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1 <?xml version="1.0" encoding="UTF-8"?>
2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
5 <book id="LKLockingGuide">
6 <bookinfo>
7 <title>Unreliable Guide To Locking</title>
9 <authorgroup>
10 <author>
11 <firstname>Rusty</firstname>
12 <surname>Russell</surname>
13 <affiliation>
14 <address>
15 <email>rusty@rustcorp.com.au</email>
16 </address>
17 </affiliation>
18 </author>
19 </authorgroup>
21 <copyright>
22 <year>2003</year>
23 <holder>Rusty Russell</holder>
24 </copyright>
26 <legalnotice>
27 <para>
28 This documentation is free software; you can redistribute
29 it and/or modify it under the terms of the GNU General Public
30 License as published by the Free Software Foundation; either
31 version 2 of the License, or (at your option) any later
32 version.
33 </para>
35 <para>
36 This program is distributed in the hope that it will be
37 useful, but WITHOUT ANY WARRANTY; without even the implied
38 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
39 See the GNU General Public License for more details.
40 </para>
42 <para>
43 You should have received a copy of the GNU General Public
44 License along with this program; if not, write to the Free
45 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
46 MA 02111-1307 USA
47 </para>
49 <para>
50 For more details see the file COPYING in the source
51 distribution of Linux.
52 </para>
53 </legalnotice>
54 </bookinfo>
56 <toc></toc>
57 <chapter id="intro">
58 <title>Introduction</title>
59 <para>
60 Welcome, to Rusty's Remarkably Unreliable Guide to Kernel
61 Locking issues. This document describes the locking systems in
62 the Linux Kernel in 2.6.
63 </para>
64 <para>
65 With the wide availability of HyperThreading, and <firstterm
66 linkend="gloss-preemption">preemption </firstterm> in the Linux
67 Kernel, everyone hacking on the kernel needs to know the
68 fundamentals of concurrency and locking for
69 <firstterm linkend="gloss-smp"><acronym>SMP</acronym></firstterm>.
70 </para>
71 </chapter>
73 <chapter id="races">
74 <title>The Problem With Concurrency</title>
75 <para>
76 (Skip this if you know what a Race Condition is).
77 </para>
78 <para>
79 In a normal program, you can increment a counter like so:
80 </para>
81 <programlisting>
82 very_important_count++;
83 </programlisting>
85 <para>
86 This is what they would expect to happen:
87 </para>
89 <table>
90 <title>Expected Results</title>
92 <tgroup cols="2" align="left">
94 <thead>
95 <row>
96 <entry>Instance 1</entry>
97 <entry>Instance 2</entry>
98 </row>
99 </thead>
101 <tbody>
102 <row>
103 <entry>read very_important_count (5)</entry>
104 <entry></entry>
105 </row>
106 <row>
107 <entry>add 1 (6)</entry>
108 <entry></entry>
109 </row>
110 <row>
111 <entry>write very_important_count (6)</entry>
112 <entry></entry>
113 </row>
114 <row>
115 <entry></entry>
116 <entry>read very_important_count (6)</entry>
117 </row>
118 <row>
119 <entry></entry>
120 <entry>add 1 (7)</entry>
121 </row>
122 <row>
123 <entry></entry>
124 <entry>write very_important_count (7)</entry>
125 </row>
126 </tbody>
128 </tgroup>
129 </table>
131 <para>
132 This is what might happen:
133 </para>
135 <table>
136 <title>Possible Results</title>
138 <tgroup cols="2" align="left">
139 <thead>
140 <row>
141 <entry>Instance 1</entry>
142 <entry>Instance 2</entry>
143 </row>
144 </thead>
146 <tbody>
147 <row>
148 <entry>read very_important_count (5)</entry>
149 <entry></entry>
150 </row>
151 <row>
152 <entry></entry>
153 <entry>read very_important_count (5)</entry>
154 </row>
155 <row>
156 <entry>add 1 (6)</entry>
157 <entry></entry>
158 </row>
159 <row>
160 <entry></entry>
161 <entry>add 1 (6)</entry>
162 </row>
163 <row>
164 <entry>write very_important_count (6)</entry>
165 <entry></entry>
166 </row>
167 <row>
168 <entry></entry>
169 <entry>write very_important_count (6)</entry>
170 </row>
171 </tbody>
172 </tgroup>
173 </table>
175 <sect1 id="race-condition">
176 <title>Race Conditions and Critical Regions</title>
177 <para>
178 This overlap, where the result depends on the
179 relative timing of multiple tasks, is called a <firstterm>race condition</firstterm>.
180 The piece of code containing the concurrency issue is called a
181 <firstterm>critical region</firstterm>. And especially since Linux starting running
182 on SMP machines, they became one of the major issues in kernel
183 design and implementation.
184 </para>
185 <para>
186 Preemption can have the same effect, even if there is only one
187 CPU: by preempting one task during the critical region, we have
188 exactly the same race condition. In this case the thread which
189 preempts might run the critical region itself.
190 </para>
191 <para>
192 The solution is to recognize when these simultaneous accesses
193 occur, and use locks to make sure that only one instance can
194 enter the critical region at any time. There are many
195 friendly primitives in the Linux kernel to help you do this.
196 And then there are the unfriendly primitives, but I'll pretend
197 they don't exist.
198 </para>
199 </sect1>
200 </chapter>
202 <chapter id="locks">
203 <title>Locking in the Linux Kernel</title>
205 <para>
206 If I could give you one piece of advice: never sleep with anyone
207 crazier than yourself. But if I had to give you advice on
208 locking: <emphasis>keep it simple</emphasis>.
209 </para>
211 <para>
212 Be reluctant to introduce new locks.
213 </para>
215 <para>
216 Strangely enough, this last one is the exact reverse of my advice when
217 you <emphasis>have</emphasis> slept with someone crazier than yourself.
218 And you should think about getting a big dog.
219 </para>
221 <sect1 id="lock-intro">
222 <title>Two Main Types of Kernel Locks: Spinlocks and Mutexes</title>
224 <para>
225 There are two main types of kernel locks. The fundamental type
226 is the spinlock
227 (<filename class="headerfile">include/asm/spinlock.h</filename>),
228 which is a very simple single-holder lock: if you can't get the
229 spinlock, you keep trying (spinning) until you can. Spinlocks are
230 very small and fast, and can be used anywhere.
231 </para>
232 <para>
233 The second type is a mutex
234 (<filename class="headerfile">include/linux/mutex.h</filename>): it
235 is like a spinlock, but you may block holding a mutex.
236 If you can't lock a mutex, your task will suspend itself, and be woken
237 up when the mutex is released. This means the CPU can do something
238 else while you are waiting. There are many cases when you simply
239 can't sleep (see <xref linkend="sleeping-things"/>), and so have to
240 use a spinlock instead.
241 </para>
242 <para>
243 Neither type of lock is recursive: see
244 <xref linkend="deadlock"/>.
245 </para>
246 </sect1>
248 <sect1 id="uniprocessor">
249 <title>Locks and Uniprocessor Kernels</title>
251 <para>
252 For kernels compiled without <symbol>CONFIG_SMP</symbol>, and
253 without <symbol>CONFIG_PREEMPT</symbol> spinlocks do not exist at
254 all. This is an excellent design decision: when no-one else can
255 run at the same time, there is no reason to have a lock.
256 </para>
258 <para>
259 If the kernel is compiled without <symbol>CONFIG_SMP</symbol>,
260 but <symbol>CONFIG_PREEMPT</symbol> is set, then spinlocks
261 simply disable preemption, which is sufficient to prevent any
262 races. For most purposes, we can think of preemption as
263 equivalent to SMP, and not worry about it separately.
264 </para>
266 <para>
267 You should always test your locking code with <symbol>CONFIG_SMP</symbol>
268 and <symbol>CONFIG_PREEMPT</symbol> enabled, even if you don't have an SMP test box, because it
269 will still catch some kinds of locking bugs.
270 </para>
272 <para>
273 Mutexes still exist, because they are required for
274 synchronization between <firstterm linkend="gloss-usercontext">user
275 contexts</firstterm>, as we will see below.
276 </para>
277 </sect1>
279 <sect1 id="usercontextlocking">
280 <title>Locking Only In User Context</title>
282 <para>
283 If you have a data structure which is only ever accessed from
284 user context, then you can use a simple mutex
285 (<filename>include/linux/mutex.h</filename>) to protect it. This
286 is the most trivial case: you initialize the mutex. Then you can
287 call <function>mutex_lock_interruptible()</function> to grab the mutex,
288 and <function>mutex_unlock()</function> to release it. There is also a
289 <function>mutex_lock()</function>, which should be avoided, because it
290 will not return if a signal is received.
291 </para>
293 <para>
294 Example: <filename>net/netfilter/nf_sockopt.c</filename> allows
295 registration of new <function>setsockopt()</function> and
296 <function>getsockopt()</function> calls, with
297 <function>nf_register_sockopt()</function>. Registration and
298 de-registration are only done on module load and unload (and boot
299 time, where there is no concurrency), and the list of registrations
300 is only consulted for an unknown <function>setsockopt()</function>
301 or <function>getsockopt()</function> system call. The
302 <varname>nf_sockopt_mutex</varname> is perfect to protect this,
303 especially since the setsockopt and getsockopt calls may well
304 sleep.
305 </para>
306 </sect1>
308 <sect1 id="lock-user-bh">
309 <title>Locking Between User Context and Softirqs</title>
311 <para>
312 If a <firstterm linkend="gloss-softirq">softirq</firstterm> shares
313 data with user context, you have two problems. Firstly, the current
314 user context can be interrupted by a softirq, and secondly, the
315 critical region could be entered from another CPU. This is where
316 <function>spin_lock_bh()</function>
317 (<filename class="headerfile">include/linux/spinlock.h</filename>) is
318 used. It disables softirqs on that CPU, then grabs the lock.
319 <function>spin_unlock_bh()</function> does the reverse. (The
320 '_bh' suffix is a historical reference to "Bottom Halves", the
321 old name for software interrupts. It should really be
322 called spin_lock_softirq()' in a perfect world).
323 </para>
325 <para>
326 Note that you can also use <function>spin_lock_irq()</function>
327 or <function>spin_lock_irqsave()</function> here, which stop
328 hardware interrupts as well: see <xref linkend="hardirq-context"/>.
329 </para>
331 <para>
332 This works perfectly for <firstterm linkend="gloss-up"><acronym>UP
333 </acronym></firstterm> as well: the spin lock vanishes, and this macro
334 simply becomes <function>local_bh_disable()</function>
335 (<filename class="headerfile">include/linux/interrupt.h</filename>), which
336 protects you from the softirq being run.
337 </para>
338 </sect1>
340 <sect1 id="lock-user-tasklet">
341 <title>Locking Between User Context and Tasklets</title>
343 <para>
344 This is exactly the same as above, because <firstterm
345 linkend="gloss-tasklet">tasklets</firstterm> are actually run
346 from a softirq.
347 </para>
348 </sect1>
350 <sect1 id="lock-user-timers">
351 <title>Locking Between User Context and Timers</title>
353 <para>
354 This, too, is exactly the same as above, because <firstterm
355 linkend="gloss-timers">timers</firstterm> are actually run from
356 a softirq. From a locking point of view, tasklets and timers
357 are identical.
358 </para>
359 </sect1>
361 <sect1 id="lock-tasklets">
362 <title>Locking Between Tasklets/Timers</title>
364 <para>
365 Sometimes a tasklet or timer might want to share data with
366 another tasklet or timer.
367 </para>
369 <sect2 id="lock-tasklets-same">
370 <title>The Same Tasklet/Timer</title>
371 <para>
372 Since a tasklet is never run on two CPUs at once, you don't
373 need to worry about your tasklet being reentrant (running
374 twice at once), even on SMP.
375 </para>
376 </sect2>
378 <sect2 id="lock-tasklets-different">
379 <title>Different Tasklets/Timers</title>
380 <para>
381 If another tasklet/timer wants
382 to share data with your tasklet or timer , you will both need to use
383 <function>spin_lock()</function> and
384 <function>spin_unlock()</function> calls.
385 <function>spin_lock_bh()</function> is
386 unnecessary here, as you are already in a tasklet, and
387 none will be run on the same CPU.
388 </para>
389 </sect2>
390 </sect1>
392 <sect1 id="lock-softirqs">
393 <title>Locking Between Softirqs</title>
395 <para>
396 Often a softirq might
397 want to share data with itself or a tasklet/timer.
398 </para>
400 <sect2 id="lock-softirqs-same">
401 <title>The Same Softirq</title>
403 <para>
404 The same softirq can run on the other CPUs: you can use a
405 per-CPU array (see <xref linkend="per-cpu"/>) for better
406 performance. If you're going so far as to use a softirq,
407 you probably care about scalable performance enough
408 to justify the extra complexity.
409 </para>
411 <para>
412 You'll need to use <function>spin_lock()</function> and
413 <function>spin_unlock()</function> for shared data.
414 </para>
415 </sect2>
417 <sect2 id="lock-softirqs-different">
418 <title>Different Softirqs</title>
420 <para>
421 You'll need to use <function>spin_lock()</function> and
422 <function>spin_unlock()</function> for shared data, whether it
423 be a timer, tasklet, different softirq or the same or another
424 softirq: any of them could be running on a different CPU.
425 </para>
426 </sect2>
427 </sect1>
428 </chapter>
430 <chapter id="hardirq-context">
431 <title>Hard IRQ Context</title>
433 <para>
434 Hardware interrupts usually communicate with a
435 tasklet or softirq. Frequently this involves putting work in a
436 queue, which the softirq will take out.
437 </para>
439 <sect1 id="hardirq-softirq">
440 <title>Locking Between Hard IRQ and Softirqs/Tasklets</title>
442 <para>
443 If a hardware irq handler shares data with a softirq, you have
444 two concerns. Firstly, the softirq processing can be
445 interrupted by a hardware interrupt, and secondly, the
446 critical region could be entered by a hardware interrupt on
447 another CPU. This is where <function>spin_lock_irq()</function> is
448 used. It is defined to disable interrupts on that cpu, then grab
449 the lock. <function>spin_unlock_irq()</function> does the reverse.
450 </para>
452 <para>
453 The irq handler does not to use
454 <function>spin_lock_irq()</function>, because the softirq cannot
455 run while the irq handler is running: it can use
456 <function>spin_lock()</function>, which is slightly faster. The
457 only exception would be if a different hardware irq handler uses
458 the same lock: <function>spin_lock_irq()</function> will stop
459 that from interrupting us.
460 </para>
462 <para>
463 This works perfectly for UP as well: the spin lock vanishes,
464 and this macro simply becomes <function>local_irq_disable()</function>
465 (<filename class="headerfile">include/asm/smp.h</filename>), which
466 protects you from the softirq/tasklet/BH being run.
467 </para>
469 <para>
470 <function>spin_lock_irqsave()</function>
471 (<filename>include/linux/spinlock.h</filename>) is a variant
472 which saves whether interrupts were on or off in a flags word,
473 which is passed to <function>spin_unlock_irqrestore()</function>. This
474 means that the same code can be used inside an hard irq handler (where
475 interrupts are already off) and in softirqs (where the irq
476 disabling is required).
477 </para>
479 <para>
480 Note that softirqs (and hence tasklets and timers) are run on
481 return from hardware interrupts, so
482 <function>spin_lock_irq()</function> also stops these. In that
483 sense, <function>spin_lock_irqsave()</function> is the most
484 general and powerful locking function.
485 </para>
487 </sect1>
488 <sect1 id="hardirq-hardirq">
489 <title>Locking Between Two Hard IRQ Handlers</title>
490 <para>
491 It is rare to have to share data between two IRQ handlers, but
492 if you do, <function>spin_lock_irqsave()</function> should be
493 used: it is architecture-specific whether all interrupts are
494 disabled inside irq handlers themselves.
495 </para>
496 </sect1>
498 </chapter>
500 <chapter id="cheatsheet">
501 <title>Cheat Sheet For Locking</title>
502 <para>
503 Pete Zaitcev gives the following summary:
504 </para>
505 <itemizedlist>
506 <listitem>
507 <para>
508 If you are in a process context (any syscall) and want to
509 lock other process out, use a mutex. You can take a mutex
510 and sleep (<function>copy_from_user*(</function> or
511 <function>kmalloc(x,GFP_KERNEL)</function>).
512 </para>
513 </listitem>
514 <listitem>
515 <para>
516 Otherwise (== data can be touched in an interrupt), use
517 <function>spin_lock_irqsave()</function> and
518 <function>spin_unlock_irqrestore()</function>.
519 </para>
520 </listitem>
521 <listitem>
522 <para>
523 Avoid holding spinlock for more than 5 lines of code and
524 across any function call (except accessors like
525 <function>readb</function>).
526 </para>
527 </listitem>
528 </itemizedlist>
530 <sect1 id="minimum-lock-reqirements">
531 <title>Table of Minimum Requirements</title>
533 <para> The following table lists the <emphasis>minimum</emphasis>
534 locking requirements between various contexts. In some cases,
535 the same context can only be running on one CPU at a time, so
536 no locking is required for that context (eg. a particular
537 thread can only run on one CPU at a time, but if it needs
538 shares data with another thread, locking is required).
539 </para>
540 <para>
541 Remember the advice above: you can always use
542 <function>spin_lock_irqsave()</function>, which is a superset
543 of all other spinlock primitives.
544 </para>
546 <table>
547 <title>Table of Locking Requirements</title>
548 <tgroup cols="11">
549 <tbody>
551 <row>
552 <entry></entry>
553 <entry>IRQ Handler A</entry>
554 <entry>IRQ Handler B</entry>
555 <entry>Softirq A</entry>
556 <entry>Softirq B</entry>
557 <entry>Tasklet A</entry>
558 <entry>Tasklet B</entry>
559 <entry>Timer A</entry>
560 <entry>Timer B</entry>
561 <entry>User Context A</entry>
562 <entry>User Context B</entry>
563 </row>
565 <row>
566 <entry>IRQ Handler A</entry>
567 <entry>None</entry>
568 </row>
570 <row>
571 <entry>IRQ Handler B</entry>
572 <entry>SLIS</entry>
573 <entry>None</entry>
574 </row>
576 <row>
577 <entry>Softirq A</entry>
578 <entry>SLI</entry>
579 <entry>SLI</entry>
580 <entry>SL</entry>
581 </row>
583 <row>
584 <entry>Softirq B</entry>
585 <entry>SLI</entry>
586 <entry>SLI</entry>
587 <entry>SL</entry>
588 <entry>SL</entry>
589 </row>
591 <row>
592 <entry>Tasklet A</entry>
593 <entry>SLI</entry>
594 <entry>SLI</entry>
595 <entry>SL</entry>
596 <entry>SL</entry>
597 <entry>None</entry>
598 </row>
600 <row>
601 <entry>Tasklet B</entry>
602 <entry>SLI</entry>
603 <entry>SLI</entry>
604 <entry>SL</entry>
605 <entry>SL</entry>
606 <entry>SL</entry>
607 <entry>None</entry>
608 </row>
610 <row>
611 <entry>Timer A</entry>
612 <entry>SLI</entry>
613 <entry>SLI</entry>
614 <entry>SL</entry>
615 <entry>SL</entry>
616 <entry>SL</entry>
617 <entry>SL</entry>
618 <entry>None</entry>
619 </row>
621 <row>
622 <entry>Timer B</entry>
623 <entry>SLI</entry>
624 <entry>SLI</entry>
625 <entry>SL</entry>
626 <entry>SL</entry>
627 <entry>SL</entry>
628 <entry>SL</entry>
629 <entry>SL</entry>
630 <entry>None</entry>
631 </row>
633 <row>
634 <entry>User Context A</entry>
635 <entry>SLI</entry>
636 <entry>SLI</entry>
637 <entry>SLBH</entry>
638 <entry>SLBH</entry>
639 <entry>SLBH</entry>
640 <entry>SLBH</entry>
641 <entry>SLBH</entry>
642 <entry>SLBH</entry>
643 <entry>None</entry>
644 </row>
646 <row>
647 <entry>User Context B</entry>
648 <entry>SLI</entry>
649 <entry>SLI</entry>
650 <entry>SLBH</entry>
651 <entry>SLBH</entry>
652 <entry>SLBH</entry>
653 <entry>SLBH</entry>
654 <entry>SLBH</entry>
655 <entry>SLBH</entry>
656 <entry>MLI</entry>
657 <entry>None</entry>
658 </row>
660 </tbody>
661 </tgroup>
662 </table>
664 <table>
665 <title>Legend for Locking Requirements Table</title>
666 <tgroup cols="2">
667 <tbody>
669 <row>
670 <entry>SLIS</entry>
671 <entry>spin_lock_irqsave</entry>
672 </row>
673 <row>
674 <entry>SLI</entry>
675 <entry>spin_lock_irq</entry>
676 </row>
677 <row>
678 <entry>SL</entry>
679 <entry>spin_lock</entry>
680 </row>
681 <row>
682 <entry>SLBH</entry>
683 <entry>spin_lock_bh</entry>
684 </row>
685 <row>
686 <entry>MLI</entry>
687 <entry>mutex_lock_interruptible</entry>
688 </row>
690 </tbody>
691 </tgroup>
692 </table>
694 </sect1>
695 </chapter>
697 <chapter id="trylock-functions">
698 <title>The trylock Functions</title>
699 <para>
700 There are functions that try to acquire a lock only once and immediately
701 return a value telling about success or failure to acquire the lock.
702 They can be used if you need no access to the data protected with the lock
703 when some other thread is holding the lock. You should acquire the lock
704 later if you then need access to the data protected with the lock.
705 </para>
707 <para>
708 <function>spin_trylock()</function> does not spin but returns non-zero if
709 it acquires the spinlock on the first try or 0 if not. This function can
710 be used in all contexts like <function>spin_lock</function>: you must have
711 disabled the contexts that might interrupt you and acquire the spin lock.
712 </para>
714 <para>
715 <function>mutex_trylock()</function> does not suspend your task
716 but returns non-zero if it could lock the mutex on the first try
717 or 0 if not. This function cannot be safely used in hardware or software
718 interrupt contexts despite not sleeping.
719 </para>
720 </chapter>
722 <chapter id="Examples">
723 <title>Common Examples</title>
724 <para>
725 Let's step through a simple example: a cache of number to name
726 mappings. The cache keeps a count of how often each of the objects is
727 used, and when it gets full, throws out the least used one.
729 </para>
731 <sect1 id="examples-usercontext">
732 <title>All In User Context</title>
733 <para>
734 For our first example, we assume that all operations are in user
735 context (ie. from system calls), so we can sleep. This means we can
736 use a mutex to protect the cache and all the objects within
737 it. Here's the code:
738 </para>
740 <programlisting>
741 #include &lt;linux/list.h&gt;
742 #include &lt;linux/slab.h&gt;
743 #include &lt;linux/string.h&gt;
744 #include &lt;linux/mutex.h&gt;
745 #include &lt;asm/errno.h&gt;
747 struct object
749 struct list_head list;
750 int id;
751 char name[32];
752 int popularity;
755 /* Protects the cache, cache_num, and the objects within it */
756 static DEFINE_MUTEX(cache_lock);
757 static LIST_HEAD(cache);
758 static unsigned int cache_num = 0;
759 #define MAX_CACHE_SIZE 10
761 /* Must be holding cache_lock */
762 static struct object *__cache_find(int id)
764 struct object *i;
766 list_for_each_entry(i, &amp;cache, list)
767 if (i-&gt;id == id) {
768 i-&gt;popularity++;
769 return i;
771 return NULL;
774 /* Must be holding cache_lock */
775 static void __cache_delete(struct object *obj)
777 BUG_ON(!obj);
778 list_del(&amp;obj-&gt;list);
779 kfree(obj);
780 cache_num--;
783 /* Must be holding cache_lock */
784 static void __cache_add(struct object *obj)
786 list_add(&amp;obj-&gt;list, &amp;cache);
787 if (++cache_num > MAX_CACHE_SIZE) {
788 struct object *i, *outcast = NULL;
789 list_for_each_entry(i, &amp;cache, list) {
790 if (!outcast || i-&gt;popularity &lt; outcast-&gt;popularity)
791 outcast = i;
793 __cache_delete(outcast);
797 int cache_add(int id, const char *name)
799 struct object *obj;
801 if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL)
802 return -ENOMEM;
804 strlcpy(obj-&gt;name, name, sizeof(obj-&gt;name));
805 obj-&gt;id = id;
806 obj-&gt;popularity = 0;
808 mutex_lock(&amp;cache_lock);
809 __cache_add(obj);
810 mutex_unlock(&amp;cache_lock);
811 return 0;
814 void cache_delete(int id)
816 mutex_lock(&amp;cache_lock);
817 __cache_delete(__cache_find(id));
818 mutex_unlock(&amp;cache_lock);
821 int cache_find(int id, char *name)
823 struct object *obj;
824 int ret = -ENOENT;
826 mutex_lock(&amp;cache_lock);
827 obj = __cache_find(id);
828 if (obj) {
829 ret = 0;
830 strcpy(name, obj-&gt;name);
832 mutex_unlock(&amp;cache_lock);
833 return ret;
835 </programlisting>
837 <para>
838 Note that we always make sure we have the cache_lock when we add,
839 delete, or look up the cache: both the cache infrastructure itself and
840 the contents of the objects are protected by the lock. In this case
841 it's easy, since we copy the data for the user, and never let them
842 access the objects directly.
843 </para>
844 <para>
845 There is a slight (and common) optimization here: in
846 <function>cache_add</function> we set up the fields of the object
847 before grabbing the lock. This is safe, as no-one else can access it
848 until we put it in cache.
849 </para>
850 </sect1>
852 <sect1 id="examples-interrupt">
853 <title>Accessing From Interrupt Context</title>
854 <para>
855 Now consider the case where <function>cache_find</function> can be
856 called from interrupt context: either a hardware interrupt or a
857 softirq. An example would be a timer which deletes object from the
858 cache.
859 </para>
860 <para>
861 The change is shown below, in standard patch format: the
862 <symbol>-</symbol> are lines which are taken away, and the
863 <symbol>+</symbol> are lines which are added.
864 </para>
865 <programlisting>
866 --- cache.c.usercontext 2003-12-09 13:58:54.000000000 +1100
867 +++ cache.c.interrupt 2003-12-09 14:07:49.000000000 +1100
868 @@ -12,7 +12,7 @@
869 int popularity;
872 -static DEFINE_MUTEX(cache_lock);
873 +static DEFINE_SPINLOCK(cache_lock);
874 static LIST_HEAD(cache);
875 static unsigned int cache_num = 0;
876 #define MAX_CACHE_SIZE 10
877 @@ -55,6 +55,7 @@
878 int cache_add(int id, const char *name)
880 struct object *obj;
881 + unsigned long flags;
883 if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL)
884 return -ENOMEM;
885 @@ -63,30 +64,33 @@
886 obj-&gt;id = id;
887 obj-&gt;popularity = 0;
889 - mutex_lock(&amp;cache_lock);
890 + spin_lock_irqsave(&amp;cache_lock, flags);
891 __cache_add(obj);
892 - mutex_unlock(&amp;cache_lock);
893 + spin_unlock_irqrestore(&amp;cache_lock, flags);
894 return 0;
897 void cache_delete(int id)
899 - mutex_lock(&amp;cache_lock);
900 + unsigned long flags;
902 + spin_lock_irqsave(&amp;cache_lock, flags);
903 __cache_delete(__cache_find(id));
904 - mutex_unlock(&amp;cache_lock);
905 + spin_unlock_irqrestore(&amp;cache_lock, flags);
908 int cache_find(int id, char *name)
910 struct object *obj;
911 int ret = -ENOENT;
912 + unsigned long flags;
914 - mutex_lock(&amp;cache_lock);
915 + spin_lock_irqsave(&amp;cache_lock, flags);
916 obj = __cache_find(id);
917 if (obj) {
918 ret = 0;
919 strcpy(name, obj-&gt;name);
921 - mutex_unlock(&amp;cache_lock);
922 + spin_unlock_irqrestore(&amp;cache_lock, flags);
923 return ret;
925 </programlisting>
927 <para>
928 Note that the <function>spin_lock_irqsave</function> will turn off
929 interrupts if they are on, otherwise does nothing (if we are already
930 in an interrupt handler), hence these functions are safe to call from
931 any context.
932 </para>
933 <para>
934 Unfortunately, <function>cache_add</function> calls
935 <function>kmalloc</function> with the <symbol>GFP_KERNEL</symbol>
936 flag, which is only legal in user context. I have assumed that
937 <function>cache_add</function> is still only called in user context,
938 otherwise this should become a parameter to
939 <function>cache_add</function>.
940 </para>
941 </sect1>
942 <sect1 id="examples-refcnt">
943 <title>Exposing Objects Outside This File</title>
944 <para>
945 If our objects contained more information, it might not be sufficient
946 to copy the information in and out: other parts of the code might want
947 to keep pointers to these objects, for example, rather than looking up
948 the id every time. This produces two problems.
949 </para>
950 <para>
951 The first problem is that we use the <symbol>cache_lock</symbol> to
952 protect objects: we'd need to make this non-static so the rest of the
953 code can use it. This makes locking trickier, as it is no longer all
954 in one place.
955 </para>
956 <para>
957 The second problem is the lifetime problem: if another structure keeps
958 a pointer to an object, it presumably expects that pointer to remain
959 valid. Unfortunately, this is only guaranteed while you hold the
960 lock, otherwise someone might call <function>cache_delete</function>
961 and even worse, add another object, re-using the same address.
962 </para>
963 <para>
964 As there is only one lock, you can't hold it forever: no-one else would
965 get any work done.
966 </para>
967 <para>
968 The solution to this problem is to use a reference count: everyone who
969 has a pointer to the object increases it when they first get the
970 object, and drops the reference count when they're finished with it.
971 Whoever drops it to zero knows it is unused, and can actually delete it.
972 </para>
973 <para>
974 Here is the code:
975 </para>
977 <programlisting>
978 --- cache.c.interrupt 2003-12-09 14:25:43.000000000 +1100
979 +++ cache.c.refcnt 2003-12-09 14:33:05.000000000 +1100
980 @@ -7,6 +7,7 @@
981 struct object
983 struct list_head list;
984 + unsigned int refcnt;
985 int id;
986 char name[32];
987 int popularity;
988 @@ -17,6 +18,35 @@
989 static unsigned int cache_num = 0;
990 #define MAX_CACHE_SIZE 10
992 +static void __object_put(struct object *obj)
994 + if (--obj-&gt;refcnt == 0)
995 + kfree(obj);
998 +static void __object_get(struct object *obj)
1000 + obj-&gt;refcnt++;
1003 +void object_put(struct object *obj)
1005 + unsigned long flags;
1007 + spin_lock_irqsave(&amp;cache_lock, flags);
1008 + __object_put(obj);
1009 + spin_unlock_irqrestore(&amp;cache_lock, flags);
1012 +void object_get(struct object *obj)
1014 + unsigned long flags;
1016 + spin_lock_irqsave(&amp;cache_lock, flags);
1017 + __object_get(obj);
1018 + spin_unlock_irqrestore(&amp;cache_lock, flags);
1021 /* Must be holding cache_lock */
1022 static struct object *__cache_find(int id)
1024 @@ -35,6 +65,7 @@
1026 BUG_ON(!obj);
1027 list_del(&amp;obj-&gt;list);
1028 + __object_put(obj);
1029 cache_num--;
1032 @@ -63,6 +94,7 @@
1033 strlcpy(obj-&gt;name, name, sizeof(obj-&gt;name));
1034 obj-&gt;id = id;
1035 obj-&gt;popularity = 0;
1036 + obj-&gt;refcnt = 1; /* The cache holds a reference */
1038 spin_lock_irqsave(&amp;cache_lock, flags);
1039 __cache_add(obj);
1040 @@ -79,18 +111,15 @@
1041 spin_unlock_irqrestore(&amp;cache_lock, flags);
1044 -int cache_find(int id, char *name)
1045 +struct object *cache_find(int id)
1047 struct object *obj;
1048 - int ret = -ENOENT;
1049 unsigned long flags;
1051 spin_lock_irqsave(&amp;cache_lock, flags);
1052 obj = __cache_find(id);
1053 - if (obj) {
1054 - ret = 0;
1055 - strcpy(name, obj-&gt;name);
1057 + if (obj)
1058 + __object_get(obj);
1059 spin_unlock_irqrestore(&amp;cache_lock, flags);
1060 - return ret;
1061 + return obj;
1063 </programlisting>
1065 <para>
1066 We encapsulate the reference counting in the standard 'get' and 'put'
1067 functions. Now we can return the object itself from
1068 <function>cache_find</function> which has the advantage that the user
1069 can now sleep holding the object (eg. to
1070 <function>copy_to_user</function> to name to userspace).
1071 </para>
1072 <para>
1073 The other point to note is that I said a reference should be held for
1074 every pointer to the object: thus the reference count is 1 when first
1075 inserted into the cache. In some versions the framework does not hold
1076 a reference count, but they are more complicated.
1077 </para>
1079 <sect2 id="examples-refcnt-atomic">
1080 <title>Using Atomic Operations For The Reference Count</title>
1081 <para>
1082 In practice, <type>atomic_t</type> would usually be used for
1083 <structfield>refcnt</structfield>. There are a number of atomic
1084 operations defined in
1086 <filename class="headerfile">include/asm/atomic.h</filename>: these are
1087 guaranteed to be seen atomically from all CPUs in the system, so no
1088 lock is required. In this case, it is simpler than using spinlocks,
1089 although for anything non-trivial using spinlocks is clearer. The
1090 <function>atomic_inc</function> and
1091 <function>atomic_dec_and_test</function> are used instead of the
1092 standard increment and decrement operators, and the lock is no longer
1093 used to protect the reference count itself.
1094 </para>
1096 <programlisting>
1097 --- cache.c.refcnt 2003-12-09 15:00:35.000000000 +1100
1098 +++ cache.c.refcnt-atomic 2003-12-11 15:49:42.000000000 +1100
1099 @@ -7,7 +7,7 @@
1100 struct object
1102 struct list_head list;
1103 - unsigned int refcnt;
1104 + atomic_t refcnt;
1105 int id;
1106 char name[32];
1107 int popularity;
1108 @@ -18,33 +18,15 @@
1109 static unsigned int cache_num = 0;
1110 #define MAX_CACHE_SIZE 10
1112 -static void __object_put(struct object *obj)
1114 - if (--obj-&gt;refcnt == 0)
1115 - kfree(obj);
1118 -static void __object_get(struct object *obj)
1120 - obj-&gt;refcnt++;
1123 void object_put(struct object *obj)
1125 - unsigned long flags;
1127 - spin_lock_irqsave(&amp;cache_lock, flags);
1128 - __object_put(obj);
1129 - spin_unlock_irqrestore(&amp;cache_lock, flags);
1130 + if (atomic_dec_and_test(&amp;obj-&gt;refcnt))
1131 + kfree(obj);
1134 void object_get(struct object *obj)
1136 - unsigned long flags;
1138 - spin_lock_irqsave(&amp;cache_lock, flags);
1139 - __object_get(obj);
1140 - spin_unlock_irqrestore(&amp;cache_lock, flags);
1141 + atomic_inc(&amp;obj-&gt;refcnt);
1144 /* Must be holding cache_lock */
1145 @@ -65,7 +47,7 @@
1147 BUG_ON(!obj);
1148 list_del(&amp;obj-&gt;list);
1149 - __object_put(obj);
1150 + object_put(obj);
1151 cache_num--;
1154 @@ -94,7 +76,7 @@
1155 strlcpy(obj-&gt;name, name, sizeof(obj-&gt;name));
1156 obj-&gt;id = id;
1157 obj-&gt;popularity = 0;
1158 - obj-&gt;refcnt = 1; /* The cache holds a reference */
1159 + atomic_set(&amp;obj-&gt;refcnt, 1); /* The cache holds a reference */
1161 spin_lock_irqsave(&amp;cache_lock, flags);
1162 __cache_add(obj);
1163 @@ -119,7 +101,7 @@
1164 spin_lock_irqsave(&amp;cache_lock, flags);
1165 obj = __cache_find(id);
1166 if (obj)
1167 - __object_get(obj);
1168 + object_get(obj);
1169 spin_unlock_irqrestore(&amp;cache_lock, flags);
1170 return obj;
1172 </programlisting>
1173 </sect2>
1174 </sect1>
1176 <sect1 id="examples-lock-per-obj">
1177 <title>Protecting The Objects Themselves</title>
1178 <para>
1179 In these examples, we assumed that the objects (except the reference
1180 counts) never changed once they are created. If we wanted to allow
1181 the name to change, there are three possibilities:
1182 </para>
1183 <itemizedlist>
1184 <listitem>
1185 <para>
1186 You can make <symbol>cache_lock</symbol> non-static, and tell people
1187 to grab that lock before changing the name in any object.
1188 </para>
1189 </listitem>
1190 <listitem>
1191 <para>
1192 You can provide a <function>cache_obj_rename</function> which grabs
1193 this lock and changes the name for the caller, and tell everyone to
1194 use that function.
1195 </para>
1196 </listitem>
1197 <listitem>
1198 <para>
1199 You can make the <symbol>cache_lock</symbol> protect only the cache
1200 itself, and use another lock to protect the name.
1201 </para>
1202 </listitem>
1203 </itemizedlist>
1205 <para>
1206 Theoretically, you can make the locks as fine-grained as one lock for
1207 every field, for every object. In practice, the most common variants
1208 are:
1209 </para>
1210 <itemizedlist>
1211 <listitem>
1212 <para>
1213 One lock which protects the infrastructure (the <symbol>cache</symbol>
1214 list in this example) and all the objects. This is what we have done
1215 so far.
1216 </para>
1217 </listitem>
1218 <listitem>
1219 <para>
1220 One lock which protects the infrastructure (including the list
1221 pointers inside the objects), and one lock inside the object which
1222 protects the rest of that object.
1223 </para>
1224 </listitem>
1225 <listitem>
1226 <para>
1227 Multiple locks to protect the infrastructure (eg. one lock per hash
1228 chain), possibly with a separate per-object lock.
1229 </para>
1230 </listitem>
1231 </itemizedlist>
1233 <para>
1234 Here is the "lock-per-object" implementation:
1235 </para>
1236 <programlisting>
1237 --- cache.c.refcnt-atomic 2003-12-11 15:50:54.000000000 +1100
1238 +++ cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100
1239 @@ -6,11 +6,17 @@
1241 struct object
1243 + /* These two protected by cache_lock. */
1244 struct list_head list;
1245 + int popularity;
1247 atomic_t refcnt;
1249 + /* Doesn't change once created. */
1250 int id;
1252 + spinlock_t lock; /* Protects the name */
1253 char name[32];
1254 - int popularity;
1257 static DEFINE_SPINLOCK(cache_lock);
1258 @@ -77,6 +84,7 @@
1259 obj-&gt;id = id;
1260 obj-&gt;popularity = 0;
1261 atomic_set(&amp;obj-&gt;refcnt, 1); /* The cache holds a reference */
1262 + spin_lock_init(&amp;obj-&gt;lock);
1264 spin_lock_irqsave(&amp;cache_lock, flags);
1265 __cache_add(obj);
1266 </programlisting>
1268 <para>
1269 Note that I decide that the <structfield>popularity</structfield>
1270 count should be protected by the <symbol>cache_lock</symbol> rather
1271 than the per-object lock: this is because it (like the
1272 <structname>struct list_head</structname> inside the object) is
1273 logically part of the infrastructure. This way, I don't need to grab
1274 the lock of every object in <function>__cache_add</function> when
1275 seeking the least popular.
1276 </para>
1278 <para>
1279 I also decided that the <structfield>id</structfield> member is
1280 unchangeable, so I don't need to grab each object lock in
1281 <function>__cache_find()</function> to examine the
1282 <structfield>id</structfield>: the object lock is only used by a
1283 caller who wants to read or write the <structfield>name</structfield>
1284 field.
1285 </para>
1287 <para>
1288 Note also that I added a comment describing what data was protected by
1289 which locks. This is extremely important, as it describes the runtime
1290 behavior of the code, and can be hard to gain from just reading. And
1291 as Alan Cox says, <quote>Lock data, not code</quote>.
1292 </para>
1293 </sect1>
1294 </chapter>
1296 <chapter id="common-problems">
1297 <title>Common Problems</title>
1298 <sect1 id="deadlock">
1299 <title>Deadlock: Simple and Advanced</title>
1301 <para>
1302 There is a coding bug where a piece of code tries to grab a
1303 spinlock twice: it will spin forever, waiting for the lock to
1304 be released (spinlocks, rwlocks and mutexes are not
1305 recursive in Linux). This is trivial to diagnose: not a
1306 stay-up-five-nights-talk-to-fluffy-code-bunnies kind of
1307 problem.
1308 </para>
1310 <para>
1311 For a slightly more complex case, imagine you have a region
1312 shared by a softirq and user context. If you use a
1313 <function>spin_lock()</function> call to protect it, it is
1314 possible that the user context will be interrupted by the softirq
1315 while it holds the lock, and the softirq will then spin
1316 forever trying to get the same lock.
1317 </para>
1319 <para>
1320 Both of these are called deadlock, and as shown above, it can
1321 occur even with a single CPU (although not on UP compiles,
1322 since spinlocks vanish on kernel compiles with
1323 <symbol>CONFIG_SMP</symbol>=n. You'll still get data corruption
1324 in the second example).
1325 </para>
1327 <para>
1328 This complete lockup is easy to diagnose: on SMP boxes the
1329 watchdog timer or compiling with <symbol>DEBUG_SPINLOCK</symbol> set
1330 (<filename>include/linux/spinlock.h</filename>) will show this up
1331 immediately when it happens.
1332 </para>
1334 <para>
1335 A more complex problem is the so-called 'deadly embrace',
1336 involving two or more locks. Say you have a hash table: each
1337 entry in the table is a spinlock, and a chain of hashed
1338 objects. Inside a softirq handler, you sometimes want to
1339 alter an object from one place in the hash to another: you
1340 grab the spinlock of the old hash chain and the spinlock of
1341 the new hash chain, and delete the object from the old one,
1342 and insert it in the new one.
1343 </para>
1345 <para>
1346 There are two problems here. First, if your code ever
1347 tries to move the object to the same chain, it will deadlock
1348 with itself as it tries to lock it twice. Secondly, if the
1349 same softirq on another CPU is trying to move another object
1350 in the reverse direction, the following could happen:
1351 </para>
1353 <table>
1354 <title>Consequences</title>
1356 <tgroup cols="2" align="left">
1358 <thead>
1359 <row>
1360 <entry>CPU 1</entry>
1361 <entry>CPU 2</entry>
1362 </row>
1363 </thead>
1365 <tbody>
1366 <row>
1367 <entry>Grab lock A -&gt; OK</entry>
1368 <entry>Grab lock B -&gt; OK</entry>
1369 </row>
1370 <row>
1371 <entry>Grab lock B -&gt; spin</entry>
1372 <entry>Grab lock A -&gt; spin</entry>
1373 </row>
1374 </tbody>
1375 </tgroup>
1376 </table>
1378 <para>
1379 The two CPUs will spin forever, waiting for the other to give up
1380 their lock. It will look, smell, and feel like a crash.
1381 </para>
1382 </sect1>
1384 <sect1 id="techs-deadlock-prevent">
1385 <title>Preventing Deadlock</title>
1387 <para>
1388 Textbooks will tell you that if you always lock in the same
1389 order, you will never get this kind of deadlock. Practice
1390 will tell you that this approach doesn't scale: when I
1391 create a new lock, I don't understand enough of the kernel
1392 to figure out where in the 5000 lock hierarchy it will fit.
1393 </para>
1395 <para>
1396 The best locks are encapsulated: they never get exposed in
1397 headers, and are never held around calls to non-trivial
1398 functions outside the same file. You can read through this
1399 code and see that it will never deadlock, because it never
1400 tries to grab another lock while it has that one. People
1401 using your code don't even need to know you are using a
1402 lock.
1403 </para>
1405 <para>
1406 A classic problem here is when you provide callbacks or
1407 hooks: if you call these with the lock held, you risk simple
1408 deadlock, or a deadly embrace (who knows what the callback
1409 will do?). Remember, the other programmers are out to get
1410 you, so don't do this.
1411 </para>
1413 <sect2 id="techs-deadlock-overprevent">
1414 <title>Overzealous Prevention Of Deadlocks</title>
1416 <para>
1417 Deadlocks are problematic, but not as bad as data
1418 corruption. Code which grabs a read lock, searches a list,
1419 fails to find what it wants, drops the read lock, grabs a
1420 write lock and inserts the object has a race condition.
1421 </para>
1423 <para>
1424 If you don't see why, please stay the fuck away from my code.
1425 </para>
1426 </sect2>
1427 </sect1>
1429 <sect1 id="racing-timers">
1430 <title>Racing Timers: A Kernel Pastime</title>
1432 <para>
1433 Timers can produce their own special problems with races.
1434 Consider a collection of objects (list, hash, etc) where each
1435 object has a timer which is due to destroy it.
1436 </para>
1438 <para>
1439 If you want to destroy the entire collection (say on module
1440 removal), you might do the following:
1441 </para>
1443 <programlisting>
1444 /* THIS CODE BAD BAD BAD BAD: IF IT WAS ANY WORSE IT WOULD USE
1445 HUNGARIAN NOTATION */
1446 spin_lock_bh(&amp;list_lock);
1448 while (list) {
1449 struct foo *next = list-&gt;next;
1450 del_timer(&amp;list-&gt;timer);
1451 kfree(list);
1452 list = next;
1455 spin_unlock_bh(&amp;list_lock);
1456 </programlisting>
1458 <para>
1459 Sooner or later, this will crash on SMP, because a timer can
1460 have just gone off before the <function>spin_lock_bh()</function>,
1461 and it will only get the lock after we
1462 <function>spin_unlock_bh()</function>, and then try to free
1463 the element (which has already been freed!).
1464 </para>
1466 <para>
1467 This can be avoided by checking the result of
1468 <function>del_timer()</function>: if it returns
1469 <returnvalue>1</returnvalue>, the timer has been deleted.
1470 If <returnvalue>0</returnvalue>, it means (in this
1471 case) that it is currently running, so we can do:
1472 </para>
1474 <programlisting>
1475 retry:
1476 spin_lock_bh(&amp;list_lock);
1478 while (list) {
1479 struct foo *next = list-&gt;next;
1480 if (!del_timer(&amp;list-&gt;timer)) {
1481 /* Give timer a chance to delete this */
1482 spin_unlock_bh(&amp;list_lock);
1483 goto retry;
1485 kfree(list);
1486 list = next;
1489 spin_unlock_bh(&amp;list_lock);
1490 </programlisting>
1492 <para>
1493 Another common problem is deleting timers which restart
1494 themselves (by calling <function>add_timer()</function> at the end
1495 of their timer function). Because this is a fairly common case
1496 which is prone to races, you should use <function>del_timer_sync()</function>
1497 (<filename class="headerfile">include/linux/timer.h</filename>)
1498 to handle this case. It returns the number of times the timer
1499 had to be deleted before we finally stopped it from adding itself back
1501 </para>
1502 </sect1>
1504 </chapter>
1506 <chapter id="Efficiency">
1507 <title>Locking Speed</title>
1509 <para>
1510 There are three main things to worry about when considering speed of
1511 some code which does locking. First is concurrency: how many things
1512 are going to be waiting while someone else is holding a lock. Second
1513 is the time taken to actually acquire and release an uncontended lock.
1514 Third is using fewer, or smarter locks. I'm assuming that the lock is
1515 used fairly often: otherwise, you wouldn't be concerned about
1516 efficiency.
1517 </para>
1518 <para>
1519 Concurrency depends on how long the lock is usually held: you should
1520 hold the lock for as long as needed, but no longer. In the cache
1521 example, we always create the object without the lock held, and then
1522 grab the lock only when we are ready to insert it in the list.
1523 </para>
1524 <para>
1525 Acquisition times depend on how much damage the lock operations do to
1526 the pipeline (pipeline stalls) and how likely it is that this CPU was
1527 the last one to grab the lock (ie. is the lock cache-hot for this
1528 CPU): on a machine with more CPUs, this likelihood drops fast.
1529 Consider a 700MHz Intel Pentium III: an instruction takes about 0.7ns,
1530 an atomic increment takes about 58ns, a lock which is cache-hot on
1531 this CPU takes 160ns, and a cacheline transfer from another CPU takes
1532 an additional 170 to 360ns. (These figures from Paul McKenney's
1533 <ulink url="http://www.linuxjournal.com/article.php?sid=6993"> Linux
1534 Journal RCU article</ulink>).
1535 </para>
1536 <para>
1537 These two aims conflict: holding a lock for a short time might be done
1538 by splitting locks into parts (such as in our final per-object-lock
1539 example), but this increases the number of lock acquisitions, and the
1540 results are often slower than having a single lock. This is another
1541 reason to advocate locking simplicity.
1542 </para>
1543 <para>
1544 The third concern is addressed below: there are some methods to reduce
1545 the amount of locking which needs to be done.
1546 </para>
1548 <sect1 id="efficiency-rwlocks">
1549 <title>Read/Write Lock Variants</title>
1551 <para>
1552 Both spinlocks and mutexes have read/write variants:
1553 <type>rwlock_t</type> and <structname>struct rw_semaphore</structname>.
1554 These divide users into two classes: the readers and the writers. If
1555 you are only reading the data, you can get a read lock, but to write to
1556 the data you need the write lock. Many people can hold a read lock,
1557 but a writer must be sole holder.
1558 </para>
1560 <para>
1561 If your code divides neatly along reader/writer lines (as our
1562 cache code does), and the lock is held by readers for
1563 significant lengths of time, using these locks can help. They
1564 are slightly slower than the normal locks though, so in practice
1565 <type>rwlock_t</type> is not usually worthwhile.
1566 </para>
1567 </sect1>
1569 <sect1 id="efficiency-read-copy-update">
1570 <title>Avoiding Locks: Read Copy Update</title>
1572 <para>
1573 There is a special method of read/write locking called Read Copy
1574 Update. Using RCU, the readers can avoid taking a lock
1575 altogether: as we expect our cache to be read more often than
1576 updated (otherwise the cache is a waste of time), it is a
1577 candidate for this optimization.
1578 </para>
1580 <para>
1581 How do we get rid of read locks? Getting rid of read locks
1582 means that writers may be changing the list underneath the
1583 readers. That is actually quite simple: we can read a linked
1584 list while an element is being added if the writer adds the
1585 element very carefully. For example, adding
1586 <symbol>new</symbol> to a single linked list called
1587 <symbol>list</symbol>:
1588 </para>
1590 <programlisting>
1591 new-&gt;next = list-&gt;next;
1592 wmb();
1593 list-&gt;next = new;
1594 </programlisting>
1596 <para>
1597 The <function>wmb()</function> is a write memory barrier. It
1598 ensures that the first operation (setting the new element's
1599 <symbol>next</symbol> pointer) is complete and will be seen by
1600 all CPUs, before the second operation is (putting the new
1601 element into the list). This is important, since modern
1602 compilers and modern CPUs can both reorder instructions unless
1603 told otherwise: we want a reader to either not see the new
1604 element at all, or see the new element with the
1605 <symbol>next</symbol> pointer correctly pointing at the rest of
1606 the list.
1607 </para>
1608 <para>
1609 Fortunately, there is a function to do this for standard
1610 <structname>struct list_head</structname> lists:
1611 <function>list_add_rcu()</function>
1612 (<filename>include/linux/list.h</filename>).
1613 </para>
1614 <para>
1615 Removing an element from the list is even simpler: we replace
1616 the pointer to the old element with a pointer to its successor,
1617 and readers will either see it, or skip over it.
1618 </para>
1619 <programlisting>
1620 list-&gt;next = old-&gt;next;
1621 </programlisting>
1622 <para>
1623 There is <function>list_del_rcu()</function>
1624 (<filename>include/linux/list.h</filename>) which does this (the
1625 normal version poisons the old object, which we don't want).
1626 </para>
1627 <para>
1628 The reader must also be careful: some CPUs can look through the
1629 <symbol>next</symbol> pointer to start reading the contents of
1630 the next element early, but don't realize that the pre-fetched
1631 contents is wrong when the <symbol>next</symbol> pointer changes
1632 underneath them. Once again, there is a
1633 <function>list_for_each_entry_rcu()</function>
1634 (<filename>include/linux/list.h</filename>) to help you. Of
1635 course, writers can just use
1636 <function>list_for_each_entry()</function>, since there cannot
1637 be two simultaneous writers.
1638 </para>
1639 <para>
1640 Our final dilemma is this: when can we actually destroy the
1641 removed element? Remember, a reader might be stepping through
1642 this element in the list right now: if we free this element and
1643 the <symbol>next</symbol> pointer changes, the reader will jump
1644 off into garbage and crash. We need to wait until we know that
1645 all the readers who were traversing the list when we deleted the
1646 element are finished. We use <function>call_rcu()</function> to
1647 register a callback which will actually destroy the object once
1648 all pre-existing readers are finished. Alternatively,
1649 <function>synchronize_rcu()</function> may be used to block until
1650 all pre-existing are finished.
1651 </para>
1652 <para>
1653 But how does Read Copy Update know when the readers are
1654 finished? The method is this: firstly, the readers always
1655 traverse the list inside
1656 <function>rcu_read_lock()</function>/<function>rcu_read_unlock()</function>
1657 pairs: these simply disable preemption so the reader won't go to
1658 sleep while reading the list.
1659 </para>
1660 <para>
1661 RCU then waits until every other CPU has slept at least once:
1662 since readers cannot sleep, we know that any readers which were
1663 traversing the list during the deletion are finished, and the
1664 callback is triggered. The real Read Copy Update code is a
1665 little more optimized than this, but this is the fundamental
1666 idea.
1667 </para>
1669 <programlisting>
1670 --- cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100
1671 +++ cache.c.rcupdate 2003-12-11 17:55:14.000000000 +1100
1672 @@ -1,15 +1,18 @@
1673 #include &lt;linux/list.h&gt;
1674 #include &lt;linux/slab.h&gt;
1675 #include &lt;linux/string.h&gt;
1676 +#include &lt;linux/rcupdate.h&gt;
1677 #include &lt;linux/mutex.h&gt;
1678 #include &lt;asm/errno.h&gt;
1680 struct object
1682 - /* These two protected by cache_lock. */
1683 + /* This is protected by RCU */
1684 struct list_head list;
1685 int popularity;
1687 + struct rcu_head rcu;
1689 atomic_t refcnt;
1691 /* Doesn't change once created. */
1692 @@ -40,7 +43,7 @@
1694 struct object *i;
1696 - list_for_each_entry(i, &amp;cache, list) {
1697 + list_for_each_entry_rcu(i, &amp;cache, list) {
1698 if (i-&gt;id == id) {
1699 i-&gt;popularity++;
1700 return i;
1701 @@ -49,19 +52,25 @@
1702 return NULL;
1705 +/* Final discard done once we know no readers are looking. */
1706 +static void cache_delete_rcu(void *arg)
1708 + object_put(arg);
1711 /* Must be holding cache_lock */
1712 static void __cache_delete(struct object *obj)
1714 BUG_ON(!obj);
1715 - list_del(&amp;obj-&gt;list);
1716 - object_put(obj);
1717 + list_del_rcu(&amp;obj-&gt;list);
1718 cache_num--;
1719 + call_rcu(&amp;obj-&gt;rcu, cache_delete_rcu);
1722 /* Must be holding cache_lock */
1723 static void __cache_add(struct object *obj)
1725 - list_add(&amp;obj-&gt;list, &amp;cache);
1726 + list_add_rcu(&amp;obj-&gt;list, &amp;cache);
1727 if (++cache_num > MAX_CACHE_SIZE) {
1728 struct object *i, *outcast = NULL;
1729 list_for_each_entry(i, &amp;cache, list) {
1730 @@ -104,12 +114,11 @@
1731 struct object *cache_find(int id)
1733 struct object *obj;
1734 - unsigned long flags;
1736 - spin_lock_irqsave(&amp;cache_lock, flags);
1737 + rcu_read_lock();
1738 obj = __cache_find(id);
1739 if (obj)
1740 object_get(obj);
1741 - spin_unlock_irqrestore(&amp;cache_lock, flags);
1742 + rcu_read_unlock();
1743 return obj;
1745 </programlisting>
1747 <para>
1748 Note that the reader will alter the
1749 <structfield>popularity</structfield> member in
1750 <function>__cache_find()</function>, and now it doesn't hold a lock.
1751 One solution would be to make it an <type>atomic_t</type>, but for
1752 this usage, we don't really care about races: an approximate result is
1753 good enough, so I didn't change it.
1754 </para>
1756 <para>
1757 The result is that <function>cache_find()</function> requires no
1758 synchronization with any other functions, so is almost as fast on SMP
1759 as it would be on UP.
1760 </para>
1762 <para>
1763 There is a furthur optimization possible here: remember our original
1764 cache code, where there were no reference counts and the caller simply
1765 held the lock whenever using the object? This is still possible: if
1766 you hold the lock, no one can delete the object, so you don't need to
1767 get and put the reference count.
1768 </para>
1770 <para>
1771 Now, because the 'read lock' in RCU is simply disabling preemption, a
1772 caller which always has preemption disabled between calling
1773 <function>cache_find()</function> and
1774 <function>object_put()</function> does not need to actually get and
1775 put the reference count: we could expose
1776 <function>__cache_find()</function> by making it non-static, and
1777 such callers could simply call that.
1778 </para>
1779 <para>
1780 The benefit here is that the reference count is not written to: the
1781 object is not altered in any way, which is much faster on SMP
1782 machines due to caching.
1783 </para>
1784 </sect1>
1786 <sect1 id="per-cpu">
1787 <title>Per-CPU Data</title>
1789 <para>
1790 Another technique for avoiding locking which is used fairly
1791 widely is to duplicate information for each CPU. For example,
1792 if you wanted to keep a count of a common condition, you could
1793 use a spin lock and a single counter. Nice and simple.
1794 </para>
1796 <para>
1797 If that was too slow (it's usually not, but if you've got a
1798 really big machine to test on and can show that it is), you
1799 could instead use a counter for each CPU, then none of them need
1800 an exclusive lock. See <function>DEFINE_PER_CPU()</function>,
1801 <function>get_cpu_var()</function> and
1802 <function>put_cpu_var()</function>
1803 (<filename class="headerfile">include/linux/percpu.h</filename>).
1804 </para>
1806 <para>
1807 Of particular use for simple per-cpu counters is the
1808 <type>local_t</type> type, and the
1809 <function>cpu_local_inc()</function> and related functions,
1810 which are more efficient than simple code on some architectures
1811 (<filename class="headerfile">include/asm/local.h</filename>).
1812 </para>
1814 <para>
1815 Note that there is no simple, reliable way of getting an exact
1816 value of such a counter, without introducing more locks. This
1817 is not a problem for some uses.
1818 </para>
1819 </sect1>
1821 <sect1 id="mostly-hardirq">
1822 <title>Data Which Mostly Used By An IRQ Handler</title>
1824 <para>
1825 If data is always accessed from within the same IRQ handler, you
1826 don't need a lock at all: the kernel already guarantees that the
1827 irq handler will not run simultaneously on multiple CPUs.
1828 </para>
1829 <para>
1830 Manfred Spraul points out that you can still do this, even if
1831 the data is very occasionally accessed in user context or
1832 softirqs/tasklets. The irq handler doesn't use a lock, and
1833 all other accesses are done as so:
1834 </para>
1836 <programlisting>
1837 spin_lock(&amp;lock);
1838 disable_irq(irq);
1840 enable_irq(irq);
1841 spin_unlock(&amp;lock);
1842 </programlisting>
1843 <para>
1844 The <function>disable_irq()</function> prevents the irq handler
1845 from running (and waits for it to finish if it's currently
1846 running on other CPUs). The spinlock prevents any other
1847 accesses happening at the same time. Naturally, this is slower
1848 than just a <function>spin_lock_irq()</function> call, so it
1849 only makes sense if this type of access happens extremely
1850 rarely.
1851 </para>
1852 </sect1>
1853 </chapter>
1855 <chapter id="sleeping-things">
1856 <title>What Functions Are Safe To Call From Interrupts?</title>
1858 <para>
1859 Many functions in the kernel sleep (ie. call schedule())
1860 directly or indirectly: you can never call them while holding a
1861 spinlock, or with preemption disabled. This also means you need
1862 to be in user context: calling them from an interrupt is illegal.
1863 </para>
1865 <sect1 id="sleeping">
1866 <title>Some Functions Which Sleep</title>
1868 <para>
1869 The most common ones are listed below, but you usually have to
1870 read the code to find out if other calls are safe. If everyone
1871 else who calls it can sleep, you probably need to be able to
1872 sleep, too. In particular, registration and deregistration
1873 functions usually expect to be called from user context, and can
1874 sleep.
1875 </para>
1877 <itemizedlist>
1878 <listitem>
1879 <para>
1880 Accesses to
1881 <firstterm linkend="gloss-userspace">userspace</firstterm>:
1882 </para>
1883 <itemizedlist>
1884 <listitem>
1885 <para>
1886 <function>copy_from_user()</function>
1887 </para>
1888 </listitem>
1889 <listitem>
1890 <para>
1891 <function>copy_to_user()</function>
1892 </para>
1893 </listitem>
1894 <listitem>
1895 <para>
1896 <function>get_user()</function>
1897 </para>
1898 </listitem>
1899 <listitem>
1900 <para>
1901 <function>put_user()</function>
1902 </para>
1903 </listitem>
1904 </itemizedlist>
1905 </listitem>
1907 <listitem>
1908 <para>
1909 <function>kmalloc(GFP_KERNEL)</function>
1910 </para>
1911 </listitem>
1913 <listitem>
1914 <para>
1915 <function>mutex_lock_interruptible()</function> and
1916 <function>mutex_lock()</function>
1917 </para>
1918 <para>
1919 There is a <function>mutex_trylock()</function> which does not
1920 sleep. Still, it must not be used inside interrupt context since
1921 its implementation is not safe for that.
1922 <function>mutex_unlock()</function> will also never sleep.
1923 It cannot be used in interrupt context either since a mutex
1924 must be released by the same task that acquired it.
1925 </para>
1926 </listitem>
1927 </itemizedlist>
1928 </sect1>
1930 <sect1 id="dont-sleep">
1931 <title>Some Functions Which Don't Sleep</title>
1933 <para>
1934 Some functions are safe to call from any context, or holding
1935 almost any lock.
1936 </para>
1938 <itemizedlist>
1939 <listitem>
1940 <para>
1941 <function>printk()</function>
1942 </para>
1943 </listitem>
1944 <listitem>
1945 <para>
1946 <function>kfree()</function>
1947 </para>
1948 </listitem>
1949 <listitem>
1950 <para>
1951 <function>add_timer()</function> and <function>del_timer()</function>
1952 </para>
1953 </listitem>
1954 </itemizedlist>
1955 </sect1>
1956 </chapter>
1958 <chapter id="apiref">
1959 <title>Mutex API reference</title>
1960 !Iinclude/linux/mutex.h
1961 !Ekernel/mutex.c
1962 </chapter>
1964 <chapter id="references">
1965 <title>Further reading</title>
1967 <itemizedlist>
1968 <listitem>
1969 <para>
1970 <filename>Documentation/spinlocks.txt</filename>:
1971 Linus Torvalds' spinlocking tutorial in the kernel sources.
1972 </para>
1973 </listitem>
1975 <listitem>
1976 <para>
1977 Unix Systems for Modern Architectures: Symmetric
1978 Multiprocessing and Caching for Kernel Programmers:
1979 </para>
1981 <para>
1982 Curt Schimmel's very good introduction to kernel level
1983 locking (not written for Linux, but nearly everything
1984 applies). The book is expensive, but really worth every
1985 penny to understand SMP locking. [ISBN: 0201633388]
1986 </para>
1987 </listitem>
1988 </itemizedlist>
1989 </chapter>
1991 <chapter id="thanks">
1992 <title>Thanks</title>
1994 <para>
1995 Thanks to Telsa Gwynne for DocBooking, neatening and adding
1996 style.
1997 </para>
1999 <para>
2000 Thanks to Martin Pool, Philipp Rumpf, Stephen Rothwell, Paul
2001 Mackerras, Ruedi Aschwanden, Alan Cox, Manfred Spraul, Tim
2002 Waugh, Pete Zaitcev, James Morris, Robert Love, Paul McKenney,
2003 John Ashby for proofreading, correcting, flaming, commenting.
2004 </para>
2006 <para>
2007 Thanks to the cabal for having no influence on this document.
2008 </para>
2009 </chapter>
2011 <glossary id="glossary">
2012 <title>Glossary</title>
2014 <glossentry id="gloss-preemption">
2015 <glossterm>preemption</glossterm>
2016 <glossdef>
2017 <para>
2018 Prior to 2.5, or when <symbol>CONFIG_PREEMPT</symbol> is
2019 unset, processes in user context inside the kernel would not
2020 preempt each other (ie. you had that CPU until you gave it up,
2021 except for interrupts). With the addition of
2022 <symbol>CONFIG_PREEMPT</symbol> in 2.5.4, this changed: when
2023 in user context, higher priority tasks can "cut in": spinlocks
2024 were changed to disable preemption, even on UP.
2025 </para>
2026 </glossdef>
2027 </glossentry>
2029 <glossentry id="gloss-bh">
2030 <glossterm>bh</glossterm>
2031 <glossdef>
2032 <para>
2033 Bottom Half: for historical reasons, functions with
2034 '_bh' in them often now refer to any software interrupt, e.g.
2035 <function>spin_lock_bh()</function> blocks any software interrupt
2036 on the current CPU. Bottom halves are deprecated, and will
2037 eventually be replaced by tasklets. Only one bottom half will be
2038 running at any time.
2039 </para>
2040 </glossdef>
2041 </glossentry>
2043 <glossentry id="gloss-hwinterrupt">
2044 <glossterm>Hardware Interrupt / Hardware IRQ</glossterm>
2045 <glossdef>
2046 <para>
2047 Hardware interrupt request. <function>in_irq()</function> returns
2048 <returnvalue>true</returnvalue> in a hardware interrupt handler.
2049 </para>
2050 </glossdef>
2051 </glossentry>
2053 <glossentry id="gloss-interruptcontext">
2054 <glossterm>Interrupt Context</glossterm>
2055 <glossdef>
2056 <para>
2057 Not user context: processing a hardware irq or software irq.
2058 Indicated by the <function>in_interrupt()</function> macro
2059 returning <returnvalue>true</returnvalue>.
2060 </para>
2061 </glossdef>
2062 </glossentry>
2064 <glossentry id="gloss-smp">
2065 <glossterm><acronym>SMP</acronym></glossterm>
2066 <glossdef>
2067 <para>
2068 Symmetric Multi-Processor: kernels compiled for multiple-CPU
2069 machines. (CONFIG_SMP=y).
2070 </para>
2071 </glossdef>
2072 </glossentry>
2074 <glossentry id="gloss-softirq">
2075 <glossterm>Software Interrupt / softirq</glossterm>
2076 <glossdef>
2077 <para>
2078 Software interrupt handler. <function>in_irq()</function> returns
2079 <returnvalue>false</returnvalue>; <function>in_softirq()</function>
2080 returns <returnvalue>true</returnvalue>. Tasklets and softirqs
2081 both fall into the category of 'software interrupts'.
2082 </para>
2083 <para>
2084 Strictly speaking a softirq is one of up to 32 enumerated software
2085 interrupts which can run on multiple CPUs at once.
2086 Sometimes used to refer to tasklets as
2087 well (ie. all software interrupts).
2088 </para>
2089 </glossdef>
2090 </glossentry>
2092 <glossentry id="gloss-tasklet">
2093 <glossterm>tasklet</glossterm>
2094 <glossdef>
2095 <para>
2096 A dynamically-registrable software interrupt,
2097 which is guaranteed to only run on one CPU at a time.
2098 </para>
2099 </glossdef>
2100 </glossentry>
2102 <glossentry id="gloss-timers">
2103 <glossterm>timer</glossterm>
2104 <glossdef>
2105 <para>
2106 A dynamically-registrable software interrupt, which is run at
2107 (or close to) a given time. When running, it is just like a
2108 tasklet (in fact, they are called from the TIMER_SOFTIRQ).
2109 </para>
2110 </glossdef>
2111 </glossentry>
2113 <glossentry id="gloss-up">
2114 <glossterm><acronym>UP</acronym></glossterm>
2115 <glossdef>
2116 <para>
2117 Uni-Processor: Non-SMP. (CONFIG_SMP=n).
2118 </para>
2119 </glossdef>
2120 </glossentry>
2122 <glossentry id="gloss-usercontext">
2123 <glossterm>User Context</glossterm>
2124 <glossdef>
2125 <para>
2126 The kernel executing on behalf of a particular process (ie. a
2127 system call or trap) or kernel thread. You can tell which
2128 process with the <symbol>current</symbol> macro.) Not to
2129 be confused with userspace. Can be interrupted by software or
2130 hardware interrupts.
2131 </para>
2132 </glossdef>
2133 </glossentry>
2135 <glossentry id="gloss-userspace">
2136 <glossterm>Userspace</glossterm>
2137 <glossdef>
2138 <para>
2139 A process executing its own code outside the kernel.
2140 </para>
2141 </glossdef>
2142 </glossentry>
2144 </glossary>
2145 </book>