diff options
| author | Linus Torvalds <torvalds@ppc970.osdl.org> | 2005-04-16 15:20:36 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@ppc970.osdl.org> | 2005-04-16 15:20:36 -0700 |
| commit | 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch) | |
| tree | 0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/DocBook/kernel-locking.tmpl | |
Linux-2.6.12-rc2
Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.
Let it rip!
Diffstat (limited to 'Documentation/DocBook/kernel-locking.tmpl')
| -rw-r--r-- | Documentation/DocBook/kernel-locking.tmpl | 2088 |
1 files changed, 2088 insertions, 0 deletions
diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl new file mode 100644 index 000000000000..90dc2de8e0af --- /dev/null +++ b/Documentation/DocBook/kernel-locking.tmpl @@ -0,0 +1,2088 @@ +<?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="LKLockingGuide"> + <bookinfo> + <title>Unreliable Guide To Locking</title> + + <authorgroup> + <author> + <firstname>Rusty</firstname> + <surname>Russell</surname> + <affiliation> + <address> + <email>rusty@rustcorp.com.au</email> + </address> + </affiliation> + </author> + </authorgroup> + + <copyright> + <year>2003</year> + <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> + </bookinfo> + + <toc></toc> + <chapter id="intro"> + <title>Introduction</title> + <para> + Welcome, to Rusty's Remarkably Unreliable Guide to Kernel + Locking issues. This document describes the locking systems in + the Linux Kernel in 2.6. + </para> + <para> + With the wide availability of HyperThreading, and <firstterm + linkend="gloss-preemption">preemption </firstterm> in the Linux + Kernel, everyone hacking on the kernel needs to know the + fundamentals of concurrency and locking for + <firstterm linkend="gloss-smp"><acronym>SMP</acronym></firstterm>. + </para> + </chapter> + + <chapter id="races"> + <title>The Problem With Concurrency</title> + <para> + (Skip this if you know what a Race Condition is). + </para> + <para> + In a normal program, you can increment a counter like so: + </para> + <programlisting> + very_important_count++; + </programlisting> + + <para> + This is what they would expect to happen: + </para> + + <table> + <title>Expected Results</title> + + <tgroup cols="2" align="left"> + + <thead> + <row> + <entry>Instance 1</entry> + <entry>Instance 2</entry> + </row> + </thead> + + <tbody> + <row> + <entry>read very_important_count (5)</entry> + <entry></entry> + </row> + <row> + <entry>add 1 (6)</entry> + <entry></entry> + </row> + <row> + <entry>write very_important_count (6)</entry> + <entry></entry> + </row> + <row> + <entry></entry> + <entry>read very_important_count (6)</entry> + </row> + <row> + <entry></entry> + <entry>add 1 (7)</entry> + </row> + <row> + <entry></entry> + <entry>write very_important_count (7)</entry> + </row> + </tbody> + + </tgroup> + </table> + + <para> + This is what might happen: + </para> + + <table> + <title>Possible Results</title> + + <tgroup cols="2" align="left"> + <thead> + <row> + <entry>Instance 1</entry> + <entry>Instance 2</entry> + </row> + </thead> + + <tbody> + <row> + <entry>read very_important_count (5)</entry> + <entry></entry> + </row> + <row> + <entry></entry> + <entry>read very_important_count (5)</entry> + </row> + <row> + <entry>add 1 (6)</entry> + <entry></entry> + </row> + <row> + <entry></entry> + <entry>add 1 (6)</entry> + </row> + <row> + <entry>write very_important_count (6)</entry> + <entry></entry> + </row> + <row> + <entry></entry> + <entry>write very_important_count (6)</entry> + </row> + </tbody> + </tgroup> + </table> + + <sect1 id="race-condition"> + <title>Race Conditions and Critical Regions</title> + <para> + This overlap, where the result depends on the + relative timing of multiple tasks, is called a <firstterm>race condition</firstterm>. + The piece of code containing the concurrency issue is called a + <firstterm>critical region</firstterm>. And especially since Linux starting running + on SMP machines, they became one of the major issues in kernel + design and implementation. + </para> + <para> + Preemption can have the same effect, even if there is only one + CPU: by preempting one task during the critical region, we have + exactly the same race condition. In this case the thread which + preempts might run the critical region itself. + </para> + <para> + The solution is to recognize when these simultaneous accesses + occur, and use locks to make sure that only one instance can + enter the critical region at any time. There are many + friendly primitives in the Linux kernel to help you do this. + And then there are the unfriendly primitives, but I'll pretend + they don't exist. + </para> + </sect1> + </chapter> + + <chapter id="locks"> + <title>Locking in the Linux Kernel</title> + + <para> + If I could give you one piece of advice: never sleep with anyone + crazier than yourself. But if I had to give you advice on + locking: <emphasis>keep it simple</emphasis>. + </para> + + <para> + Be reluctant to introduce new locks. + </para> + + <para> + Strangely enough, this last one is the exact reverse of my advice when + you <emphasis>have</emphasis> slept with someone crazier than yourself. + And you should think about getting a big dog. + </para> + + <sect1 id="lock-intro"> + <title>Two Main Types of Kernel Locks: Spinlocks and Semaphores</title> + + <para> + There are two main types of kernel locks. The fundamental type + is the spinlock + (<filename class="headerfile">include/asm/spinlock.h</filename>), + which is a very simple single-holder lock: if you can't get the + spinlock, you keep trying (spinning) until you can. Spinlocks are + very small and fast, and can be used anywhere. + </para> + <para> + The second type is a semaphore + (<filename class="headerfile">include/asm/semaphore.h</filename>): it + can have more than one holder at any time (the number decided at + initialization time), although it is most commonly used as a + single-holder lock (a mutex). If you can't get a semaphore, + your task will put itself on the queue, and be woken up when the + semaphore is released. This means the CPU will do something + else while you are waiting, but there are many cases when you + simply can't sleep (see <xref linkend="sleeping-things"/>), and so + have to use a spinlock instead. + </para> + <para> + Neither type of lock is recursive: see + <xref linkend="deadlock"/>. + </para> + </sect1> + + <sect1 id="uniprocessor"> + <title>Locks and Uniprocessor Kernels</title> + + <para> + For kernels compiled without <symbol>CONFIG_SMP</symbol>, and + without <symbol>CONFIG_PREEMPT</symbol> spinlocks do not exist at + all. This is an excellent design decision: when no-one else can + run at the same time, there is no reason to have a lock. + </para> + + <para> + If the kernel is compiled without <symbol>CONFIG_SMP</symbol>, + but <symbol>CONFIG_PREEMPT</symbol> is set, then spinlocks + simply disable preemption, which is sufficient to prevent any + races. For most purposes, we can think of preemption as + equivalent to SMP, and not worry about it separately. + </para> + + <para> + You should always test your locking code with <symbol>CONFIG_SMP</symbol> + and <symbol>CONFIG_PREEMPT</symbol> enabled, even if you don't have an SMP test box, because it + will still catch some kinds of locking bugs. + </para> + + <para> + Semaphores still exist, because they are required for + synchronization between <firstterm linkend="gloss-usercontext">user + contexts</firstterm>, as we will see below. + </para> + </sect1> + + <sect1 id="usercontextlocking"> + <title>Locking Only In User Context</title> + + <para> + If you have a data structure which is only ever accessed from + user context, then you can use a simple semaphore + (<filename>linux/asm/semaphore.h</filename>) to protect it. This + is the most trivial case: you initialize the semaphore to the number + of resources available (usually 1), and call + <function>down_interruptible()</function> to grab the semaphore, and + <function>up()</function> to release it. There is also a + <function>down()</function>, which should be avoided, because it + will not return if a signal is received. + </para> + + <para> + Example: <filename>linux/net/core/netfilter.c</filename> allows + registration of new <function>setsockopt()</function> and + <function>getsockopt()</function> calls, with + <function>nf_register_sockopt()</function>. Registration and + de-registration are only done on module load and unload (and boot + time, where there is no concurrency), and the list of registrations + is only consulted for an unknown <function>setsockopt()</function> + or <function>getsockopt()</function> system call. The + <varname>nf_sockopt_mutex</varname> is perfect to protect this, + especially since the setsockopt and getsockopt calls may well + sleep. + </para> + </sect1> + + <sect1 id="lock-user-bh"> + <title>Locking Between User Context and Softirqs</title> + + <para> + If a <firstterm linkend="gloss-softirq">softirq</firstterm> shares + data with user context, you have two problems. Firstly, the current + user context can be interrupted by a softirq, and secondly, the + critical region could be entered from another CPU. This is where + <function>spin_lock_bh()</function> + (<filename class="headerfile">include/linux/spinlock.h</filename>) is + used. It disables softirqs on that CPU, then grabs the lock. + <function>spin_unlock_bh()</function> does the reverse. (The + '_bh' suffix is a historical reference to "Bottom Halves", the + old name for software interrupts. It should really be + called spin_lock_softirq()' in a perfect world). + </para> + + <para> + Note that you can also use <function>spin_lock_irq()</function> + or <function>spin_lock_irqsave()</function> here, which stop + hardware interrupts as well: see <xref linkend="hardirq-context"/>. + </para> + + <para> + This works perfectly for <firstterm linkend="gloss-up"><acronym>UP + </acronym></firstterm> as well: the spin lock vanishes, and this macro + simply becomes <function>local_bh_disable()</function> + (<filename class="headerfile">include/linux/interrupt.h</filename>), which + protects you from the softirq being run. + </para> + </sect1> + + <sect1 id="lock-user-tasklet"> + <title>Locking Between User Context and Tasklets</title> + + <para> + This is exactly the same as above, because <firstterm + linkend="gloss-tasklet">tasklets</firstterm> are actually run + from a softirq. + </para> + </sect1> + + <sect1 id="lock-user-timers"> + <title>Locking Between User Context and Timers</title> + + <para> + This, too, is exactly the same as above, because <firstterm + linkend="gloss-timers">timers</firstterm> are actually run from + a softirq. From a locking point of view, tasklets and timers + are identical. + </para> + </sect1> + + <sect1 id="lock-tasklets"> + <title>Locking Between Tasklets/Timers</title> + + <para> + Sometimes a tasklet or timer might want to share data with + another tasklet or timer. + </para> + + <sect2 id="lock-tasklets-same"> + <title>The Same Tasklet/Timer</title> + <para> + Since a tasklet is never run on two CPUs at once, you don't + need to worry about your tasklet being reentrant (running + twice at once), even on SMP. + </para> + </sect2> + + <sect2 id="lock-tasklets-different"> + <title>Different Tasklets/Timers</title> + <para> + If another tasklet/timer wants + to share data with your tasklet or timer , you will both need to use + <function>spin_lock()</function> and + <function>spin_unlock()</function> calls. + <function>spin_lock_bh()</function> is + unnecessary here, as you are already in a tasklet, and + none will be run on the same CPU. + </para> + </sect2> + </sect1> + + <sect1 id="lock-softirqs"> + <title>Locking Between Softirqs</title> + + <para> + Often a softirq might + want to share data with itself or a tasklet/timer. + </para> + + <sect2 id="lock-softirqs-same"> + <title>The Same Softirq</title> + + <para> + The same softirq can run on the other CPUs: you can use a + per-CPU array (see <xref linkend="per-cpu"/>) for better + performance. If you're going so far as to use a softirq, + you probably care about scalable performance enough + to justify the extra complexity. + </para> + + <para> + You'll need to use <function>spin_lock()</function> and + <function>spin_unlock()</function> for shared data. + </para> + </sect2> + + <sect2 id="lock-softirqs-different"> + <title>Different Softirqs</title> + + <para> + You'll need to use <function>spin_lock()</function> and + <function>spin_unlock()</function> for shared data, whether it + be a timer, tasklet, different softirq or the same or another + softirq: any of them could be running on a different CPU. + </para> + </sect2> + </sect1> + </chapter> + + <chapter id="hardirq-context"> + <title>Hard IRQ Context</title> + + <para> + Hardware interrupts usually communicate with a + tasklet or softirq. Frequently this involves putting work in a + queue, which the softirq will take out. + </para> + + <sect1 id="hardirq-softirq"> + <title>Locking Between Hard IRQ and Softirqs/Tasklets</title> + + <para> + If a hardware irq handler shares data with a softirq, you have + two concerns. Firstly, the softirq processing can be + interrupted by a hardware interrupt, and secondly, the + critical region could be entered by a hardware interrupt on + another CPU. This is where <function>spin_lock_irq()</function> is + used. It is defined to disable interrupts on that cpu, then grab + the lock. <function>spin_unlock_irq()</function> does the reverse. + </para> + + <para> + The irq handler does not to use + <function>spin_lock_irq()</function>, because the softirq cannot + run while the irq handler is running: it can use + <function>spin_lock()</function>, which is slightly faster. The + only exception would be if a different hardware irq handler uses + the same lock: <function>spin_lock_irq()</function> will stop + that from interrupting us. + </para> + + <para> + This works perfectly for UP as well: the spin lock vanishes, + and this macro simply becomes <function>local_irq_disable()</function> + (<filename class="headerfile">include/asm/smp.h</filename>), which + protects you from the softirq/tasklet/BH being run. + </para> + + <para> + <function>spin_lock_irqsave()</function> + (<filename>include/linux/spinlock.h</filename>) is a variant + which saves whether interrupts were on or off in a flags word, + which is passed to <function>spin_unlock_irqrestore()</function>. This + means that the same code can be used inside an hard irq handler (where + interrupts are already off) and in softirqs (where the irq + disabling is required). + </para> + + <para> + Note that softirqs (and hence tasklets and timers) are run on + return from hardware interrupts, so + <function>spin_lock_irq()</function> also stops these. In that + sense, <function>spin_lock_irqsave()</function> is the most + general and powerful locking function. + </para> + + </sect1> + <sect1 id="hardirq-hardirq"> + <title>Locking Between Two Hard IRQ Handlers</title> + <para> + It is rare to have to share data between two IRQ handlers, but + if you do, <function>spin_lock_irqsave()</function> should be + used: it is architecture-specific whether all interrupts are + disabled inside irq handlers themselves. + </para> + </sect1> + + </chapter> + + <chapter id="cheatsheet"> + <title>Cheat Sheet For Locking</title> + <para> + Pete Zaitcev gives the following summary: + </para> + <itemizedlist> + <listitem> + <para> + If you are in a process context (any syscall) and want to + lock other process out, use a semaphore. You can take a semaphore + and sleep (<function>copy_from_user*(</function> or + <function>kmalloc(x,GFP_KERNEL)</function>). + </para> + </listitem> + <listitem> + <para> + Otherwise (== data can be touched in an interrupt), use + <function>spin_lock_irqsave()</function> and + <function>spin_unlock_irqrestore()</function>. + </para> + </listitem> + <listitem> + <para> + Avoid holding spinlock for more than 5 lines of code and + across any function call (except accessors like + <function>readb</function>). + </para> + </listitem> + </itemizedlist> + + <sect1 id="minimum-lock-reqirements"> + <title>Table of Minimum Requirements</title> + + <para> The following table lists the <emphasis>minimum</emphasis> + locking requirements between various contexts. In some cases, + the same context can only be running on one CPU at a time, so + no locking is required for that context (eg. a particular + thread can only run on one CPU at a time, but if it needs + shares data with another thread, locking is required). + </para> + <para> + Remember the advice above: you can always use + <function>spin_lock_irqsave()</function>, which is a superset + of all other spinlock primitives. + </para> + <table> +<title>Table of Locking Requirements</title> +<tgroup cols="11"> +<tbody> +<row> +<entry></entry> +<entry>IRQ Handler A</entry> +<entry>IRQ Handler B</entry> +<entry>Softirq A</entry> +<entry>Softirq B</entry> +<entry>Tasklet A</entry> +<entry>Tasklet B</entry> +<entry>Timer A</entry> +<entry>Timer B</entry> +<entry>User Context A</entry> +<entry>User Context B</entry> +</row> + +<row> +<entry>IRQ Handler A</entry> +<entry>None</entry> +</row> + +<row> +<entry>IRQ Handler B</entry> +<entry>spin_lock_irqsave</entry> +<entry>None</entry> +</row> + +<row> +<entry>Softirq A</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock</entry> +</row> + +<row> +<entry>Softirq B</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +</row> + +<row> +<entry>Tasklet A</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>None</entry> +</row> + +<row> +<entry>Tasklet B</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>None</entry> +</row> + +<row> +<entry>Timer A</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>None</entry> +</row> + +<row> +<entry>Timer B</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>spin_lock</entry> +<entry>None</entry> +</row> + +<row> +<entry>User Context A</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>None</entry> +</row> + +<row> +<entry>User Context B</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_irq</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>spin_lock_bh</entry> +<entry>down_interruptible</entry> +<entry>None</entry> +</row> + +</tbody> +</tgroup> +</table> +</sect1> +</chapter> + + <chapter id="Examples"> + <title>Common Examples</title> + <para> +Let's step through a simple example: a cache of number to name +mappings. The cache keeps a count of how often each of the objects is +used, and when it gets full, throws out the least used one. + + </para> + + <sect1 id="examples-usercontext"> + <title>All In User Context</title> + <para> +For our first example, we assume that all operations are in user +context (ie. from system calls), so we can sleep. This means we can +use a semaphore to protect the cache and all the objects within +it. Here's the code: + </para> + + <programlisting> +#include <linux/list.h> +#include <linux/slab.h> +#include <linux/string.h> +#include <asm/semaphore.h> +#include <asm/errno.h> + +struct object +{ + struct list_head list; + int id; + char name[32]; + int popularity; +}; + +/* Protects the cache, cache_num, and the objects within it */ +static DECLARE_MUTEX(cache_lock); +static LIST_HEAD(cache); +static unsigned int cache_num = 0; +#define MAX_CACHE_SIZE 10 + +/* Must be holding cache_lock */ +static struct object *__cache_find(int id) +{ + struct object *i; + + list_for_each_entry(i, &cache, list) + if (i->id == id) { + i->popularity++; + return i; + } + return NULL; +} + +/* Must be holding cache_lock */ +static void __cache_delete(struct object *obj) +{ + BUG_ON(!obj); + list_del(&obj->list); + kfree(obj); + cache_num--; +} + +/* Must be holding cache_lock */ +static void __cache_add(struct object *obj) +{ + list_add(&obj->list, &cache); + if (++cache_num > MAX_CACHE_SIZE) { + struct object *i, *outcast = NULL; + list_for_each_entry(i, &cache, list) { + if (!outcast || i->popularity < outcast->popularity) + outcast = i; + } + __cache_delete(outcast); + } +} + +int cache_add(int id, const char *name) +{ + struct object *obj; + + if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL) + return -ENOMEM; + + strlcpy(obj->name, name, sizeof(obj->name)); + obj->id = id; + obj->popularity = 0; + + down(&cache_lock); + __cache_add(obj); + up(&cache_lock); + return 0; +} + +void cache_delete(int id) +{ + down(&cache_lock); + __cache_delete(__cache_find(id)); + up(&cache_lock); +} + +int cache_find(int id, char *name) +{ + struct object *obj; + int ret = -ENOENT; + + down(&cache_lock); + obj = __cache_find(id); + if (obj) { + ret = 0; + strcpy(name, obj->name); + } + up(&cache_lock); + return ret; +} +</programlisting> + + <para> +Note that we always make sure we have the cache_lock when we add, +delete, or look up the cache: both the cache infrastructure itself and +the contents of the objects are protected by the lock. In this case +it's easy, since we copy the data for the user, and never let them +access the objects directly. + </para> + <para> +There is a slight (and common) optimization here: in +<function>cache_add</function> we set up the fields of the object +before grabbing the lock. This is safe, as no-one else can access it +until we put it in cache. + </para> + </sect1> + + <sect1 id="examples-interrupt"> + <title>Accessing From Interrupt Context</title> + <para> +Now consider the case where <function>cache_find</function> can be +called from interrupt context: either a hardware interrupt or a +softirq. An example would be a timer which deletes object from the +cache. + </para> + <para> +The change is shown below, in standard patch format: the +<symbol>-</symbol> are lines which are taken away, and the +<symbol>+</symbol> are lines which are added. + </para> +<programlisting> +--- cache.c.usercontext 2003-12-09 13:58:54.000000000 +1100 ++++ cache.c.interrupt 2003-12-09 14:07:49.000000000 +1100 +@@ -12,7 +12,7 @@ + int popularity; + }; + +-static DECLARE_MUTEX(cache_lock); ++static spinlock_t cache_lock = SPIN_LOCK_UNLOCKED; + static LIST_HEAD(cache); + static unsigned int cache_num = 0; + #define MAX_CACHE_SIZE 10 +@@ -55,6 +55,7 @@ + int cache_add(int id, const char *name) + { + struct object *obj; ++ unsigned long flags; + + if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL) + return -ENOMEM; +@@ -63,30 +64,33 @@ + obj->id = id; + obj->popularity = 0; + +- down(&cache_lock); ++ spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); +- up(&cache_lock); ++ spin_unlock_irqrestore(&cache_lock, flags); + return 0; + } + + void cache_delete(int id) + { +- down(&cache_lock); ++ unsigned long flags; ++ ++ spin_lock_irqsave(&cache_lock, flags); + __cache_delete(__cache_find(id)); +- up(&cache_lock); ++ spin_unlock_irqrestore(&cache_lock, flags); + } + + int cache_find(int id, char *name) + { + struct object *obj; + int ret = -ENOENT; ++ unsigned long flags; + +- down(&cache_lock); ++ spin_lock_irqsave(&cache_lock, flags); + obj = __cache_find(id); + if (obj) { + ret = 0; + strcpy(name, obj->name); + } +- up(&cache_lock); ++ spin_unlock_irqrestore(&cache_lock, flags); + return ret; + } +</programlisting> + + <para> +Note that the <function>spin_lock_irqsave</function> will turn off +interrupts if they are on, otherwise does nothing (if we are already +in an interrupt handler), hence these functions are safe to call from +any context. + </para> + <para> +Unfortunately, <function>cache_add</function> calls +<function>kmalloc</function> with the <symbol>GFP_KERNEL</symbol> +flag, which is only legal in user context. I have assumed that +<function>cache_add</function> is still only called in user context, +otherwise this should become a parameter to +<function>cache_add</function>. + </para> + </sect1> + <sect1 id="examples-refcnt"> + <title>Exposing Objects Outside This File</title> + <para> +If our objects contained more information, it might not be sufficient +to copy the information in and out: other parts of the code might want +to keep pointers to these objects, for example, rather than looking up +the id every time. This produces two problems. + </para> + <para> +The first problem is that we use the <symbol>cache_lock</symbol> to +protect objects: we'd need to make this non-static so the rest of the +code can use it. This makes locking trickier, as it is no longer all +in one place. + </para> + <para> +The second problem is the lifetime problem: if another structure keeps +a pointer to an object, it presumably expects that pointer to remain +valid. Unfortunately, this is only guaranteed while you hold the +lock, otherwise someone might call <function>cache_delete</function> +and even worse, add another object, re-using the same address. + </para> + <para> +As there is only one lock, you can't hold it forever: no-one else would +get any work done. + </para> + <para> +The solution to this problem is to use a reference count: everyone who +has a pointer to the object increases it when they first get the +object, and drops the reference count when they're finished with it. +Whoever drops it to zero knows it is unused, and can actually delete it. + </para> + <para> +Here is the code: + </para> + +<programlisting> +--- cache.c.interrupt 2003-12-09 14:25:43.000000000 +1100 ++++ cache.c.refcnt 2003-12-09 14:33:05.000000000 +1100 +@@ -7,6 +7,7 @@ + struct object + { + struct list_head list; ++ unsigned int refcnt; + int id; + char name[32]; + int popularity; +@@ -17,6 +18,35 @@ + static unsigned int cache_num = 0; + #define MAX_CACHE_SIZE 10 + ++static void __object_put(struct object *obj) ++{ ++ if (--obj->refcnt == 0) ++ kfree(obj); ++} ++ ++static void __object_get(struct object *obj) ++{ ++ obj->refcnt++; ++} ++ ++void object_put(struct object *obj) ++{ ++ unsigned long flags; ++ ++ spin_lock_irqsave(&cache_lock, flags); ++ __object_put(obj); ++ spin_unlock_irqrestore(&cache_lock, flags); ++} ++ ++void object_get(struct object *obj) ++{ ++ unsigned long flags; ++ ++ spin_lock_irqsave(&cache_lock, flags); ++ __object_get(obj); ++ spin_unlock_irqrestore(&cache_lock, flags); ++} ++ + /* Must be holding cache_lock */ + static struct object *__cache_find(int id) + { +@@ -35,6 +65,7 @@ + { + BUG_ON(!obj); + list_del(&obj->list); ++ __object_put(obj); + cache_num--; + } + +@@ -63,6 +94,7 @@ + strlcpy(obj->name, name, sizeof(obj->name)); + obj->id = id; + obj->popularity = 0; ++ obj->refcnt = 1; /* The cache holds a reference */ + + spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); +@@ -79,18 +111,15 @@ + spin_unlock_irqrestore(&cache_lock, flags); + } + +-int cache_find(int id, char *name) ++struct object *cache_find(int id) + { + struct object *obj; +- int ret = -ENOENT; + unsigned long flags; + + spin_lock_irqsave(&cache_lock, flags); + obj = __cache_find(id); +- if (obj) { +- ret = 0; +- strcpy(name, obj->name); +- } ++ if (obj) ++ __object_get(obj); + spin_unlock_irqrestore(&cache_lock, flags); +- return ret; ++ return obj; + } +</programlisting> + +<para> +We encapsulate the reference counting in the standard 'get' and 'put' +functions. Now we can return the object itself from +<function>cache_find</function> which has the advantage that the user +can now sleep holding the object (eg. to +<function>copy_to_user</function> to name to userspace). +</para> +<para> +The other point to note is that I said a reference should be held for +every pointer to the object: thus the reference count is 1 when first +inserted into the cache. In some versions the framework does not hold +a reference count, but they are more complicated. +</para> + + <sect2 id="examples-refcnt-atomic"> + <title>Using Atomic Operations For The Reference Count</title> +<para> +In practice, <type>atomic_t</type> would usually be used for +<structfield>refcnt</structfield>. There are a number of atomic +operations defined in + +<filename class="headerfile">include/asm/atomic.h</filename>: these are +guaranteed to be seen atomically from all CPUs in the system, so no +lock is required. In this case, it is simpler than using spinlocks, +although for anything non-trivial using spinlocks is clearer. The +<function>atomic_inc</function> and +<function>atomic_dec_and_test</function> are used instead of the +standard increment and decrement operators, and the lock is no longer +used to protect the reference count itself. +</para> + +<programlisting> +--- cache.c.refcnt 2003-12-09 15:00:35.000000000 +1100 ++++ cache.c.refcnt-atomic 2003-12-11 15:49:42.000000000 +1100 +@@ -7,7 +7,7 @@ + struct object + { + struct list_head list; +- unsigned int refcnt; ++ atomic_t refcnt; + int id; + char name[32]; + int popularity; +@@ -18,33 +18,15 @@ + static unsigned int cache_num = 0; + #define MAX_CACHE_SIZE 10 + +-static void __object_put(struct object *obj) +-{ +- if (--obj->refcnt == 0) +- kfree(obj); +-} +- +-static void __object_get(struct object *obj) +-{ +- obj->refcnt++; +-} +- + void object_put(struct object *obj) + { +- unsigned long flags; +- +- spin_lock_irqsave(&cache_lock, flags); +- __object_put(obj); +- spin_unlock_irqrestore(&cache_lock, flags); ++ if (atomic_dec_and_test(&obj->refcnt)) ++ kfree(obj); + } + + void object_get(struct object *obj) + { +- unsigned long flags; +- +- spin_lock_irqsave(&cache_lock, flags); +- __object_get(obj); +- spin_unlock_irqrestore(&cache_lock, flags); ++ atomic_inc(&obj->refcnt); + } + + /* Must be holding cache_lock */ +@@ -65,7 +47,7 @@ + { + BUG_ON(!obj); + list_del(&obj->list); +- __object_put(obj); ++ object_put(obj); + cache_num--; + } + +@@ -94,7 +76,7 @@ + strlcpy(obj->name, name, sizeof(obj->name)); + obj->id = id; + obj->popularity = 0; +- obj->refcnt = 1; /* The cache holds a reference */ ++ atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ + + spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); +@@ -119,7 +101,7 @@ + spin_lock_irqsave(&cache_lock, flags); + obj = __cache_find(id); + if (obj) +- __object_get(obj); ++ object_get(obj); + spin_unlock_irqrestore(&cache_lock, flags); + return obj; + } +</programlisting> +</sect2> +</sect1> + + <sect1 id="examples-lock-per-obj"> + <title>Protecting The Objects Themselves</title> + <para> +In these examples, we assumed that the objects (except the reference +counts) never changed once they are created. If we wanted to allow +the name to change, there are three possibilities: + </para> + <itemizedlist> + <listitem> + <para> +You can make <symbol>cache_lock</symbol> non-static, and tell people +to grab that lock before changing the name in any object. + </para> + </listitem> + <listitem> + <para> +You can provide a <function>cache_obj_rename</function> which grabs +this lock and changes the name for the caller, and tell everyone to +use that function. + </para> + </listitem> + <listitem> + <para> +You can make the <symbol>cache_lock</symbol> protect only the cache +itself, and use another lock to protect the name. + </para> + </listitem> + </itemizedlist> + + <para> +Theoretically, you can make the locks as fine-grained as one lock for +every field, for every object. In practice, the most common variants +are: +</para> + <itemizedlist> + <listitem> + <para> +One lock which protects the infrastructure (the <symbol>cache</symbol> +list in this example) and all the objects. This is what we have done +so far. + </para> + </listitem> + <listitem> + <para> +One lock which protects the infrastructure (including the list +pointers inside the objects), and one lock inside the object which +protects the rest of that object. + </para> + </listitem> + <listitem> + <para> +Multiple locks to protect the infrastructure (eg. one lock per hash +chain), possibly with a separate per-object lock. + </para> + </listitem> + </itemizedlist> + +<para> +Here is the "lock-per-object" implementation: +</para> +<programlisting> +--- cache.c.refcnt-atomic 2003-12-11 15:50:54.000000000 +1100 ++++ cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100 +@@ -6,11 +6,17 @@ + + struct object + { ++ /* These two protected by cache_lock. */ + struct list_head list; ++ int popularity; ++ + atomic_t refcnt; ++ ++ /* Doesn't change once created. */ + int id; ++ ++ spinlock_t lock; /* Protects the name */ + char name[32]; +- int popularity; + }; + + static spinlock_t cache_lock = SPIN_LOCK_UNLOCKED; +@@ -77,6 +84,7 @@ + obj->id = id; + obj->popularity = 0; + atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ ++ spin_lock_init(&obj->lock); + + spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); +</programlisting> + +<para> +Note that I decide that the <structfield>popularity</structfield> +count should be protected by the <symbol>cache_lock</symbol> rather +than the per-object lock: this is because it (like the +<structname>struct list_head</structname> inside the object) is +logically part of the infrastructure. This way, I don't need to grab +the lock of every object in <function>__cache_add</function> when +seeking the least popular. +</para> + +<para> +I also decided that the <structfield>id</structfield> member is +unchangeable, so I don't need to grab each object lock in +<function>__cache_find()</function> to examine the +<structfield>id</structfield>: the object lock is only used by a +caller who wants to read or write the <structfield>name</structfield> +field. +</para> + +<para> +Note also that I added a comment describing what data was protected by +which locks. This is extremely important, as it describes the runtime +behavior of the code, and can be hard to gain from just reading. And +as Alan Cox says, <quote>Lock data, not code</quote>. +</para> +</sect1> +</chapter> + + <chapter id="common-problems"> + <title>Common Problems</title> + <sect1 id="deadlock"> + <title>Deadlock: Simple and Advanced</title> + + <para> + There is a coding bug where a piece of code tries to grab a + spinlock twice: it will spin forever, waiting for the lock to + be released (spinlocks, rwlocks and semaphores are not + recursive in Linux). This is trivial to diagnose: not a + stay-up-five-nights-talk-to-fluffy-code-bunnies kind of + problem. + </para> + + <para> + For a slightly more complex case, imagine you have a region + shared by a softirq and user context. If you use a + <function>spin_lock()</function> call to protect it, it is + possible that the user context will be interrupted by the softirq + while it holds the lock, and the softirq will then spin + forever trying to get the same lock. + </para> + + <para> + Both of these are called deadlock, and as shown above, it can + occur even with a single CPU (although not on UP compiles, + since spinlocks vanish on kernel compiles with + <symbol>CONFIG_SMP</symbol>=n. You'll still get data corruption + in the second example). + </para> + + <para> + This complete lockup is easy to diagnose: on SMP boxes the + watchdog timer or compiling with <symbol>DEBUG_SPINLOCKS</symbol> set + (<filename>include/linux/spinlock.h</filename>) will show this up + immediately when it happens. + </para> + + <para> + A more complex problem is the so-called 'deadly embrace', + involving two or more locks. Say you have a hash table: each + entry in the table is a spinlock, and a chain of hashed + objects. Inside a softirq handler, you sometimes want to + alter an object from one place in the hash to another: you + grab the spinlock of the old hash chain and the spinlock of + the new hash chain, and delete the object from the old one, + and insert it in the new one. + </para> + + <para> + There are two problems here. First, if your code ever + tries to move the object to the same chain, it will deadlock + with itself as it tries to lock it twice. Secondly, if the + same softirq on another CPU is trying to move another object + in the reverse direction, the following could happen: + </para> + + <table> + <title>Consequences</title> + + <tgroup cols="2" align="left"> + + <thead> + <row> + <entry>CPU 1</entry> + <entry>CPU 2</entry> + </row> + </thead> + + <tbody> + <row> + <entry>Grab lock A -> OK</entry> + <entry>Grab lock B -> OK</entry> + </row> + <row> + <entry>Grab lock B -> spin</entry> + <entry>Grab lock A -> spin</entry> + </row> + </tbody> + </tgroup> + </table> + + <para> + The two CPUs will spin forever, waiting for the other to give up + their lock. It will look, smell, and feel like a crash. + </para> + </sect1> + + <sect1 id="techs-deadlock-prevent"> + <title>Preventing Deadlock</title> + + <para> + Textbooks will tell you that if you always lock in the same + order, you will never get this kind of deadlock. Practice + will tell you that this approach doesn't scale: when I + create a new lock, I don't understand enough of the kernel + to figure out where in the 5000 lock hierarchy it will fit. + </para> + + <para> + The best locks are encapsulated: they never get exposed in + headers, and are never held around calls to non-trivial + functions outside the same file. You can read through this + code and see that it will never deadlock, because it never + tries to grab another lock while it has that one. People + using your code don't even need to know you are using a + lock. + </para> + + <para> + A classic problem here is when you provide callbacks or + hooks: if you call these with the lock held, you risk simple + deadlock, or a deadly embrace (who knows what the callback + will do?). Remember, the other programmers are out to get + you, so don't do this. + </para> + + <sect2 id="techs-deadlock-overprevent"> + <title>Overzealous Prevention Of Deadlocks</title> + + <para> + Deadlocks are problematic, but not as bad as data + corruption. Code which grabs a read lock, searches a list, + fails to find what it wants, drops the read lock, grabs a + write lock and inserts the object has a race condition. + </para> + + <para> + If you don't see why, please stay the fuck away from my code. + </para> + </sect2> + </sect1> + + <sect1 id="racing-timers"> + <title>Racing Timers: A Kernel Pastime</title> + + <para> + Timers can produce their own special problems with races. + Consider a collection of objects (list, hash, etc) where each + object has a timer which is due to destroy it. + </para> + + <para> + If you want to destroy the entire collection (say on module + removal), you might do the following: + </para> + + <programlisting> + /* THIS CODE BAD BAD BAD BAD: IF IT WAS ANY WORSE IT WOULD USE + HUNGARIAN NOTATION */ + spin_lock_bh(&list_lock); + + while (list) { + struct foo *next = list->next; + del_timer(&list->timer); + kfree(list); + list = next; + } + + spin_unlock_bh(&list_lock); + </programlisting> + + <para> + Sooner or later, this will crash on SMP, because a timer can + have just gone off before the <function>spin_lock_bh()</function>, + and it will only get the lock after we + <function>spin_unlock_bh()</function>, and then try to free + the element (which has already been freed!). + </para> + + <para> + This can be avoided by checking the result of + <function>del_timer()</function>: if it returns + <returnvalue>1</returnvalue>, the timer has been deleted. + If <returnvalue>0</returnvalue>, it means (in this + case) that it is currently running, so we can do: + </para> + + <programlisting> + retry: + spin_lock_bh(&list_lock); + + while (list) { + struct foo *next = list->next; + if (!del_timer(&list->timer)) { + /* Give timer a chance to delete this */ + spin_unlock_bh(&list_lock); + goto retry; + } + kfree(list); + list = next; + } + + spin_unlock_bh(&list_lock); + </programlisting> + + <para> + Another common problem is deleting timers which restart + themselves (by calling <function>add_timer()</function> at the end + of their timer function). Because this is a fairly common case + which is prone to races, you should use <function>del_timer_sync()</function> + (<filename class="headerfile">include/linux/timer.h</filename>) + to handle this case. It returns the number of times the timer + had to be deleted before we finally stopped it from adding itself back + in. + </para> + </sect1> + + </chapter> + + <chapter id="Efficiency"> + <title>Locking Speed</title> + + <para> +There are three main things to worry about when considering speed of +some code which does locking. First is concurrency: how many things +are going to be waiting while someone else is holding a lock. Second +is the time taken to actually acquire and release an uncontended lock. +Third is using fewer, or smarter locks. I'm assuming that the lock is +used fairly often: otherwise, you wouldn't be concerned about +efficiency. +</para> + <para> +Concurrency depends on how long the lock is usually held: you should +hold the lock for as long as needed, but no longer. In the cache +example, we always create the object without the lock held, and then +grab the lock only when we are ready to insert it in the list. +</para> + <para> +Acquisition times depend on how much damage the lock operations do to +the pipeline (pipeline stalls) and how likely it is that this CPU was +the last one to grab the lock (ie. is the lock cache-hot for this +CPU): on a machine with more CPUs, this likelihood drops fast. +Consider a 700MHz Intel Pentium III: an instruction takes about 0.7ns, +an atomic increment takes about 58ns, a lock which is cache-hot on +this CPU takes 160ns, and a cacheline transfer from another CPU takes +an additional 170 to 360ns. (These figures from Paul McKenney's +<ulink url="http://www.linuxjournal.com/article.php?sid=6993"> Linux +Journal RCU article</ulink>). +</para> + <para> +These two aims conflict: holding a lock for a short time might be done +by splitting locks into parts (such as in our final per-object-lock +example), but this increases the number of lock acquisitions, and the +results are often slower than having a single lock. This is another +reason to advocate locking simplicity. +</para> + <para> +The third concern is addressed below: there are some methods to reduce +the amount of locking which needs to be done. +</para> + + <sect1 id="efficiency-rwlocks"> + <title>Read/Write Lock Variants</title> + + <para> + Both spinlocks and semaphores have read/write variants: + <type>rwlock_t</type> and <structname>struct rw_semaphore</structname>. + These divide users into two classes: the readers and the writers. If + you are only reading the data, you can get a read lock, but to write to + the data you need the write lock. Many people can hold a read lock, + but a writer must be sole holder. + </para> + + <para> + If your code divides neatly along reader/writer lines (as our + cache code does), and the lock is held by readers for + significant lengths of time, using these locks can help. They + are slightly slower than the normal locks though, so in practice + <type>rwlock_t</type> is not usually worthwhile. + </para> + </sect1> + + <sect1 id="efficiency-read-copy-update"> + <title>Avoiding Locks: Read Copy Update</title> + + <para> + There is a special method of read/write locking called Read Copy + Update. Using RCU, the readers can avoid taking a lock + altogether: as we expect our cache to be read more often than + updated (otherwise the cache is a waste of time), it is a + candidate for this optimization. + </para> + + <para> + How do we get rid of read locks? Getting rid of read locks + means that writers may be changing the list underneath the + readers. That is actually quite simple: we can read a linked + list while an element is being added if the writer adds the + element very carefully. For example, adding + <symbol>new</symbol> to a single linked list called + <symbol>list</symbol>: + </para> + + <programlisting> + new->next = list->next; + wmb(); + list->next = new; + </programlisting> + + <para> + The <function>wmb()</function> is a write memory barrier. It + ensures that the first operation (setting the new element's + <symbol>next</symbol> pointer) is complete and will be seen by + all CPUs, before the second operation is (putting the new + element into the list). This is important, since modern + compilers and modern CPUs can both reorder instructions unless + told otherwise: we want a reader to either not see the new + element at all, or see the new element with the + <symbol>next</symbol> pointer correctly pointing at the rest of + the list. + </para> + <para> + Fortunately, there is a function to do this for standard + <structname>struct list_head</structname> lists: + <function>list_add_rcu()</function> + (<filename>include/linux/list.h</filename>). + </para> + <para> + Removing an element from the list is even simpler: we replace + the pointer to the old element with a pointer to its successor, + and readers will either see it, or skip over it. + </para> + <programlisting> + list->next = old->next; + </programlisting> + <para> + There is <function>list_del_rcu()</function> + (<filename>include/linux/list.h</filename>) which does this (the + normal version poisons the old object, which we don't want). + </para> + <para> + The reader must also be careful: some CPUs can look through the + <symbol>next</symbol> pointer to start reading the contents of + the next element early, but don't realize that the pre-fetched + contents is wrong when the <symbol>next</symbol> pointer changes + underneath them. Once again, there is a + <function>list_for_each_entry_rcu()</function> + (<filename>include/linux/list.h</filename>) to help you. Of + course, writers can just use + <function>list_for_each_entry()</function>, since there cannot + be two simultaneous writers. + </para> + <para> + Our final dilemma is this: when can we actually destroy the + removed element? Remember, a reader might be stepping through + this element in the list right now: it we free this element and + the <symbol>next</symbol> pointer changes, the reader will jump + off into garbage and crash. We need to wait until we know that + all the readers who were traversing the list when we deleted the + element are finished. We use <function>call_rcu()</function> to + register a callback which will actually destroy the object once + the readers are finished. + </para> + <para> + But how does Read Copy Update know when the readers are + finished? The method is this: firstly, the readers always + traverse the list inside + <function>rcu_read_lock()</function>/<function>rcu_read_unlock()</function> + pairs: these simply disable preemption so the reader won't go to + sleep while reading the list. + </para> + <para> + RCU then waits until every other CPU has slept at least once: + since readers cannot sleep, we know that any readers which were + traversing the list during the deletion are finished, and the + callback is triggered. The real Read Copy Update code is a + little more optimized than this, but this is the fundamental + idea. + </para> + +<programlisting> +--- cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100 ++++ cache.c.rcupdate 2003-12-11 17:55:14.000000000 +1100 +@@ -1,15 +1,18 @@ + #include <linux/list.h> + #include <linux/slab.h> + #include <linux/string.h> ++#include <linux/rcupdate.h> + #include <asm/semaphore.h> + #include <asm/errno.h> + + struct object + { +- /* These two protected by cache_lock. */ ++ /* This is protected by RCU */ + struct list_head list; + int popularity; + ++ struct rcu_head rcu; ++ + atomic_t refcnt; + + /* Doesn't change once created. */ +@@ -40,7 +43,7 @@ + { + struct object *i; + +- list_for_each_entry(i, &cache, list) { ++ list_for_each_entry_rcu(i, &cache, list) { + if (i->id == id) { + i->popularity++; + return i; +@@ -49,19 +52,25 @@ + return NULL; + } + ++/* Final discard done once we know no readers are looking. */ ++static void cache_delete_rcu(void *arg) ++{ ++ object_put(arg); ++} ++ + /* Must be holding cache_lock */ + static void __cache_delete(struct object *obj) + { + BUG_ON(!obj); +- list_del(&obj->list); +- object_put(obj); ++ list_del_rcu(&obj->list); + cache_num--; ++ call_rcu(&obj->rcu, cache_delete_rcu, obj); + } + + /* Must be holding cache_lock */ + static void __cache_add(struct object *obj) + { +- list_add(&obj->list, &cache); ++ list_add_rcu(&obj->list, &cache); + if (++cache_num > MAX_CACHE_SIZE) { + struct object *i, *outcast = NULL; + list_for_each_entry(i, &cache, list) { +@@ -85,6 +94,7 @@ + obj->popularity = 0; + atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ + spin_lock_init(&obj->lock); ++ INIT_RCU_HEAD(&obj->rcu); + + spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); +@@ -104,12 +114,11 @@ + struct object *cache_find(int id) + { + struct object *obj; +- unsigned long flags; + +- spin_lock_irqsave(&cache_lock, flags); ++ rcu_read_lock(); + obj = __cache_find(id); + if (obj) + object_get(obj); +- spin_unlock_irqrestore(&cache_lock, flags); ++ rcu_read_unlock(); + return obj; + } +</programlisting> + +<para> +Note that the reader will alter the +<structfield>popularity</structfield> member in +<function>__cache_find()</function>, and now it doesn't hold a lock. +One solution would be to make it an <type>atomic_t</type>, but for +this usage, we don't really care about races: an approximate result is +good enough, so I didn't change it. +</para> + +<para> +The result is that <function>cache_find()</function> requires no +synchronization with any other functions, so is almost as fast on SMP +as it would be on UP. +</para> + +<para> +There is a furthur optimization possible here: remember our original +cache code, where there were no reference counts and the caller simply +held the lock whenever using the object? This is still possible: if +you hold the lock, noone can delete the object, so you don't need to +get and put the reference count. +</para> + +<para> +Now, because the 'read lock' in RCU is simply disabling preemption, a +caller which always has preemption disabled between calling +<function>cache_find()</function> and +<function>object_put()</function> does not need to actually get and +put the reference count: we could expose +<function>__cache_find()</function> by making it non-static, and +such callers could simply call that. +</para> +<para> +The benefit here is that the reference count is not written to: the +object is not altered in any way, which is much faster on SMP +machines due to caching. +</para> + </sect1> + + <sect1 id="per-cpu"> + <title>Per-CPU Data</title> + + <para> + Another technique for avoiding locking which is used fairly + widely is to duplicate information for each CPU. For example, + if you wanted to keep a count of a common condition, you could + use a spin lock and a single counter. Nice and simple. + </para> + + <para> + If that was too slow (it's usually not, but if you've got a + really big machine to test on and can show that it is), you + could instead use a counter for each CPU, then none of them need + an exclusive lock. See <function>DEFINE_PER_CPU()</function>, + <function>get_cpu_var()</function> and + <function>put_cpu_var()</function> + (<filename class="headerfile">include/linux/percpu.h</filename>). + </para> + + <para> + Of particular use for simple per-cpu counters is the + <type>local_t</type> type, and the + <function>cpu_local_inc()</function> and related functions, + which are more efficient than simple code on some architectures + (<filename class="headerfile">include/asm/local.h</filename>). + </para> + + <para> + Note that there is no simple, reliable way of getting an exact + value of such a counter, without introducing more locks. This + is not a problem for some uses. + </para> + </sect1> + + <sect1 id="mostly-hardirq"> + <title>Data Which Mostly Used By An IRQ Handler</title> + + <para> + If data is always accessed from within the same IRQ handler, you + don't need a lock at all: the kernel already guarantees that the + irq handler will not run simultaneously on multiple CPUs. + </para> + <para> + Manfred Spraul points out that you can still do this, even if + the data is very occasionally accessed in user context or + softirqs/tasklets. The irq handler doesn't use a lock, and + all other accesses are done as so: + </para> + +<programlisting> + spin_lock(&lock); + disable_irq(irq); + ... + enable_irq(irq); + spin_unlock(&lock); +</programlisting> + <para> + The <function>disable_irq()</function> prevents the irq handler + from running (and waits for it to finish if it's currently + running on other CPUs). The spinlock prevents any other + accesses happening at the same time. Naturally, this is slower + than just a <function>spin_lock_irq()</function> call, so it + only makes sense if this type of access happens extremely + rarely. + </para> + </sect1> + </chapter> + + <chapter id="sleeping-things"> + <title>What Functions Are Safe To Call From Interrupts?</title> + + <para> + Many functions in the kernel sleep (ie. call schedule()) + directly or indirectly: you can never call them while holding a + spinlock, or with preemption disabled. This also means you need + to be in user context: calling them from an interrupt is illegal. + </para> + + <sect1 id="sleeping"> + <title>Some Functions Which Sleep</title> + + <para> + The most common ones are listed below, but you usually have to + read the code to find out if other calls are safe. If everyone + else who calls it can sleep, you probably need to be able to + sleep, too. In particular, registration and deregistration + functions usually expect to be called from user context, and can + sleep. + </para> + + <itemizedlist> + <listitem> + <para> + Accesses to + <firstterm linkend="gloss-userspace">userspace</firstterm>: + </para> + <itemizedlist> + <listitem> + <para> + <function>copy_from_user()</function> + </para> + </listitem> + <listitem> + <para> + <function>copy_to_user()</function> + </para> + </listitem> + <listitem> + <para> + <function>get_user()</function> + </para> + </listitem> + <listitem> + <para> + <function> put_user()</function> + </para> + </listitem> + </itemizedlist> + </listitem> + + <listitem> + <para> + <function>kmalloc(GFP_KERNEL)</function> + </para> + </listitem> + + <listitem> + <para> + <function>down_interruptible()</function> and + <function>down()</function> + </para> + <para> + There is a <function>down_trylock()</function> which can be + used inside interrupt context, as it will not sleep. + <function>up()</function> will also never sleep. + </para> + </listitem> + </itemizedlist> + </sect1> + + <sect1 id="dont-sleep"> + <title>Some Functions Which Don't Sleep</title> + + <para> + Some functions are safe to call from any context, or holding + almost any lock. + </para> + + <itemizedlist> + <listitem> + <para> + <function>printk()</function> + </para> + </listitem> + <listitem> + <para> + <function>kfree()</function> + </para> + </listitem> + <listitem> + <para> + <function>add_timer()</function> and <function>del_timer()</function> + </para> + </listitem> + </itemizedlist> + </sect1> + </chapter> + + <chapter id="references"> + <title>Further reading</title> + + <itemizedlist> + <listitem> + <para> + <filename>Documentation/spinlocks.txt</filename>: + Linus Torvalds' spinlocking tutorial in the kernel sources. + </para> + </listitem> + + <listitem> + <para> + Unix Systems for Modern Architectures: Symmetric + Multiprocessing and Caching for Kernel Programmers: + </para> + + <para> + Curt Schimmel's very good introduction to kernel level + locking (not written for Linux, but nearly everything + applies). The book is expensive, but really worth every + penny to understand SMP locking. [ISBN: 0201633388] + </para> + </listitem> + </itemizedlist> + </chapter> + + <chapter id="thanks"> + <title>Thanks</title> + + <para> + Thanks to Telsa Gwynne for DocBooking, neatening and adding + style. + </para> + + <para> + Thanks to Martin Pool, Philipp Rumpf, Stephen Rothwell, Paul + Mackerras, Ruedi Aschwanden, Alan Cox, Manfred Spraul, Tim + Waugh, Pete Zaitcev, James Morris, Robert Love, Paul McKenney, + John Ashby for proofreading, correcting, flaming, commenting. + </para> + + <para> + Thanks to the cabal for having no influence on this document. + </para> + </chapter> + + <glossary id="glossary"> + <title>Glossary</title> + + <glossentry id="gloss-preemption"> + <glossterm>preemption</glossterm> + <glossdef> + <para> + Prior to 2.5, or when <symbol>CONFIG_PREEMPT</symbol> is + unset, processes in user context inside the kernel would not + preempt each other (ie. you had that CPU until you have it up, + except for interrupts). With the addition of + <symbol>CONFIG_PREEMPT</symbol> in 2.5.4, this changed: when + in user context, higher priority tasks can "cut in": spinlocks + were changed to disable preemption, even on UP. + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-bh"> + <glossterm>bh</glossterm> + <glossdef> + <para> + Bottom Half: for historical reasons, functions with + '_bh' in them often now refer to any software interrupt, e.g. + <function>spin_lock_bh()</function> blocks any software interrupt + on the current CPU. Bottom halves are deprecated, and will + eventually be replaced by tasklets. Only one bottom half will be + running at any time. + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-hwinterrupt"> + <glossterm>Hardware Interrupt / Hardware IRQ</glossterm> + <glossdef> + <para> + Hardware interrupt request. <function>in_irq()</function> returns + <returnvalue>true</returnvalue> in a hardware interrupt handler. + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-interruptcontext"> + <glossterm>Interrupt Context</glossterm> + <glossdef> + <para> + Not user context: processing a hardware irq or software irq. + Indicated by the <function>in_interrupt()</function> macro + returning <returnvalue>true</returnvalue>. + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-smp"> + <glossterm><acronym>SMP</acronym></glossterm> + <glossdef> + <para> + Symmetric Multi-Processor: kernels compiled for multiple-CPU + machines. (CONFIG_SMP=y). + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-softirq"> + <glossterm>Software Interrupt / softirq</glossterm> + <glossdef> + <para> + Software interrupt handler. <function>in_irq()</function> returns + <returnvalue>false</returnvalue>; <function>in_softirq()</function> + returns <returnvalue>true</returnvalue>. Tasklets and softirqs + both fall into the category of 'software interrupts'. + </para> + <para> + Strictly speaking a softirq is one of up to 32 enumerated software + interrupts which can run on multiple CPUs at once. + Sometimes used to refer to tasklets as + well (ie. all software interrupts). + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-tasklet"> + <glossterm>tasklet</glossterm> + <glossdef> + <para> + A dynamically-registrable software interrupt, + which is guaranteed to only run on one CPU at a time. + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-timers"> + <glossterm>timer</glossterm> + <glossdef> + <para> + A dynamically-registrable software interrupt, which is run at + (or close to) a given time. When running, it is just like a + tasklet (in fact, they are called from the TIMER_SOFTIRQ). + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-up"> + <glossterm><acronym>UP</acronym></glossterm> + <glossdef> + <para> + Uni-Processor: Non-SMP. (CONFIG_SMP=n). + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-usercontext"> + <glossterm>User Context</glossterm> + <glossdef> + <para> + The kernel executing on behalf of a particular process (ie. a + system call or trap) or kernel thread. You can tell which + process with the <symbol>current</symbol> macro.) Not to + be confused with userspace. Can be interrupted by software or + hardware interrupts. + </para> + </glossdef> + </glossentry> + + <glossentry id="gloss-userspace"> + <glossterm>Userspace</glossterm> + <glossdef> + <para> + A process executing its own code outside the kernel. + </para> + </glossdef> + </glossentry> + + </glossary> +</book> + |