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-<?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 Mutexes</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 mutex
- (<filename class="headerfile">include/linux/mutex.h</filename>): it
- is like a spinlock, but you may block holding a mutex.
- If you can't lock a mutex, your task will suspend itself, and be woken
- up when the mutex is released. This means the CPU can do something
- else while you are waiting. 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>
- Mutexes 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 mutex
- (<filename>include/linux/mutex.h</filename>) to protect it. This
- is the most trivial case: you initialize the mutex. Then you can
- call <function>mutex_lock_interruptible()</function> to grab the mutex,
- and <function>mutex_unlock()</function> to release it. There is also a
- <function>mutex_lock()</function>, which should be avoided, because it
- will not return if a signal is received.
- </para>
-
- <para>
- Example: <filename>net/netfilter/nf_sockopt.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 mutex. You can take a mutex
- 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>SLIS</entry>
-<entry>None</entry>
-</row>
-
-<row>
-<entry>Softirq A</entry>
-<entry>SLI</entry>
-<entry>SLI</entry>
-<entry>SL</entry>
-</row>
-
-<row>
-<entry>Softirq B</entry>
-<entry>SLI</entry>
-<entry>SLI</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-</row>
-
-<row>
-<entry>Tasklet A</entry>
-<entry>SLI</entry>
-<entry>SLI</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>None</entry>
-</row>
-
-<row>
-<entry>Tasklet B</entry>
-<entry>SLI</entry>
-<entry>SLI</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>None</entry>
-</row>
-
-<row>
-<entry>Timer A</entry>
-<entry>SLI</entry>
-<entry>SLI</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>None</entry>
-</row>
-
-<row>
-<entry>Timer B</entry>
-<entry>SLI</entry>
-<entry>SLI</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>SL</entry>
-<entry>None</entry>
-</row>
-
-<row>
-<entry>User Context A</entry>
-<entry>SLI</entry>
-<entry>SLI</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>None</entry>
-</row>
-
-<row>
-<entry>User Context B</entry>
-<entry>SLI</entry>
-<entry>SLI</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>SLBH</entry>
-<entry>MLI</entry>
-<entry>None</entry>
-</row>
-
-</tbody>
-</tgroup>
-</table>
-
- <table>
-<title>Legend for Locking Requirements Table</title>
-<tgroup cols="2">
-<tbody>
-
-<row>
-<entry>SLIS</entry>
-<entry>spin_lock_irqsave</entry>
-</row>
-<row>
-<entry>SLI</entry>
-<entry>spin_lock_irq</entry>
-</row>
-<row>
-<entry>SL</entry>
-<entry>spin_lock</entry>
-</row>
-<row>
-<entry>SLBH</entry>
-<entry>spin_lock_bh</entry>
-</row>
-<row>
-<entry>MLI</entry>
-<entry>mutex_lock_interruptible</entry>
-</row>
-
-</tbody>
-</tgroup>
-</table>
-
-</sect1>
-</chapter>
-
-<chapter id="trylock-functions">
- <title>The trylock Functions</title>
- <para>
- There are functions that try to acquire a lock only once and immediately
- return a value telling about success or failure to acquire the lock.
- They can be used if you need no access to the data protected with the lock
- when some other thread is holding the lock. You should acquire the lock
- later if you then need access to the data protected with the lock.
- </para>
-
- <para>
- <function>spin_trylock()</function> does not spin but returns non-zero if
- it acquires the spinlock on the first try or 0 if not. This function can
- be used in all contexts like <function>spin_lock</function>: you must have
- disabled the contexts that might interrupt you and acquire the spin lock.
- </para>
-
- <para>
- <function>mutex_trylock()</function> does not suspend your task
- but returns non-zero if it could lock the mutex on the first try
- or 0 if not. This function cannot be safely used in hardware or software
- interrupt contexts despite not sleeping.
- </para>
-</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 mutex to protect the cache and all the objects within
-it. Here's the code:
- </para>
-
- <programlisting>
-#include &lt;linux/list.h&gt;
-#include &lt;linux/slab.h&gt;
-#include &lt;linux/string.h&gt;
-#include &lt;linux/mutex.h&gt;
-#include &lt;asm/errno.h&gt;
-
-struct object
-{
- struct list_head list;
- int id;
- char name[32];
- int popularity;
-};
-
-/* Protects the cache, cache_num, and the objects within it */
-static DEFINE_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, &amp;cache, list)
- if (i-&gt;id == id) {
- i-&gt;popularity++;
- return i;
- }
- return NULL;
-}
-
-/* Must be holding cache_lock */
-static void __cache_delete(struct object *obj)
-{
- BUG_ON(!obj);
- list_del(&amp;obj-&gt;list);
- kfree(obj);
- cache_num--;
-}
-
-/* Must be holding cache_lock */
-static void __cache_add(struct object *obj)
-{
- list_add(&amp;obj-&gt;list, &amp;cache);
- if (++cache_num > MAX_CACHE_SIZE) {
- struct object *i, *outcast = NULL;
- list_for_each_entry(i, &amp;cache, list) {
- if (!outcast || i-&gt;popularity &lt; outcast-&gt;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-&gt;name, name, sizeof(obj-&gt;name));
- obj-&gt;id = id;
- obj-&gt;popularity = 0;
-
- mutex_lock(&amp;cache_lock);
- __cache_add(obj);
- mutex_unlock(&amp;cache_lock);
- return 0;
-}
-
-void cache_delete(int id)
-{
- mutex_lock(&amp;cache_lock);
- __cache_delete(__cache_find(id));
- mutex_unlock(&amp;cache_lock);
-}
-
-int cache_find(int id, char *name)
-{
- struct object *obj;
- int ret = -ENOENT;
-
- mutex_lock(&amp;cache_lock);
- obj = __cache_find(id);
- if (obj) {
- ret = 0;
- strcpy(name, obj-&gt;name);
- }
- mutex_unlock(&amp;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 DEFINE_MUTEX(cache_lock);
-+static DEFINE_SPINLOCK(cache_lock);
- 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-&gt;id = id;
- obj-&gt;popularity = 0;
-
-- mutex_lock(&amp;cache_lock);
-+ spin_lock_irqsave(&amp;cache_lock, flags);
- __cache_add(obj);
-- mutex_unlock(&amp;cache_lock);
-+ spin_unlock_irqrestore(&amp;cache_lock, flags);
- return 0;
- }
-
- void cache_delete(int id)
- {
-- mutex_lock(&amp;cache_lock);
-+ unsigned long flags;
-+
-+ spin_lock_irqsave(&amp;cache_lock, flags);
- __cache_delete(__cache_find(id));
-- mutex_unlock(&amp;cache_lock);
-+ spin_unlock_irqrestore(&amp;cache_lock, flags);
- }
-
- int cache_find(int id, char *name)
- {
- struct object *obj;
- int ret = -ENOENT;
-+ unsigned long flags;
-
-- mutex_lock(&amp;cache_lock);
-+ spin_lock_irqsave(&amp;cache_lock, flags);
- obj = __cache_find(id);
- if (obj) {
- ret = 0;
- strcpy(name, obj-&gt;name);
- }
-- mutex_unlock(&amp;cache_lock);
-+ spin_unlock_irqrestore(&amp;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-&gt;refcnt == 0)
-+ kfree(obj);
-+}
-+
-+static void __object_get(struct object *obj)
-+{
-+ obj-&gt;refcnt++;
-+}
-+
-+void object_put(struct object *obj)
-+{
-+ unsigned long flags;
-+
-+ spin_lock_irqsave(&amp;cache_lock, flags);
-+ __object_put(obj);
-+ spin_unlock_irqrestore(&amp;cache_lock, flags);
-+}
-+
-+void object_get(struct object *obj)
-+{
-+ unsigned long flags;
-+
-+ spin_lock_irqsave(&amp;cache_lock, flags);
-+ __object_get(obj);
-+ spin_unlock_irqrestore(&amp;cache_lock, flags);
-+}
-+
- /* Must be holding cache_lock */
- static struct object *__cache_find(int id)
- {
-@@ -35,6 +65,7 @@
- {
- BUG_ON(!obj);
- list_del(&amp;obj-&gt;list);
-+ __object_put(obj);
- cache_num--;
- }
-
-@@ -63,6 +94,7 @@
- strlcpy(obj-&gt;name, name, sizeof(obj-&gt;name));
- obj-&gt;id = id;
- obj-&gt;popularity = 0;
-+ obj-&gt;refcnt = 1; /* The cache holds a reference */
-
- spin_lock_irqsave(&amp;cache_lock, flags);
- __cache_add(obj);
-@@ -79,18 +111,15 @@
- spin_unlock_irqrestore(&amp;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(&amp;cache_lock, flags);
- obj = __cache_find(id);
-- if (obj) {
-- ret = 0;
-- strcpy(name, obj-&gt;name);
-- }
-+ if (obj)
-+ __object_get(obj);
- spin_unlock_irqrestore(&amp;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-&gt;refcnt == 0)
-- kfree(obj);
--}
--
--static void __object_get(struct object *obj)
--{
-- obj-&gt;refcnt++;
--}
--
- void object_put(struct object *obj)
- {
-- unsigned long flags;
--
-- spin_lock_irqsave(&amp;cache_lock, flags);
-- __object_put(obj);
-- spin_unlock_irqrestore(&amp;cache_lock, flags);
-+ if (atomic_dec_and_test(&amp;obj-&gt;refcnt))
-+ kfree(obj);
- }
-
- void object_get(struct object *obj)
- {
-- unsigned long flags;
--
-- spin_lock_irqsave(&amp;cache_lock, flags);
-- __object_get(obj);
-- spin_unlock_irqrestore(&amp;cache_lock, flags);
-+ atomic_inc(&amp;obj-&gt;refcnt);
- }
-
- /* Must be holding cache_lock */
-@@ -65,7 +47,7 @@
- {
- BUG_ON(!obj);
- list_del(&amp;obj-&gt;list);
-- __object_put(obj);
-+ object_put(obj);
- cache_num--;
- }
-
-@@ -94,7 +76,7 @@
- strlcpy(obj-&gt;name, name, sizeof(obj-&gt;name));
- obj-&gt;id = id;
- obj-&gt;popularity = 0;
-- obj-&gt;refcnt = 1; /* The cache holds a reference */
-+ atomic_set(&amp;obj-&gt;refcnt, 1); /* The cache holds a reference */
-
- spin_lock_irqsave(&amp;cache_lock, flags);
- __cache_add(obj);
-@@ -119,7 +101,7 @@
- spin_lock_irqsave(&amp;cache_lock, flags);
- obj = __cache_find(id);
- if (obj)
-- __object_get(obj);
-+ object_get(obj);
- spin_unlock_irqrestore(&amp;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 DEFINE_SPINLOCK(cache_lock);
-@@ -77,6 +84,7 @@
- obj-&gt;id = id;
- obj-&gt;popularity = 0;
- atomic_set(&amp;obj-&gt;refcnt, 1); /* The cache holds a reference */
-+ spin_lock_init(&amp;obj-&gt;lock);
-
- spin_lock_irqsave(&amp;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 mutexes 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_SPINLOCK</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 -&gt; OK</entry>
- <entry>Grab lock B -&gt; OK</entry>
- </row>
- <row>
- <entry>Grab lock B -&gt; spin</entry>
- <entry>Grab lock A -&gt; 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(&amp;list_lock);
-
- while (list) {
- struct foo *next = list-&gt;next;
- del_timer(&amp;list-&gt;timer);
- kfree(list);
- list = next;
- }
-
- spin_unlock_bh(&amp;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(&amp;list_lock);
-
- while (list) {
- struct foo *next = list-&gt;next;
- if (!del_timer(&amp;list-&gt;timer)) {
- /* Give timer a chance to delete this */
- spin_unlock_bh(&amp;list_lock);
- goto retry;
- }
- kfree(list);
- list = next;
- }
-
- spin_unlock_bh(&amp;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 mutexes 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-&gt;next = list-&gt;next;
- wmb();
- list-&gt;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-&gt;next = old-&gt;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: if 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
- all pre-existing readers are finished. Alternatively,
- <function>synchronize_rcu()</function> may be used to block until
- all pre-existing 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 &lt;linux/list.h&gt;
- #include &lt;linux/slab.h&gt;
- #include &lt;linux/string.h&gt;
-+#include &lt;linux/rcupdate.h&gt;
- #include &lt;linux/mutex.h&gt;
- #include &lt;asm/errno.h&gt;
-
- 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, &amp;cache, list) {
-+ list_for_each_entry_rcu(i, &amp;cache, list) {
- if (i-&gt;id == id) {
- i-&gt;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(&amp;obj-&gt;list);
-- object_put(obj);
-+ list_del_rcu(&amp;obj-&gt;list);
- cache_num--;
-+ call_rcu(&amp;obj-&gt;rcu, cache_delete_rcu);
- }
-
- /* Must be holding cache_lock */
- static void __cache_add(struct object *obj)
- {
-- list_add(&amp;obj-&gt;list, &amp;cache);
-+ list_add_rcu(&amp;obj-&gt;list, &amp;cache);
- if (++cache_num > MAX_CACHE_SIZE) {
- struct object *i, *outcast = NULL;
- list_for_each_entry(i, &amp;cache, list) {
-@@ -104,12 +114,11 @@
- struct object *cache_find(int id)
- {
- struct object *obj;
-- unsigned long flags;
-
-- spin_lock_irqsave(&amp;cache_lock, flags);
-+ rcu_read_lock();
- obj = __cache_find(id);
- if (obj)
- object_get(obj);
-- spin_unlock_irqrestore(&amp;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, no one 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(&amp;lock);
- disable_irq(irq);
- ...
- enable_irq(irq);
- spin_unlock(&amp;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>mutex_lock_interruptible()</function> and
- <function>mutex_lock()</function>
- </para>
- <para>
- There is a <function>mutex_trylock()</function> which does not
- sleep. Still, it must not be used inside interrupt context since
- its implementation is not safe for that.
- <function>mutex_unlock()</function> will also never sleep.
- It cannot be used in interrupt context either since a mutex
- must be released by the same task that acquired it.
- </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="apiref">
- <title>Mutex API reference</title>
-!Iinclude/linux/mutex.h
-!Ekernel/mutex.c
- </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 gave 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>
-