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| author | Mauro Carvalho Chehab <mchehab@s-opensource.com> | 2017-05-11 09:55:30 -0300 |
|---|---|---|
| committer | Mauro Carvalho Chehab <mchehab@s-opensource.com> | 2017-05-16 08:00:58 -0300 |
| commit | e548cdeffcd8ab8d3551a539890e682c08ab7828 (patch) | |
| tree | 6749befde851c8656e6c240073f0a283b8280b76 | |
| parent | dca1e58e3f1c82a840abaafb9328f84ae69a9926 (diff) | |
| download | linux-e548cdeffcd8ab8d3551a539890e682c08ab7828.tar.bz2 | |
docs-rst: convert kernel-locking to ReST
Use pandoc to convert documentation to ReST by calling
Documentation/sphinx/tmplcvt script.
- Manually adjust tables with got broken by conversion
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
| -rw-r--r-- | Documentation/DocBook/Makefile | 1 | ||||
| -rw-r--r-- | Documentation/DocBook/kernel-locking.tmpl | 2151 | ||||
| -rw-r--r-- | Documentation/kernel-hacking/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/kernel-hacking/locking.rst | 1453 |
4 files changed, 1454 insertions, 2152 deletions
diff --git a/Documentation/DocBook/Makefile b/Documentation/DocBook/Makefile index 7d7482b5ad92..9df94f7c2003 100644 --- a/Documentation/DocBook/Makefile +++ b/Documentation/DocBook/Makefile @@ -7,7 +7,6 @@ # list of DOCBOOKS. DOCBOOKS := z8530book.xml \ - kernel-locking.xml \ networking.xml \ filesystems.xml lsm.xml kgdb.xml \ libata.xml mtdnand.xml librs.xml rapidio.xml \ diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl deleted file mode 100644 index 7c9cc4846cb6..000000000000 --- a/Documentation/DocBook/kernel-locking.tmpl +++ /dev/null @@ -1,2151 +0,0 @@ -<?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 <linux/list.h> -#include <linux/slab.h> -#include <linux/string.h> -#include <linux/mutex.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 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, &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; - - mutex_lock(&cache_lock); - __cache_add(obj); - mutex_unlock(&cache_lock); - return 0; -} - -void cache_delete(int id) -{ - mutex_lock(&cache_lock); - __cache_delete(__cache_find(id)); - mutex_unlock(&cache_lock); -} - -int cache_find(int id, char *name) -{ - struct object *obj; - int ret = -ENOENT; - - mutex_lock(&cache_lock); - obj = __cache_find(id); - if (obj) { - ret = 0; - strcpy(name, obj->name); - } - mutex_unlock(&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->id = id; - obj->popularity = 0; - -- mutex_lock(&cache_lock); -+ spin_lock_irqsave(&cache_lock, flags); - __cache_add(obj); -- mutex_unlock(&cache_lock); -+ spin_unlock_irqrestore(&cache_lock, flags); - return 0; - } - - void cache_delete(int id) - { -- mutex_lock(&cache_lock); -+ unsigned long flags; -+ -+ spin_lock_irqsave(&cache_lock, flags); - __cache_delete(__cache_find(id)); -- mutex_unlock(&cache_lock); -+ spin_unlock_irqrestore(&cache_lock, flags); - } - - int cache_find(int id, char *name) - { - struct object *obj; - int ret = -ENOENT; -+ unsigned long flags; - -- mutex_lock(&cache_lock); -+ spin_lock_irqsave(&cache_lock, flags); - obj = __cache_find(id); - if (obj) { - ret = 0; - strcpy(name, obj->name); - } -- mutex_unlock(&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 DEFINE_SPINLOCK(cache_lock); -@@ -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 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 -> 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 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->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: 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 <linux/list.h> - #include <linux/slab.h> - #include <linux/string.h> -+#include <linux/rcupdate.h> - #include <linux/mutex.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); - } - - /* 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) { -@@ -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 further 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(&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>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-mutex"> - <title>Mutex API reference</title> -!Iinclude/linux/mutex.h -!Ekernel/locking/mutex.c - </chapter> - - <chapter id="apiref-futex"> - <title>Futex API reference</title> -!Ikernel/futex.c - </chapter> - - <chapter id="references"> - <title>Further reading</title> - - <itemizedlist> - <listitem> - <para> - <filename>Documentation/locking/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> - diff --git a/Documentation/kernel-hacking/index.rst b/Documentation/kernel-hacking/index.rst index ba208bf03ce9..fcb0eda3cca3 100644 --- a/Documentation/kernel-hacking/index.rst +++ b/Documentation/kernel-hacking/index.rst @@ -6,3 +6,4 @@ Kernel Hacking Guides :maxdepth: 2 hacking + locking diff --git a/Documentation/kernel-hacking/locking.rst b/Documentation/kernel-hacking/locking.rst new file mode 100644 index 000000000000..976b2703df75 --- /dev/null +++ b/Documentation/kernel-hacking/locking.rst @@ -0,0 +1,1453 @@ +=========================== +Unreliable Guide To Locking +=========================== + +:Author: Rusty Russell + +Introduction +============ + +Welcome, to Rusty's Remarkably Unreliable Guide to Kernel Locking +issues. This document describes the locking systems in the Linux Kernel +in 2.6. + +With the wide availability of HyperThreading, and preemption in the +Linux Kernel, everyone hacking on the kernel needs to know the +fundamentals of concurrency and locking for SMP. + +The Problem With Concurrency +============================ + +(Skip this if you know what a Race Condition is). + +In a normal program, you can increment a counter like so: + +:: + + very_important_count++; + + +This is what they would expect to happen: + ++------------------------------------+------------------------------------+ +| Instance 1 | Instance 2 | ++====================================+====================================+ +| read very_important_count (5) | | ++------------------------------------+------------------------------------+ +| add 1 (6) | | ++------------------------------------+------------------------------------+ +| write very_important_count (6) | | ++------------------------------------+------------------------------------+ +| | read very_important_count (6) | ++------------------------------------+------------------------------------+ +| | add 1 (7) | ++------------------------------------+------------------------------------+ +| | write very_important_count (7) | ++------------------------------------+------------------------------------+ + +Table: Expected Results + +This is what might happen: + ++------------------------------------+------------------------------------+ +| Instance 1 | Instance 2 | ++====================================+====================================+ +| read very_important_count (5) | | ++------------------------------------+------------------------------------+ +| | read very_important_count (5) | ++------------------------------------+------------------------------------+ +| add 1 (6) | | ++------------------------------------+------------------------------------+ +| | add 1 (6) | ++------------------------------------+------------------------------------+ +| write very_important_count (6) | | ++------------------------------------+------------------------------------+ +| | write very_important_count (6) | ++------------------------------------+------------------------------------+ + +Table: Possible Results + +Race Conditions and Critical Regions +------------------------------------ + +This overlap, where the result depends on the relative timing of +multiple tasks, is called a race condition. The piece of code containing +the concurrency issue is called a critical region. And especially since +Linux starting running on SMP machines, they became one of the major +issues in kernel design and implementation. + +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. + +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. + +Locking in the Linux Kernel +=========================== + +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: *keep it +simple*. + +Be reluctant to introduce new locks. + +Strangely enough, this last one is the exact reverse of my advice when +you *have* slept with someone crazier than yourself. And you should +think about getting a big dog. + +Two Main Types of Kernel Locks: Spinlocks and Mutexes +----------------------------------------------------- + +There are two main types of kernel locks. The fundamental type is the +spinlock (``include/asm/spinlock.h``), 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. + +The second type is a mutex (``include/linux/mutex.h``): 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 +`What Functions Are Safe To Call From Interrupts? <#sleeping-things>`__), +and so have to use a spinlock instead. + +Neither type of lock is recursive: see +`Deadlock: Simple and Advanced <#deadlock>`__. + +Locks and Uniprocessor Kernels +------------------------------ + +For kernels compiled without ``CONFIG_SMP``, and without +``CONFIG_PREEMPT`` 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. + +If the kernel is compiled without ``CONFIG_SMP``, but ``CONFIG_PREEMPT`` +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. + +You should always test your locking code with ``CONFIG_SMP`` and +``CONFIG_PREEMPT`` enabled, even if you don't have an SMP test box, +because it will still catch some kinds of locking bugs. + +Mutexes still exist, because they are required for synchronization +between user contexts, as we will see below. + +Locking Only In User Context +---------------------------- + +If you have a data structure which is only ever accessed from user +context, then you can use a simple mutex (``include/linux/mutex.h``) to +protect it. This is the most trivial case: you initialize the mutex. +Then you can call :c:func:`mutex_lock_interruptible()` to grab the +mutex, and :c:func:`mutex_unlock()` to release it. There is also a +:c:func:`mutex_lock()`, which should be avoided, because it will +not return if a signal is received. + +Example: ``net/netfilter/nf_sockopt.c`` allows registration of new +:c:func:`setsockopt()` and :c:func:`getsockopt()` calls, with +:c:func:`nf_register_sockopt()`. 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 :c:func:`setsockopt()` or :c:func:`getsockopt()` system +call. The ``nf_sockopt_mutex`` is perfect to protect this, especially +since the setsockopt and getsockopt calls may well sleep. + +Locking Between User Context and Softirqs +----------------------------------------- + +If a softirq 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 :c:func:`spin_lock_bh()` (``include/linux/spinlock.h``) is +used. It disables softirqs on that CPU, then grabs the lock. +:c:func:`spin_unlock_bh()` 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). + +Note that you can also use :c:func:`spin_lock_irq()` or +:c:func:`spin_lock_irqsave()` here, which stop hardware interrupts +as well: see `Hard IRQ Context <#hardirq-context>`__. + +This works perfectly for UP as well: the spin lock vanishes, and this +macro simply becomes :c:func:`local_bh_disable()` +(``include/linux/interrupt.h``), which protects you from the softirq +being run. + +Locking Between User Context and Tasklets +----------------------------------------- + +This is exactly the same as above, because tasklets are actually run +from a softirq. + +Locking Between User Context and Timers +--------------------------------------- + +This, too, is exactly the same as above, because timers are actually run +from a softirq. From a locking point of view, tasklets and timers are +identical. + +Locking Between Tasklets/Timers +------------------------------- + +Sometimes a tasklet or timer might want to share data with another +tasklet or timer. + +The Same Tasklet/Timer +~~~~~~~~~~~~~~~~~~~~~~ + +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. + +Different Tasklets/Timers +~~~~~~~~~~~~~~~~~~~~~~~~~ + +If another tasklet/timer wants to share data with your tasklet or timer +, you will both need to use :c:func:`spin_lock()` and +:c:func:`spin_unlock()` calls. :c:func:`spin_lock_bh()` is +unnecessary here, as you are already in a tasklet, and none will be run +on the same CPU. + +Locking Between Softirqs +------------------------ + +Often a softirq might want to share data with itself or a tasklet/timer. + +The Same Softirq +~~~~~~~~~~~~~~~~ + +The same softirq can run on the other CPUs: you can use a per-CPU array +(see `Per-CPU Data <#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. + +You'll need to use :c:func:`spin_lock()` and +:c:func:`spin_unlock()` for shared data. + +Different Softirqs +~~~~~~~~~~~~~~~~~~ + +You'll need to use :c:func:`spin_lock()` and +:c:func:`spin_unlock()` 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. + +Hard IRQ Context +================ + +Hardware interrupts usually communicate with a tasklet or softirq. +Frequently this involves putting work in a queue, which the softirq will +take out. + +Locking Between Hard IRQ and Softirqs/Tasklets +---------------------------------------------- + +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 +:c:func:`spin_lock_irq()` is used. It is defined to disable +interrupts on that cpu, then grab the lock. +:c:func:`spin_unlock_irq()` does the reverse. + +The irq handler does not to use :c:func:`spin_lock_irq()`, because +the softirq cannot run while the irq handler is running: it can use +:c:func:`spin_lock()`, which is slightly faster. The only exception +would be if a different hardware irq handler uses the same lock: +:c:func:`spin_lock_irq()` will stop that from interrupting us. + +This works perfectly for UP as well: the spin lock vanishes, and this +macro simply becomes :c:func:`local_irq_disable()` +(``include/asm/smp.h``), which protects you from the softirq/tasklet/BH +being run. + +:c:func:`spin_lock_irqsave()` (``include/linux/spinlock.h``) is a +variant which saves whether interrupts were on or off in a flags word, +which is passed to :c:func:`spin_unlock_irqrestore()`. 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). + +Note that softirqs (and hence tasklets and timers) are run on return +from hardware interrupts, so :c:func:`spin_lock_irq()` also stops +these. In that sense, :c:func:`spin_lock_irqsave()` is the most +general and powerful locking function. + +Locking Between Two Hard IRQ Handlers +------------------------------------- + +It is rare to have to share data between two IRQ handlers, but if you +do, :c:func:`spin_lock_irqsave()` should be used: it is +architecture-specific whether all interrupts are disabled inside irq +handlers themselves. + +Cheat Sheet For Locking +======================= + +Pete Zaitcev gives the following summary: + +- 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 + (``copy_from_user*(`` or ``kmalloc(x,GFP_KERNEL)``). + +- Otherwise (== data can be touched in an interrupt), use + :c:func:`spin_lock_irqsave()` and + :c:func:`spin_unlock_irqrestore()`. + +- Avoid holding spinlock for more than 5 lines of code and across any + function call (except accessors like :c:func:`readb()`). + +Table of Minimum Requirements +----------------------------- + +The following table lists the *minimum* 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). + +Remember the advice above: you can always use +:c:func:`spin_lock_irqsave()`, which is a superset of all other +spinlock primitives. + ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| | IRQ Handler A | IRQ Handler B | Softirq A | Softirq B | Tasklet A | Tasklet B | Timer A | Timer B | User Context A | User Context B | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| IRQ Handler A | None | | | | | | | | | | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| IRQ Handler B | SLIS | None | | | | | | | | | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| Softirq A | SLI | SLI | SL | | | | | | | | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| Softirq B | SLI | SLI | SL | SL | | | | | | | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| Tasklet A | SLI | SLI | SL | SL | None | | | | | | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| Tasklet B | SLI | SLI | SL | SL | SL | None | | | | | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| Timer A | SLI | SLI | SL | SL | SL | SL | None | | | | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| Timer B | SLI | SLI | SL | SL | SL | SL | SL | None | | | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| User Context A | SLI | SLI | SLBH | SLBH | SLBH | SLBH | SLBH | SLBH | None | | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ +| User Context B | SLI | SLI | SLBH | SLBH | SLBH | SLBH | SLBH | SLBH | MLI | None | ++------------------+-----------------+-----------------+-------------+-------------+-------------+-------------+-----------+-----------+------------------+------------------+ + +Table: Table of Locking Requirements + ++--------+----------------------------+ +| SLIS | spin_lock_irqsave | ++--------+----------------------------+ +| SLI | spin_lock_irq | ++--------+----------------------------+ +| SL | spin_lock | ++--------+----------------------------+ +| SLBH | spin_lock_bh | ++--------+----------------------------+ +| MLI | mutex_lock_interruptible | ++--------+----------------------------+ + +Table: Legend for Locking Requirements Table + +The trylock Functions +===================== + +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. + +:c:func:`spin_trylock()` 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 :c:func:`spin_lock()`: you must have +disabled the contexts that might interrupt you and acquire the spin +lock. + +:c:func:`mutex_trylock()` 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. + +Common Examples +=============== + +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. + +All In User Context +------------------- + +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:: + + #include <linux/list.h> + #include <linux/slab.h> + #include <linux/string.h> + #include <linux/mutex.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 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, &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; + + mutex_lock(&cache_lock); + __cache_add(obj); + mutex_unlock(&cache_lock); + return 0; + } + + void cache_delete(int id) + { + mutex_lock(&cache_lock); + __cache_delete(__cache_find(id)); + mutex_unlock(&cache_lock); + } + + int cache_find(int id, char *name) + { + struct object *obj; + int ret = -ENOENT; + + mutex_lock(&cache_lock); + obj = __cache_find(id); + if (obj) { + ret = 0; + strcpy(name, obj->name); + } + mutex_unlock(&cache_lock); + return ret; + } + +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. + +There is a slight (and common) optimization here: in +:c:func:`cache_add()` 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. + +Accessing From Interrupt Context +-------------------------------- + +Now consider the case where :c:func:`cache_find()` 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. + +The change is shown below, in standard patch format: the ``-`` are lines +which are taken away, and the ``+`` are lines which are added. + +:: + + --- 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->id = id; + obj->popularity = 0; + + - mutex_lock(&cache_lock); + + spin_lock_irqsave(&cache_lock, flags); + __cache_add(obj); + - mutex_unlock(&cache_lock); + + spin_unlock_irqrestore(&cache_lock, flags); + return 0; + } + + void cache_delete(int id) + { + - mutex_lock(&cache_lock); + + unsigned long flags; + + + + spin_lock_irqsave(&cache_lock, flags); + __cache_delete(__cache_find(id)); + - mutex_unlock(&cache_lock); + + spin_unlock_irqrestore(&cache_lock, flags); + } + + int cache_find(int id, char *name) + { + struct object *obj; + int ret = -ENOENT; + + unsigned long flags; + + - mutex_lock(&cache_lock); + + spin_lock_irqsave(&cache_lock, flags); + obj = __cache_find(id); + if (obj) { + ret = 0; + strcpy(name, obj->name); + } + - mutex_unlock(&cache_lock); + + spin_unlock_irqrestore(&cache_lock, flags); + return ret; + } + +Note that the :c:func:`spin_lock_irqsave()` 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. + +Unfortunately, :c:func:`cache_add()` calls :c:func:`kmalloc()` +with the ``GFP_KERNEL`` flag, which is only legal in user context. I +have assumed that :c:func:`cache_add()` is still only called in +user context, otherwise this should become a parameter to +:c:func:`cache_add()`. + +Exposing Objects Outside This File +---------------------------------- + +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. + +The first problem is that we use the ``cache_lock`` 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. + +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 :c:func:`cache_delete()` and even +worse, add another object, re-using the same address. + +As there is only one lock, you can't hold it forever: no-one else would +get any work done. + +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. + +Here is the code:: + + --- 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; + } + +We encapsulate the reference counting in the standard 'get' and 'put' +functions. Now we can return the object itself from +:c:func:`cache_find()` which has the advantage that the user can +now sleep holding the object (eg. to :c:func:`copy_to_user()` to +name to userspace). + +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. + +Using Atomic Operations For The Reference Count +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +In practice, ``atomic_t`` would usually be used for refcnt. There are a +number of atomic operations defined in ``include/asm/atomic.h``: 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 +:c:func:`atomic_inc()` and :c:func:`atomic_dec_and_test()` +are used instead of the standard increment and decrement operators, and +the lock is no longer used to protect the reference count itself. + +:: + + --- 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; + } + +Protecting The Objects Themselves +--------------------------------- + +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: + +- You can make ``cache_lock`` non-static, and tell people to grab that + lock before changing the name in any object. + +- You can provide a :c:func:`cache_obj_rename()` which grabs this + lock and changes the name for the caller, and tell everyone to use + that function. + +- You can make the ``cache_lock`` protect only the cache itself, and + use another lock to protect the name. + +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: + +- One lock which protects the infrastructure (the ``cache`` list in + this example) and all the objects. This is what we have done so far. + +- 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. + +- Multiple locks to protect the infrastructure (eg. one lock per hash + chain), possibly with a separate per-object lock. + +Here is the "lock-per-object" implementation: + +:: + + --- 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->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); + +Note that I decide that the popularity count should be protected by the +``cache_lock`` rather than the per-object lock: this is because it (like +the :c:type:`struct list_head <list_head>` inside the object) +is logically part of the infrastructure. This way, I don't need to grab +the lock of every object in :c:func:`__cache_add()` when seeking +the least popular. + +I also decided that the id member is unchangeable, so I don't need to +grab each object lock in :c:func:`__cache_find()` to examine the +id: the object lock is only used by a caller who wants to read or write +the name field. + +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, “Lock data, not code”. + +Common Problems +=============== + +Deadlock: Simple and Advanced +----------------------------- + +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. + +For a slightly more complex case, imagine you have a region shared by a +softirq and user context. If you use a :c:func:`spin_lock()` 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. + +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 ``CONFIG_SMP``\ =n. You'll still get data +corruption in the second example). + +This complete lockup is easy to diagnose: on SMP boxes the watchdog +timer or compiling with ``DEBUG_SPINLOCK`` set +(``include/linux/spinlock.h``) will show this up immediately when it +happens. + +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. + +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: + ++-----------------------+-----------------------+ +| CPU 1 | CPU 2 | ++=======================+=======================+ +| Grab lock A -> OK | Grab lock B -> OK | ++-----------------------+-----------------------+ +| Grab lock B -> spin | Grab lock A -> spin | ++-----------------------+-----------------------+ + +Table: Consequences + +The two CPUs will spin forever, waiting for the other to give up their +lock. It will look, smell, and feel like a crash. + +Preventing Deadlock +------------------- + +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. + +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. + +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. + +Overzealous Prevention Of Deadlocks +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +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. + +If you don't see why, please stay the fuck away from my code. + +Racing Timers: A Kernel Pastime +------------------------------- + +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. + +If you want to destroy the entire collection (say on module removal), +you might do the following:: + + /* 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); + + +Sooner or later, this will crash on SMP, because a timer can have just +gone off before the :c:func:`spin_lock_bh()`, and it will only get +the lock after we :c:func:`spin_unlock_bh()`, and then try to free +the element (which has already been freed!). + +This can be avoided by checking the result of +:c:func:`del_timer()`: if it returns 1, the timer has been deleted. +If 0, it means (in this case) that it is currently running, so we can +do:: + + 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); + + +Another common problem is deleting timers which restart themselves (by +calling :c:func:`add_timer()` at the end of their timer function). +Because this is a fairly common case which is prone to races, you should +use :c:func:`del_timer_sync()` (``include/linux/timer.h``) 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. + +Locking Speed +============= + +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. + +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. + +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 `Linux Journal RCU +article <http://www.linuxjournal.com/article.php?sid=6993>`__). + +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. + +The third concern is addressed below: there are some methods to reduce +the amount of locking which needs to be done. + +Read/Write Lock Variants +------------------------ + +Both spinlocks and mutexes have read/write variants: ``rwlock_t`` and +:c:type:`struct rw_semaphore <rw_semaphore>`. 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. + +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 ``rwlock_t`` is not usually worthwhile. + +Avoiding Locks: Read Copy Update +-------------------------------- + +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. + +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 ``new`` to a single linked list called ``list``:: + + new->next = list->next; + wmb(); + list->next = new; + + +The :c:func:`wmb()` is a write memory barrier. It ensures that the +first operation (setting the new element's ``next`` 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 ``next`` pointer correctly pointing at the +rest of the list. + +Fortunately, there is a function to do this for standard +:c:type:`struct list_head <list_head>` lists: +:c:func:`list_add_rcu()` (``include/linux/list.h``). + +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. + +:: + + list->next = old->next; + + +There is :c:func:`list_del_rcu()` (``include/linux/list.h``) which +does this (the normal version poisons the old object, which we don't +want). + +The reader must also be careful: some CPUs can look through the ``next`` +pointer to start reading the contents of the next element early, but +don't realize that the pre-fetched contents is wrong when the ``next`` +pointer changes underneath them. Once again, there is a +:c:func:`list_for_each_entry_rcu()` (``include/linux/list.h``) +to help you. Of course, writers can just use +:c:func:`list_for_each_entry()`, since there cannot be two +simultaneous writers. + +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 ``next`` 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 +:c:func:`call_rcu()` to register a callback which will actually +destroy the object once all pre-existing readers are finished. +Alternatively, :c:func:`synchronize_rcu()` may be used to block +until all pre-existing are finished. + +But how does Read Copy Update know when the readers are finished? The +method is this: firstly, the readers always traverse the list inside +:c:func:`rcu_read_lock()`/:c:func:`rcu_read_unlock()` pairs: +these simply disable preemption so the reader won't go to sleep while +reading the list. + +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. + +:: + + --- 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 <linux/mutex.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); + } + + /* 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) { + @@ -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; + } + +Note that the reader will alter the popularity member in +:c:func:`__cache_find()`, and now it doesn't hold a lock. One +solution would be to make it an ``atomic_t``, but for this usage, we +don't really care about races: an approximate result is good enough, so +I didn't change it. + +The result is that :c:func:`cache_find()` requires no +synchronization with any other functions, so is almost as fast on SMP as +it would be on UP. + +There is a further 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. + +Now, because the 'read lock' in RCU is simply disabling preemption, a +caller which always has preemption disabled between calling +:c:func:`cache_find()` and :c:func:`object_put()` does not +need to actually get and put the reference count: we could expose +:c:func:`__cache_find()` by making it non-static, and such +callers could simply call that. + +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. + +Per-CPU Data +------------ + +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. + +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 +:c:func:`DEFINE_PER_CPU()`, :c:func:`get_cpu_var()` and +:c:func:`put_cpu_var()` (``include/linux/percpu.h``). + +Of particular use for simple per-cpu counters is the ``local_t`` type, +and the :c:func:`cpu_local_inc()` and related functions, which are +more efficient than simple code on some architectures +(``include/asm/local.h``). + +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. + +Data Which Mostly Used By An IRQ Handler +---------------------------------------- + +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. + +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:: + + spin_lock(&lock); + disable_irq(irq); + ... + enable_irq(irq); + spin_unlock(&lock); + +The :c:func:`disable_irq()` 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 :c:func:`spin_lock_irq()` +call, so it only makes sense if this type of access happens extremely +rarely. + +What Functions Are Safe To Call From Interrupts? +================================================ + +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. + +Some Functions Which Sleep +-------------------------- + +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. + +- Accesses to userspace: + + - :c:func:`copy_from_user()` + + - :c:func:`copy_to_user()` + + - :c:func:`get_user()` + + - :c:func:`put_user()` + +- ``kmalloc(GFP_KERNEL)`` + +- :c:func:`mutex_lock_interruptible()` and + :c:func:`mutex_lock()` + + There is a :c:func:`mutex_trylock()` which does not sleep. + Still, it must not be used inside interrupt context since its + implementation is not safe for that. :c:func:`mutex_unlock()` + 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. + +Some Functions Which Don't Sleep +-------------------------------- + +Some functions are safe to call from any context, or holding almost any +lock. + +- :c:func:`printk()` + +- :c:func:`kfree()` + +- :c:func:`add_timer()` and :c:func:`del_timer()` + +Mutex API reference +=================== + +.. kernel-doc:: include/linux/mutex.h + :internal: + +.. kernel-doc:: kernel/locking/mutex.c + :export: + +Futex API reference +=================== + +.. kernel-doc:: kernel/futex.c + :internal: + +Further reading +=============== + +- ``Documentation/locking/spinlocks.txt``: Linus Torvalds' spinlocking + tutorial in the kernel sources. + +- Unix Systems for Modern Architectures: Symmetric Multiprocessing and + Caching for Kernel Programmers: + + 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] + +Thanks +====== + +Thanks to Telsa Gwynne for DocBooking, neatening and adding style. + +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. + +Thanks to the cabal for having no influence on this document. + +Glossary +======== + +preemption + Prior to 2.5, or when ``CONFIG_PREEMPT`` 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 + ``CONFIG_PREEMPT`` 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. + +bh + Bottom Half: for historical reasons, functions with '_bh' in them often + now refer to any software interrupt, e.g. :c:func:`spin_lock_bh()` + 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. + +Hardware Interrupt / Hardware IRQ + Hardware interrupt request. :c:func:`in_irq()` returns true in a + hardware interrupt handler. + +Interrupt Context + Not user context: processing a hardware irq or software irq. Indicated + by the :c:func:`in_interrupt()` macro returning true. + +SMP + Symmetric Multi-Processor: kernels compiled for multiple-CPU machines. + (``CONFIG_SMP=y``). + +Software Interrupt / softirq + Software interrupt handler. :c:func:`in_irq()` returns false; + :c:func:`in_softirq()` returns true. Tasklets and softirqs both + fall into the category of 'software interrupts'. + + 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). + +tasklet + A dynamically-registrable software interrupt, which is guaranteed to + only run on one CPU at a time. + +timer + 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). + +UP + Uni-Processor: Non-SMP. (CONFIG_SMP=n). + +User Context + 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 + ``current`` macro.) Not to be confused with userspace. Can be + interrupted by software or hardware interrupts. + +Userspace + A process executing its own code outside the kernel. |