 bits long), and you must use |
1da177e4c Linux-2.6.12-rc2 |
890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 |
these functions to manipulate or read atomic_t variables. <function>atomic_read()</function> and <function>atomic_set()</function> get and set the counter, <function>atomic_add()</function>, <function>atomic_sub()</function>, <function>atomic_inc()</function>, <function>atomic_dec()</function>, and <function>atomic_dec_and_test()</function> (returns <returnvalue>true</returnvalue> if it was decremented to zero). </para> <para> Yes. It returns <returnvalue>true</returnvalue> (i.e. != 0) if the atomic variable is zero. </para> <para> Note that these functions are slower than normal arithmetic, and |
b6c17ea4e [PATCH] Update Do... |
908 |
so should not be used unnecessarily. |
1da177e4c Linux-2.6.12-rc2 |
909 910 911 |
</para> <para> |
b6c17ea4e [PATCH] Update Do... |
912 913 |
The second class of atomic operations is atomic bit operations on an <type>unsigned long</type>, defined in |
1da177e4c Linux-2.6.12-rc2 |
914 915 916 917 918 919 920 921 922 923 |
<filename class="headerfile">include/linux/bitops.h</filename>. These operations generally take a pointer to the bit pattern, and a bit number: 0 is the least significant bit. <function>set_bit()</function>, <function>clear_bit()</function> and <function>change_bit()</function> set, clear, and flip the given bit. <function>test_and_set_bit()</function>, <function>test_and_clear_bit()</function> and <function>test_and_change_bit()</function> do the same thing, except return true if the bit was previously set; these are |
b6c17ea4e [PATCH] Update Do... |
924 |
particularly useful for atomically setting flags. |
1da177e4c Linux-2.6.12-rc2 |
925 926 927 928 929 930 931 |
</para> <para> It is possible to call these operations with bit indices greater than BITS_PER_LONG. The resulting behavior is strange on big-endian platforms though so it is a good idea not to do this. </para> |
1da177e4c Linux-2.6.12-rc2 |
932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 |
</chapter> <chapter id="symbols"> <title>Symbols</title> <para> Within the kernel proper, the normal linking rules apply (ie. unless a symbol is declared to be file scope with the <type>static</type> keyword, it can be used anywhere in the kernel). However, for modules, a special exported symbol table is kept which limits the entry points to the kernel proper. Modules can also export symbols. </para> <sect1 id="sym-exportsymbols"> <title><function>EXPORT_SYMBOL()</function> <filename class="headerfile">include/linux/module.h</filename></title> <para> |
b6c17ea4e [PATCH] Update Do... |
951 952 |
This is the classic method of exporting a symbol: dynamically loaded modules will be able to use the symbol as normal. |
1da177e4c Linux-2.6.12-rc2 |
953 954 955 956 957 958 959 960 961 962 963 964 |
</para> </sect1> <sect1 id="sym-exportsymbols-gpl"> <title><function>EXPORT_SYMBOL_GPL()</function> <filename class="headerfile">include/linux/module.h</filename></title> <para> Similar to <function>EXPORT_SYMBOL()</function> except that the symbols exported by <function>EXPORT_SYMBOL_GPL()</function> can only be seen by modules with a <function>MODULE_LICENSE()</function> that specifies a GPL |
b6c17ea4e [PATCH] Update Do... |
965 966 |
compatible license. It implies that the function is considered an internal implementation issue, and not really an interface. |
1da177e4c Linux-2.6.12-rc2 |
967 968 969 970 971 972 973 974 975 976 977 978 |
</para> </sect1> </chapter> <chapter id="conventions"> <title>Routines and Conventions</title> <sect1 id="conventions-doublelinkedlist"> <title>Double-linked lists <filename class="headerfile">include/linux/list.h</filename></title> <para> |
b6c17ea4e [PATCH] Update Do... |
979 980 981 982 983 984 985 |
There used to be three sets of linked-list routines in the kernel headers, but this one is the winner. If you don't have some particular pressing need for a single list, it's a good choice. </para> <para> In particular, <function>list_for_each_entry</function> is useful. |
1da177e4c Linux-2.6.12-rc2 |
986 987 988 989 990 991 992 993 994 995 996 |
</para> </sect1> <sect1 id="convention-returns"> <title>Return Conventions</title> <para> For code called in user context, it's very common to defy C convention, and return <returnvalue>0</returnvalue> for success, and a negative error number (eg. <returnvalue>-EFAULT</returnvalue>) for failure. This can be |
b6c17ea4e [PATCH] Update Do... |
997 |
unintuitive at first, but it's fairly widespread in the kernel. |
1da177e4c Linux-2.6.12-rc2 |
998 999 1000 |
</para> <para> |
b6c17ea4e [PATCH] Update Do... |
1001 |
Using <function>ERR_PTR()</function> |
1da177e4c Linux-2.6.12-rc2 |
1002 |
|
b6c17ea4e [PATCH] Update Do... |
1003 |
<filename class="headerfile">include/linux/err.h</filename>; to |
1da177e4c Linux-2.6.12-rc2 |
1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 |
encode a negative error number into a pointer, and <function>IS_ERR()</function> and <function>PTR_ERR()</function> to get it back out again: avoids a separate pointer parameter for the error number. Icky, but in a good way. </para> </sect1> <sect1 id="conventions-borkedcompile"> <title>Breaking Compilation</title> <para> Linus and the other developers sometimes change function or structure names in development kernels; this is not done just to keep everyone on their toes: it reflects a fundamental change (eg. can no longer be called with interrupts on, or does extra checks, or doesn't do checks which were caught before). Usually this is accompanied by a fairly complete note to the linux-kernel mailing list; search the archive. Simply doing a global replace on the file usually makes things <emphasis>worse</emphasis>. </para> </sect1> <sect1 id="conventions-initialising"> <title>Initializing structure members</title> <para> The preferred method of initializing structures is to use designated initialisers, as defined by ISO C99, eg: </para> <programlisting> static struct block_device_operations opt_fops = { .open = opt_open, .release = opt_release, .ioctl = opt_ioctl, .check_media_change = opt_media_change, }; </programlisting> <para> This makes it easy to grep for, and makes it clear which structure fields are set. You should do this because it looks cool. </para> </sect1> <sect1 id="conventions-gnu-extns"> <title>GNU Extensions</title> <para> GNU Extensions are explicitly allowed in the Linux kernel. Note that some of the more complex ones are not very well supported, due to lack of general use, but the following are considered standard (see the GCC info page section "C Extensions" for more details - Yes, really the info page, the |
b6c17ea4e [PATCH] Update Do... |
1057 |
man page is only a short summary of the stuff in info). |
1da177e4c Linux-2.6.12-rc2 |
1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 |
</para> <itemizedlist> <listitem> <para> Inline functions </para> </listitem> <listitem> <para> Statement expressions (ie. the ({ and }) constructs). </para> </listitem> <listitem> <para> Declaring attributes of a function / variable / type (__attribute__) </para> </listitem> <listitem> <para> typeof </para> </listitem> <listitem> <para> Zero length arrays </para> </listitem> <listitem> <para> Macro varargs </para> </listitem> <listitem> <para> Arithmetic on void pointers </para> </listitem> <listitem> <para> Non-Constant initializers </para> </listitem> <listitem> <para> Assembler Instructions (not outside arch/ and include/asm/) </para> </listitem> <listitem> <para> |
653c03168 misc: replace rem... |
1108 |
Function names as strings (__func__). |
1da177e4c Linux-2.6.12-rc2 |
1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 |
</para> </listitem> <listitem> <para> __builtin_constant_p() </para> </listitem> </itemizedlist> <para> Be wary when using long long in the kernel, the code gcc generates for it is horrible and worse: division and multiplication does not work on i386 because the GCC runtime functions for it are missing from the kernel environment. </para> <!-- FIXME: add a note about ANSI aliasing cleanness --> </sect1> <sect1 id="conventions-cplusplus"> <title>C++</title> <para> Using C++ in the kernel is usually a bad idea, because the kernel does not provide the necessary runtime environment and the include files are not tested for it. It is still possible, but not recommended. If you really want to do this, forget about exceptions at least. </para> </sect1> <sect1 id="conventions-ifdef"> <title>#if</title> <para> It is generally considered cleaner to use macros in header files (or at the top of .c files) to abstract away functions rather than using `#if' pre-processor statements throughout the source code. </para> </sect1> </chapter> <chapter id="submitting"> <title>Putting Your Stuff in the Kernel</title> <para> In order to get your stuff into shape for official inclusion, or even to make a neat patch, there's administrative work to be done: </para> <itemizedlist> <listitem> <para> Figure out whose pond you've been pissing in. Look at the top of the source files, inside the <filename>MAINTAINERS</filename> file, and last of all in the <filename>CREDITS</filename> file. You should coordinate with this person to make sure you're not duplicating effort, or trying something that's already been rejected. </para> <para> Make sure you put your name and EMail address at the top of any files you create or mangle significantly. This is the first place people will look when they find a bug, or when <emphasis>they</emphasis> want to make a change. </para> </listitem> <listitem> <para> Usually you want a configuration option for your kernel hack. |
b6c17ea4e [PATCH] Update Do... |
1181 1182 1183 1184 |
Edit <filename>Kconfig</filename> in the appropriate directory. The Config language is simple to use by cut and paste, and there's complete documentation in <filename>Documentation/kbuild/kconfig-language.txt</filename>. |
1da177e4c Linux-2.6.12-rc2 |
1185 1186 1187 1188 1189 1190 |
</para> <para> You may well want to make your CONFIG option only visible if <symbol>CONFIG_EXPERIMENTAL</symbol> is enabled: this serves as a warning to users. There many other fancy things you can do: see |
b6c17ea4e [PATCH] Update Do... |
1191 |
the various <filename>Kconfig</filename> files for ideas. |
1da177e4c Linux-2.6.12-rc2 |
1192 |
</para> |
1da177e4c Linux-2.6.12-rc2 |
1193 |
|
1da177e4c Linux-2.6.12-rc2 |
1194 |
<para> |
b6c17ea4e [PATCH] Update Do... |
1195 1196 1197 1198 1199 1200 |
In your description of the option, make sure you address both the expert user and the user who knows nothing about your feature. Mention incompatibilities and issues here. <emphasis> Definitely </emphasis> end your description with <quote> if in doubt, say N </quote> (or, occasionally, `Y'); this is for people who have no idea what you are talking about. |
1da177e4c Linux-2.6.12-rc2 |
1201 1202 1203 1204 1205 |
</para> </listitem> <listitem> <para> |
b6c17ea4e [PATCH] Update Do... |
1206 1207 1208 1209 |
Edit the <filename>Makefile</filename>: the CONFIG variables are exported here so you can usually just add a "obj-$(CONFIG_xxx) += xxx.o" line. The syntax is documented in <filename>Documentation/kbuild/makefiles.txt</filename>. |
1da177e4c Linux-2.6.12-rc2 |
1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 |
</para> </listitem> <listitem> <para> Put yourself in <filename>CREDITS</filename> if you've done something noteworthy, usually beyond a single file (your name should be at the top of the source files anyway). <filename>MAINTAINERS</filename> means you want to be consulted when changes are made to a subsystem, and hear about bugs; it implies a more-than-passing commitment to some part of the code. </para> </listitem> <listitem> <para> Finally, don't forget to read <filename>Documentation/SubmittingPatches</filename> and possibly <filename>Documentation/SubmittingDrivers</filename>. </para> </listitem> </itemizedlist> </chapter> <chapter id="cantrips"> <title>Kernel Cantrips</title> <para> Some favorites from browsing the source. Feel free to add to this list. </para> <para> |
a1a739c56 docbooks: fix fat... |
1242 |
<filename>arch/x86/include/asm/delay.h:</filename> |
1da177e4c Linux-2.6.12-rc2 |
1243 1244 |
</para> <programlisting> |
b6c17ea4e [PATCH] Update Do... |
1245 1246 1247 |
#define ndelay(n) (__builtin_constant_p(n) ? \ ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \ __ndelay(n)) |
1da177e4c Linux-2.6.12-rc2 |
1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 |
</programlisting> <para> <filename>include/linux/fs.h</filename>: </para> <programlisting> /* * Kernel pointers have redundant information, so we can use a * scheme where we can return either an error code or a dentry * pointer with the same return value. * * This should be a per-architecture thing, to allow different * error and pointer decisions. */ #define ERR_PTR(err) ((void *)((long)(err))) #define PTR_ERR(ptr) ((long)(ptr)) #define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000)) </programlisting> <para> |
a1a739c56 docbooks: fix fat... |
1268 |
<filename>arch/x86/include/asm/uaccess_32.h:</filename> |
1da177e4c Linux-2.6.12-rc2 |
1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 |
</para> <programlisting> #define copy_to_user(to,from,n) \ (__builtin_constant_p(n) ? \ __constant_copy_to_user((to),(from),(n)) : \ __generic_copy_to_user((to),(from),(n))) </programlisting> <para> <filename>arch/sparc/kernel/head.S:</filename> </para> <programlisting> /* * Sun people can't spell worth damn. "compatability" indeed. * At least we *know* we can't spell, and use a spell-checker. */ /* Uh, actually Linus it is I who cannot spell. Too much murky * Sparc assembly will do this to ya. */ C_LABEL(cputypvar): .asciz "compatability" /* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */ .align 4 C_LABEL(cputypvar_sun4m): .asciz "compatible" </programlisting> <para> <filename>arch/sparc/lib/checksum.S:</filename> </para> <programlisting> /* Sun, you just can't beat me, you just can't. Stop trying, * give up. I'm serious, I am going to kick the living shit * out of you, game over, lights out. */ </programlisting> </chapter> <chapter id="credits"> <title>Thanks</title> <para> Thanks to Andi Kleen for the idea, answering my questions, fixing my mistakes, filling content, etc. Philipp Rumpf for more spelling and clarity fixes, and some excellent non-obvious points. Werner Almesberger for giving me a great summary of <function>disable_irq()</function>, and Jes Sorensen and Andrea Arcangeli added caveats. Michael Elizabeth Chastain for checking and adding to the Configure section. <!-- Rusty insisted on this bit; I didn't do it! --> Telsa Gwynne for teaching me DocBook. </para> </chapter> </book> |