<authorgroup>
<author>
- <firstname>Paul</firstname>
- <othername>Rusty</othername>
+ <firstname>Rusty</firstname>
<surname>Russell</surname>
<affiliation>
<address>
</authorgroup>
<copyright>
- <year>2000</year>
- <holder>Paul Russell</holder>
+ <year>2003</year>
+ <holder>Rusty Russell</holder>
</copyright>
<legalnotice>
<para>
Welcome, to Rusty's Remarkably Unreliable Guide to Kernel
Locking issues. This document describes the locking systems in
- the Linux Kernel as we approach 2.4.
+ the Linux Kernel in 2.6.
</para>
<para>
- It looks like <firstterm linkend="gloss-smp"><acronym>SMP</acronym>
- </firstterm> is here to stay; so everyone hacking on the kernel
- these days needs to know the fundamentals of concurrency and locking
- for SMP.
+ 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>
- <sect1 id="races">
+ <chapter id="races">
<title>The Problem With Concurrency</title>
<para>
(Skip this if you know what a Race Condition is).
</tgroup>
</table>
+ <sect1 id="race-condition">
+ <title>Race Conditions and Critical Regions</title>
<para>
- This overlap, where what actually happens depends on the
- relative timing of multiple tasks, is called a race condition.
+ 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
- critical region. And especially since Linux starting running
+ <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
And then there are the unfriendly primitives, but I'll pretend
they don't exist.
</para>
- </sect1>
</chapter>
<chapter id="locks">
+ <title>Locking in the Linux Kernel</title>
+
+ <para>
+ If I could give you one piece of advice: never sleep with anyone
+ crazier than yourself. But if I had to give you advice on
+ locking: <emphasis>keep it simple</emphasis>.
+ </para>
+
+ <para>
+ Be reluctant to introduce new locks.
+ </para>
+
+ <para>
+ Strangely enough, this last one is the exact reverse of my advice when
+ you <emphasis>have</emphasis> slept with someone crazier than yourself.
+ And you should think about getting a big dog.
+ </para>
+
+ <sect1 id="lock-intro">
<title>Two Main Types of Kernel Locks: Spinlocks and Semaphores</title>
<para>
</para>
<para>
Neither type of lock is recursive: see
- <xref linkend="techniques-deadlocks">.
+ <xref linkend="deadlock">.
</para>
+ </sect1>
<sect1 id="uniprocessor">
<title>Locks and Uniprocessor Kernels</title>
<para>
- For kernels compiled without <symbol>CONFIG_SMP</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 at all.
+ 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>
- enabled, even if you don't have an SMP test box, because it
- will still catch some (simple) kinds of deadlock.
+ 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>
</para>
</sect1>
- <sect1 id="rwlocks">
- <title>Read/Write Lock Variants</title>
-
- <para>
- Both spinlocks and semaphores have read/write variants:
- <type>rwlock_t</type> and <structname>struct rw_semaphore</structname>.
- These divide users into two classes: the readers and the writers. If
- you are only reading the data, you can get a read lock, but to write to
- the data you need the write lock. Many people can hold a read lock,
- but a writer must be sole holder.
- </para>
-
- <para>
- This means much smoother locking if your code divides up
- neatly along reader/writer lines. All the discussions below
- also apply to read/write variants.
- </para>
- </sect1>
-
<sect1 id="usercontextlocking">
<title>Locking Only In User Context</title>
</sect1>
<sect1 id="lock-user-bh">
- <title>Locking Between User Context and BHs</title>
+ <title>Locking Between User Context and Softirqs</title>
<para>
- If a <firstterm linkend="gloss-bh">bottom half</firstterm> shares
+ 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 bottom half, and secondly, the
+ 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 bottom halves on that CPU, then grabs the lock.
- <function>spin_unlock_bh()</function> does the reverse.
+ 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>
</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 bottom half being run.
+ protects you from the softirq being run.
</para>
</sect1>
<sect1 id="lock-user-tasklet">
- <title>Locking Between User Context and Tasklets/Soft IRQs</title>
+ <title>Locking Between User Context and Tasklets</title>
<para>
- This is exactly the same as above, because
- <function>local_bh_disable()</function> actually disables all
- softirqs and <firstterm linkend="gloss-tasklet">tasklets</firstterm>
- on that CPU as well. It should really be
- called `local_softirq_disable()', but the name has been preserved
- for historical reasons. Similarly,
- <function>spin_lock_bh()</function> would now be called
- spin_lock_softirq() in a perfect world.
+ 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-bh">
- <title>Locking Between Bottom Halves</title>
+ <sect1 id="lock-user-timers">
+ <title>Locking Between User Context and Timers</title>
<para>
- Sometimes a bottom half might want to share data with
- another bottom half (especially remember that timers are run
- off a bottom half).
+ 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>
-
- <sect2 id="lock-bh-same">
- <title>The Same BH</title>
-
- <para>
- Since a bottom half is never run on two CPUs at once, you
- don't need to worry about your bottom half being run twice
- at once, even on SMP.
- </para>
- </sect2>
-
- <sect2 id="lock-bh-different">
- <title>Different BHs</title>
-
- <para>
- Since only one bottom half ever runs at a time once, you
- don't need to worry about race conditions with other bottom
- halves. Beware that things might change under you, however,
- if someone changes your bottom half to a tasklet. If you
- want to make your code future-proof, pretend you're already
- running from a tasklet (see below), and doing the extra
- locking. Of course, if it's five years before that happens,
- you're gonna look like a damn fool.
- </para>
- </sect2>
</sect1>
<sect1 id="lock-tasklets">
- <title>Locking Between Tasklets</title>
+ <title>Locking Between Tasklets/Timers</title>
<para>
- Sometimes a tasklet might want to share data with another
- tasklet, or a bottom half.
+ 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</title>
+ <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
</sect2>
<sect2 id="lock-tasklets-different">
- <title>Different Tasklets</title>
+ <title>Different Tasklets/Timers</title>
<para>
- If another tasklet (or bottom half, such as a timer) wants
- to share data with your tasklet, you will both need to use
+ 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
<title>Locking Between Softirqs</title>
<para>
- Often a <firstterm linkend="gloss-softirq">softirq</firstterm> might
- want to share data with itself, a tasklet, or a bottom half.
+ Often a softirq might
+ want to share data with itself or a tasklet/timer.
</para>
<sect2 id="lock-softirqs-same">
<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 (which can be running on a different CPU), bottom half,
- tasklet or the same or another softirq.
+ 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>
<title>Hard IRQ Context</title>
<para>
- Hardware interrupts usually communicate with a bottom half,
+ Hardware interrupts usually communicate with a
tasklet or softirq. Frequently this involves putting work in a
- queue, which the BH/softirq will take out.
+ queue, which the softirq will take out.
</para>
<sect1 id="hardirq-softirq">
- <title>Locking Between Hard IRQ and Softirqs/Tasklets/BHs</title>
+ <title>Locking Between Hard IRQ and Softirqs/Tasklets</title>
<para>
If a hardware irq handler shares data with a softirq, you have
</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
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="common-techniques">
- <title>Common Techniques</title>
+ </chapter>
+ <chapter id="cheatsheet">
+ <title>Cheat Sheet For Locking</title>
<para>
- This section lists some common dilemmas and the standard
- solutions used in the Linux kernel code. If you use these,
- people will find your code simpler to understand.
+ Pete Zaitcev gives the following summary:
</para>
+ <itemizedlist>
+ <listitem>
+ <para>
+ If you are in a process context (any syscall) and want to
+ lock other process out, use a semaphore. You can take a semaphore
+ and sleep (<function>copy_from_user*(</function> or
+ <function>kmalloc(x,GFP_KERNEL)</function>).
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Otherwise (== data can be touched in an interrupt), use
+ <function>spin_lock_irqsave()</function> and
+ <function>spin_unlock_irqrestore()</function>.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Avoid holding spinlock for more than 5 lines of code and
+ across any function call (except accessors like
+ <function>readb</function>).
+ </para>
+ </listitem>
+ </itemizedlist>
- <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>
+ <sect1 id="minimum-lock-reqirements">
+ <title>Table of Minimum Requirements</title>
- <para>
- Lock data, not code.
+ <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>
- Be reluctant to introduce new locks.
+ Remember the advice above: you can always use
+ <function>spin_lock_irqsave()</function>, which is a superset
+ of all other spinlock primitives.
</para>
+ <table>
+<title>Table of Locking Requirements</title>
+<TGROUP COLS=11>
+<TBODY>
+<ROW>
+<ENTRY></ENTRY>
+<ENTRY>IRQ Handler A</ENTRY>
+<ENTRY>IRQ Handler B</ENTRY>
+<ENTRY>Softirq A</ENTRY>
+<ENTRY>Softirq B</ENTRY>
+<ENTRY>Tasklet A</ENTRY>
+<ENTRY>Tasklet B</ENTRY>
+<ENTRY>Timer A</ENTRY>
+<ENTRY>Timer B</ENTRY>
+<ENTRY>User Context A</ENTRY>
+<ENTRY>User Context B</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>IRQ Handler A</ENTRY>
+<ENTRY>None</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>IRQ Handler B</ENTRY>
+<ENTRY>spin_lock_irqsave</ENTRY>
+<ENTRY>None</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>Softirq A</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>Softirq B</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>Tasklet A</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>None</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>Tasklet B</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>None</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>Timer A</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>None</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>Timer B</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>spin_lock</ENTRY>
+<ENTRY>None</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>User Context A</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>None</ENTRY>
+</ROW>
+
+<ROW>
+<ENTRY>User Context B</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_irq</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>spin_lock_bh</ENTRY>
+<ENTRY>down_interruptible</ENTRY>
+<ENTRY>None</ENTRY>
+</ROW>
+
+</TBODY>
+</TGROUP>
+</TABLE>
+</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>
- Strangely enough, this is the exact reverse of my advice when
- you <emphasis>have</emphasis> slept with someone crazier than yourself.
- </para>
+ </para>
- <sect1 id="techniques-no-writers">
- <title>No Writers in Interrupt Context</title>
+ <sect1 id="examples-usercontext">
+ <title>All In User Context</title>
+ <para>
+For our first example, we assume that all operations are in user
+context (ie. from system calls), so we can sleep. This means we can
+use a semaphore to protect the cache and all the objects within
+it. Here's the code:
+ </para>
+ <programlisting>
+#include <linux/list.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <asm/semaphore.h>
+#include <asm/errno.h>
+
+struct object
+{
+ struct list_head list;
+ int id;
+ char name[32];
+ int popularity;
+};
+
+/* Protects the cache, cache_num, and the objects within it */
+static DECLARE_MUTEX(cache_lock);
+static LIST_HEAD(cache);
+static unsigned int cache_num = 0;
+#define MAX_CACHE_SIZE 10
+
+/* Must be holding cache_lock */
+static struct object *__cache_find(int id)
+{
+ struct object *i;
+
+ list_for_each_entry(i, &cache, list)
+ if (i->id == id) {
+ i->popularity++;
+ return i;
+ }
+ return NULL;
+}
+
+/* Must be holding cache_lock */
+static void __cache_delete(struct object *obj)
+{
+ BUG_ON(!obj);
+ list_del(&obj->list);
+ kfree(obj);
+ cache_num--;
+}
+
+/* Must be holding cache_lock */
+static void __cache_add(struct object *obj)
+{
+ list_add(&obj->list, &cache);
+ if (++cache_num > MAX_CACHE_SIZE) {
+ struct object *i, *outcast = NULL;
+ list_for_each_entry(i, &cache, list) {
+ if (!outcast || i->popularity < outcast->popularity)
+ outcast = i;
+ }
+ __cache_delete(outcast);
+ }
+}
+
+int cache_add(int id, const char *name)
+{
+ struct object *obj;
+
+ if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL)
+ return -ENOMEM;
+
+ strlcpy(obj->name, name, sizeof(obj->name));
+ obj->id = id;
+ obj->popularity = 0;
+
+ down(&cache_lock);
+ __cache_add(obj);
+ up(&cache_lock);
+ return 0;
+}
+
+void cache_delete(int id)
+{
+ down(&cache_lock);
+ __cache_delete(__cache_find(id));
+ up(&cache_lock);
+}
+
+int cache_find(int id, char *name)
+{
+ struct object *obj;
+ int ret = -ENOENT;
+
+ down(&cache_lock);
+ obj = __cache_find(id);
+ if (obj) {
+ ret = 0;
+ strcpy(name, obj->name);
+ }
+ up(&cache_lock);
+ return ret;
+}
+</programlisting>
+
+ <para>
+Note that we always make sure we have the cache_lock when we add,
+delete, or look up the cache: both the cache infrastructure itself and
+the contents of the objects are protected by the lock. In this case
+it's easy, since we copy the data for the user, and never let them
+access the objects directly.
+ </para>
<para>
- There is a fairly common case where an interrupt handler needs
- access to a critical region, but does not need write access.
- In this case, you do not need to use
- <function>read_lock_irq()</function>, but only
- <function>read_lock()</function> everywhere (since if an interrupt
- occurs, the irq handler will only try to grab a read lock, which
- won't deadlock). You will still need to use
- <function>write_lock_irq()</function>.
+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>
- Similar logic applies to locking between softirqs/tasklets/BHs
- which never need a write lock, and user context:
- <function>read_lock()</function> and
- <function>write_lock_bh()</function>.
+Now consider the case where <function>cache_find</function> can be
+called from interrupt context: either a hardware interrupt or a
+softirq. An example would be a timer which deletes object from the
+cache.
+ </para>
+ <para>
+The change is shown below, in standard patch format: the
+<symbol>-</symbol> are lines which are taken away, and the
+<symbol>+</symbol> are lines which are added.
+ </para>
+<programlisting>
+--- cache.c.usercontext 2003-12-09 13:58:54.000000000 +1100
++++ cache.c.interrupt 2003-12-09 14:07:49.000000000 +1100
+@@ -12,7 +12,7 @@
+ int popularity;
+ };
+
+-static DECLARE_MUTEX(cache_lock);
++static spinlock_t cache_lock = SPIN_LOCK_UNLOCKED;
+ static LIST_HEAD(cache);
+ static unsigned int cache_num = 0;
+ #define MAX_CACHE_SIZE 10
+@@ -55,6 +55,7 @@
+ int cache_add(int id, const char *name)
+ {
+ struct object *obj;
++ unsigned long flags;
+
+ if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL)
+ return -ENOMEM;
+@@ -63,30 +64,33 @@
+ obj->id = id;
+ obj->popularity = 0;
+
+- down(&cache_lock);
++ spin_lock_irqsave(&cache_lock, flags);
+ __cache_add(obj);
+- up(&cache_lock);
++ spin_unlock_irqrestore(&cache_lock, flags);
+ return 0;
+ }
+
+ void cache_delete(int id)
+ {
+- down(&cache_lock);
++ unsigned long flags;
++
++ spin_lock_irqsave(&cache_lock, flags);
+ __cache_delete(__cache_find(id));
+- up(&cache_lock);
++ spin_unlock_irqrestore(&cache_lock, flags);
+ }
+
+ int cache_find(int id, char *name)
+ {
+ struct object *obj;
+ int ret = -ENOENT;
++ unsigned long flags;
+
+- down(&cache_lock);
++ spin_lock_irqsave(&cache_lock, flags);
+ obj = __cache_find(id);
+ if (obj) {
+ ret = 0;
+ strcpy(name, obj->name);
+ }
+- up(&cache_lock);
++ spin_unlock_irqrestore(&cache_lock, flags);
+ return ret;
+ }
+</programlisting>
+
+ <para>
+Note that the <function>spin_lock_irqsave</function> will turn off
+interrupts if they are on, otherwise does nothing (if we are already
+in an interrupt handler), hence these functions are safe to call from
+any context.
+ </para>
+ <para>
+Unfortunately, <function>cache_add</function> calls
+<function>kmalloc</function> with the <symbol>GFP_KERNEL</symbol>
+flag, which is only legal in user context. I have assumed that
+<function>cache_add</function> is still only called in user context,
+otherwise this should become a parameter to
+<function>cache_add</function>.
+ </para>
+ </sect1>
+ <sect1 id="examples-refcnt">
+ <title>Exposing Objects Outside This File</title>
+ <para>
+If our objects contained more information, it might not be sufficient
+to copy the information in and out: other parts of the code might want
+to keep pointers to these objects, for example, rather than looking up
+the id every time. This produces two problems.
+ </para>
+ <para>
+The first problem is that we use the <symbol>cache_lock</symbol> to
+protect objects: we'd need to make this non-static so the rest of the
+code can use it. This makes locking trickier, as it is no longer all
+in one place.
+ </para>
+ <para>
+The second problem is the lifetime problem: if another structure keeps
+a pointer to an object, it presumably expects that pointer to remain
+valid. Unfortunately, this is only guaranteed while you hold the
+lock, otherwise someone might call <function>cache_delete</function>
+and even worse, add another object, re-using the same address.
+ </para>
+ <para>
+As there is only one lock, you can't hold it forever: no-one else would
+get any work done.
+ </para>
+ <para>
+The solution to this problem is to use a reference count: everyone who
+has a pointer to the object increases it when they first get the
+object, and drops the reference count when they're finished with it.
+Whoever drops it to zero knows it is unused, and can actually delete it.
+ </para>
+ <para>
+Here is the code:
</para>
- </sect1>
- <sect1 id="techniques-deadlocks">
+<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>
+</sect1>
+
+ <sect1 id="examples-lock-per-obj">
+ <title>Protecting The Objects Themselves</title>
+ <para>
+In these examples, we assumed that the objects (except the reference
+counts) never changed once they are created. If we wanted to allow
+the name to change, there are three possibilities:
+ </para>
+ <itemizedlist>
+ <listitem>
+ <para>
+You can make <symbol>cache_lock</symbol> non-static, and tell people
+to grab that lock before changing the name in any object.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+You can provide a <function>cache_obj_rename</function> which grabs
+this lock and changes the name for the caller, and tell everyone to
+use that function.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+You can make the <symbol>cache_lock</symbol> protect only the cache
+itself, and use another lock to protect the name.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+Theoretically, you can make the locks as fine-grained as one lock for
+every field, for every object. In practice, the most common variants
+are:
+</para>
+ <itemizedlist>
+ <listitem>
+ <para>
+One lock which protects the infrastructure (the <symbol>cache</symbol>
+list in this example) and all the objects. This is what we have done
+so far.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+One lock which protects the infrastructure (including the list
+pointers inside the objects), and one lock inside the object which
+protects the rest of that object.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+Multiple locks to protect the infrastructure (eg. one lock per hash
+chain), possibly with a separate per-object lock.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+<para>
+Here is the "lock-per-object" implementation:
+</para>
+<programlisting>
+--- cache.c.refcnt-atomic 2003-12-11 15:50:54.000000000 +1100
++++ cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100
+@@ -6,11 +6,17 @@
+
+ struct object
+ {
++ /* These two protected by cache_lock. */
+ struct list_head list;
++ int popularity;
++
+ atomic_t refcnt;
++
++ /* Doesn't change once created. */
+ int id;
++
++ spinlock_t lock; /* Protects the name */
+ char name[32];
+- int popularity;
+ };
+
+ static spinlock_t cache_lock = SPIN_LOCK_UNLOCKED;
+@@ -77,6 +84,7 @@
+ obj->id = id;
+ obj->popularity = 0;
+ atomic_set(&obj->refcnt, 1); /* The cache holds a reference */
++ spin_lock_init(&obj->lock);
+
+ spin_lock_irqsave(&cache_lock, flags);
+ __cache_add(obj);
+</programlisting>
+
+<para>
+Note that I decide that the <structfield>popularity</structfield>
+count should be protected by the <symbol>cache_lock</symbol> rather
+than the per-object lock: this is because it (like the
+<structname>struct list_head</structname> inside the object) is
+logically part of the infrastructure. This way, I don't need to grab
+the lock of every object in <function>__cache_add</function> when
+seeking the least popular.
+</para>
+
+<para>
+I also decided that the <structfield>id</structfield> member is
+unchangeable, so I don't need to grab each object lock in
+<function>__cache_find()</function> to examine the
+<structfield>id</structfield>: the object lock is only used by a
+caller who wants to read or write the <structfield>name</structfield>
+field.
+</para>
+
+<para>
+Note also that I added a comment describing what data was protected by
+which locks. This is extremely important, as it describes the runtime
+behavior of the code, and can be hard to gain from just reading. And
+as Alan Cox says, <quote>Lock data, not code</quote>.
+</para>
+</chapter>
+
+ <chapter id="common-problems">
+ <title>Common Problems</title>
+ <sect1 id="deadlock">
<title>Deadlock: Simple and Advanced</title>
<para>
<para>
For a slightly more complex case, imagine you have a region
- shared by a bottom half and user context. If you use a
+ 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 bottom
- half while it holds the lock, and the bottom half will then spin
+ 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>
<para>
- A more complex problem is the so-called `deadly embrace',
+ 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
their lock. It will look, smell, and feel like a crash.
</para>
- <sect2 id="techs-deadlock-prevent">
+ <sect1 id="techs-deadlock-prevent">
<title>Preventing Deadlock</title>
<para>
will do?). Remember, the other programmers are out to get
you, so don't do this.
</para>
- </sect2>
<sect2 id="techs-deadlock-overprevent">
<title>Overzealous Prevention Of Deadlocks</title>
If you don't see why, please stay the fuck away from my code.
</para>
</sect2>
- </sect1>
+ </sect1>
+
+ <sect1 id="racing-timers">
+ <title>Racing Timers: A Kernel Pastime</title>
- <sect1 id="per-cpu">
- <title>Per-CPU Data</title>
-
<para>
- A great 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.
+ 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 that was too slow [it's probably not], you could instead
- use a counter for each CPU [don't], then none of them need an
- exclusive lock [you're wasting your time here]. To make sure
- the CPUs don't have to synchronize caches all the time, align
- the counters to cache boundaries by appending
- `__cacheline_aligned' to the declaration
- (<filename class=headerfile>include/linux/cache.h</filename>).
- [Can't you think of anything better to do?]
+ 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>
- They will need a read lock to access their own counters,
- however. That way you can use a write lock to grant exclusive
- access to all of them at once, to tally them up.
+ 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>
- <sect1 id="brlock">
- <title>Big Reader Locks</title>
+ <sect1 id="sparc">
+ <title>The Fucked Up Sparc</title>
<para>
- A classic example of per-CPU information is Ingo's `big
- reader' locks
- (<filename class=headerfile>linux/include/brlock.h</filename>). These
- use the Per-CPU Data techniques described above to create a lock which
- is very fast to get a read lock, but agonizingly slow for a write
- lock.
+ Alan Cox says <quote>the irq disable/enable is in the register
+ window on a sparc</quote>. Andi Kleen says <quote>when you do
+ restore_flags in a different function you mess up all the
+ register windows</quote>.
</para>
<para>
- Fortunately, there are a limited number of these locks
- available: you have to go through a strict interview process
- to get one.
+ So never pass the flags word set by
+ <function>spin_lock_irqsave()</function> and brethren to another
+ function (unless it's declared <type>inline</type>). Usually no-one
+ does this, but now you've been warned. Dave Miller can never do
+ anything in a straightforward manner (I can say that, because I have
+ pictures of him and a certain PowerPC maintainer in a compromising
+ position).
</para>
</sect1>
- <sect1 id="lock-avoidance-rw">
- <title>Avoiding Locks: Read And Write Ordering</title>
+ </chapter>
+
+ <chapter id="Efficiency">
+ <title>Locking Speed</title>
<para>
- Sometimes it is possible to avoid locking. Consider the
- following case from the 2.2 firewall code, which inserted an
- element into a single linked list in user context:
+There are three main things to worry about when considering speed of
+some code which does locking. First is concurrency: how many things
+are going to be waiting while someone else is holding a lock. Second
+is the time taken to actually acquire and release an uncontended lock.
+Third is using fewer, or smarter locks. I'm assuming that the lock is
+used fairly often: otherwise, you wouldn't be concerned about
+efficiency.
+</para>
+ <para>
+Concurrency depends on how long the lock is usually held: you should
+hold the lock for as long as needed, but no longer. In the cache
+example, we always create the object without the lock held, and then
+grab the lock only when we are ready to insert it in the list.
+</para>
+ <para>
+Acquisition times depend on how much damage the lock operations do to
+the pipeline (pipeline stalls) and how likely it is that this CPU was
+the last one to grab the lock (ie. is the lock cache-hot for this
+CPU): on a machine with more CPUs, this likelihood drops fast.
+Consider a 700MHz Intel Pentium III: an instruction takes about 0.7ns,
+an atomic increment takes about 58ns, a lock which is cache-hot on
+this CPU takes 160ns, and a cacheline transfer from another CPU takes
+an additional 170 to 360ns. (These figures from Paul McKenney's
+<ulink url="http://www.linuxjournal.com/article.php?sid=6993"> Linux
+Journal RCU article</ulink>).
+</para>
+ <para>
+These two aims conflict: holding a lock for a short time might be done
+by splitting locks into parts (such as in our final per-object-lock
+example), but this increases the number of lock acquisitions, and the
+results are often slower than having a single lock. This is another
+reason to advocate locking simplicity.
+</para>
+ <para>
+The third concern is addressed below: there are some methods to reduce
+the amount of locking which needs to be done.
+</para>
+
+ <sect1 id="efficiency-rwlocks">
+ <title>Read/Write Lock Variants</title>
+
+ <para>
+ Both spinlocks and semaphores have read/write variants:
+ <type>rwlock_t</type> and <structname>struct rw_semaphore</structname>.
+ These divide users into two classes: the readers and the writers. If
+ you are only reading the data, you can get a read lock, but to write to
+ the data you need the write lock. Many people can hold a read lock,
+ but a writer must be sole holder.
</para>
- <programlisting>
- new->next = i->next;
- i->next = new;
- </programlisting>
+ <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>
- Here the author (Alan Cox, who knows what he's doing) assumes
- that the pointer assignments are atomic. This is important,
- because networking packets would traverse this list on bottom
- halves without a lock. Depending on their exact timing, they
- would either see the new element in the list with a valid
- <structfield>next</structfield> pointer, or it would not be in the
- list yet. A lock is still required against other CPUs inserting
- or deleting from the list, of course.
+ 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>
- Of course, the writes <emphasis>must</emphasis> be in this
- order, otherwise the new element appears in the list with an
- invalid <structfield>next</structfield> pointer, and any other
- CPU iterating at the wrong time will jump through it into garbage.
- Because modern CPUs reorder, Alan's code actually read as follows:
+ 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 = i->next;
+ new->next = list->next;
wmb();
- i->next = new;
+ list->next = new;
</programlisting>
<para>
- The <function>wmb()</function> is a write memory barrier
- (<filename class=headerfile>include/asm/system.h</filename>): neither
- the compiler nor the CPU will allow any writes to memory after the
- <function>wmb()</function> to be visible to other hardware
- before any of the writes before the <function>wmb()</function>.
+ 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>
- As i386 does not do write reordering, this bug would never
- show up on that platform. On other SMP platforms, however, it
- will.
+ 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>
- There is also <function>rmb()</function> for read ordering: to ensure
- any previous variable reads occur before following reads. The simple
- <function>mb()</function> macro combines both
- <function>rmb()</function> and <function>wmb()</function>.
+ 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>
- Some atomic operations are defined to act as a memory barrier
- (ie. as per the <function>mb()</function> macro, but if in
- doubt, be explicit.
- <!-- Rusty Russell 2 May 2001, 2.4.4 -->
- Also,
- spinlock operations act as partial barriers: operations after
- gaining a spinlock will never be moved to precede the
- <function>spin_lock()</function> call, and operations before
- releasing a spinlock will never be moved after the
- <function>spin_unlock()</function> call.
- <!-- Manfred Spraul <manfreds@colorfullife.com>
- 24 May 2000 2.3.99-pre9 -->
+ 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>
- </sect1>
-
- <sect1 id="lock-avoidance-atomic-ops">
- <title>Avoiding Locks: Atomic Operations</title>
-
<para>
- 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, thus
- avoiding races. If your shared data consists of a single counter, say,
- these operations might be simpler than using spinlocks (although for
- anything non-trivial using spinlocks is clearer).
+ 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>
- Note that the atomic operations do in general not act as memory
- barriers. Instead you can insert a memory barrier before or
- after <function>atomic_inc()</function> or
- <function>atomic_dec()</function> by inserting
- <function>smp_mb__before_atomic_inc()</function>,
- <function>smp_mb__after_atomic_inc()</function>,
- <function>smp_mb__before_atomic_dec()</function> or
- <function>smp_mb__after_atomic_dec()</function>
- respectively. The advantage of using those macros instead of
- <function>smp_mb()</function> is, that they are cheaper on some
- platforms.
- <!-- Sebastian Wilhelmi <seppi@seppi.de> 2002-03-04 -->
+ Our final dilemma is this: when can we actually destroy the
+ removed element? Remember, a reader might be stepping through
+ this element in the list right now: it we free this element and
+ the <symbol>next</symbol> pointer changes, the reader will jump
+ off into garbage and crash. We need to wait until we know that
+ all the readers who were traversing the list when we deleted the
+ element are finished. We use <function>call_rcu()</function> to
+ register a callback which will actually destroy the object once
+ the readers are finished.
+ </para>
+ <para>
+ But how does Read Copy Update know when the readers are
+ finished? The method is this: firstly, the readers always
+ traverse the list inside
+ <function>rcu_read_lock()</function>/<function>rcu_read_unlock()</function>
+ pairs: these simply disable preemption so the reader won't go to
+ sleep while reading the list.
+ </para>
+ <para>
+ RCU then waits until every other CPU has slept at least once:
+ since readers cannot sleep, we know that any readers which were
+ traversing the list during the deletion are finished, and the
+ callback is triggered. The real Read Copy Update code is a
+ little more optimized than this, but this is the fundamental
+ idea.
</para>
- </sect1>
- <sect1 id="ref-counts">
- <title>Protecting A Collection of Objects: Reference Counts</title>
+<programlisting>
+--- cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100
++++ cache.c.rcupdate 2003-12-11 17:55:14.000000000 +1100
+@@ -1,15 +1,18 @@
+ #include <linux/list.h>
+ #include <linux/slab.h>
+ #include <linux/string.h>
++#include <linux/rcupdate.h>
+ #include <asm/semaphore.h>
+ #include <asm/errno.h>
+
+ struct object
+ {
+- /* These two protected by cache_lock. */
++ /* This is protected by RCU */
+ struct list_head list;
+ int popularity;
+
++ struct rcu_head rcu;
++
+ atomic_t refcnt;
+
+ /* Doesn't change once created. */
+@@ -40,7 +43,7 @@
+ {
+ struct object *i;
+
+- list_for_each_entry(i, &cache, list) {
++ list_for_each_entry_rcu(i, &cache, list) {
+ if (i->id == id) {
+ i->popularity++;
+ return i;
+@@ -49,19 +52,25 @@
+ return NULL;
+ }
+
++/* Final discard done once we know no readers are looking. */
++static void cache_delete_rcu(void *arg)
++{
++ object_put(arg);
++}
++
+ /* Must be holding cache_lock */
+ static void __cache_delete(struct object *obj)
+ {
+ BUG_ON(!obj);
+- list_del(&obj->list);
+- object_put(obj);
++ list_del_rcu(&obj->list);
+ cache_num--;
++ call_rcu(&obj->rcu, cache_delete_rcu, obj);
+ }
+
+ /* Must be holding cache_lock */
+ static void __cache_add(struct object *obj)
+ {
+- list_add(&obj->list, &cache);
++ list_add_rcu(&obj->list, &cache);
+ if (++cache_num > MAX_CACHE_SIZE) {
+ struct object *i, *outcast = NULL;
+ list_for_each_entry(i, &cache, list) {
+@@ -85,6 +94,7 @@
+ obj->popularity = 0;
+ atomic_set(&obj->refcnt, 1); /* The cache holds a reference */
+ spin_lock_init(&obj->lock);
++ INIT_RCU_HEAD(&obj->rcu);
+
+ spin_lock_irqsave(&cache_lock, flags);
+ __cache_add(obj);
+@@ -104,12 +114,11 @@
+ struct object *cache_find(int id)
+ {
+ struct object *obj;
+- unsigned long flags;
+
+- spin_lock_irqsave(&cache_lock, flags);
++ rcu_read_lock();
+ obj = __cache_find(id);
+ if (obj)
+ object_get(obj);
+- spin_unlock_irqrestore(&cache_lock, flags);
++ rcu_read_unlock();
+ return obj;
+ }
+</programlisting>
+
+<para>
+Note that the reader will alter the
+<structfield>popularity</structfield> member in
+<function>__cache_find()</function>, and now it doesn't hold a lock.
+One solution would be to make it an <type>atomic_t</type>, but for
+this usage, we don't really care about races: an approximate result is
+good enough, so I didn't change it.
+</para>
+
+<para>
+The result is that <function>cache_find()</function> requires no
+synchronization with any other functions, so is almost as fast on SMP
+as it would be on UP.
+</para>
+
+<para>
+There is a furthur optimization possible here: remember our original
+cache code, where there were no reference counts and the caller simply
+held the lock whenever using the object? This is still possible: if
+you hold the lock, noone can delete the object, so you don't need to
+get and put the reference count.
+</para>
+
+<para>
+Now, because the 'read lock' in RCU is simply disabling preemption, a
+caller which always has preemption disabled between calling
+<function>cache_find()</function> and
+<function>object_put()</function> does not need to actually get and
+put the reference count: we could expose
+<function>__cache_find()</function> by making it non-static, and
+such callers could simply call that.
+</para>
+<para>
+The benefit here is that the reference count is not written to: the
+object is not altered in any way, which is much faster on SMP
+machines due to caching.
+</para>
+ </sect1>
+
+ <sect1 id="per-cpu">
+ <title>Per-CPU Data</title>
<para>
- Locking a collection of objects is fairly easy: you get a
- single spinlock, and you make sure you grab it before
- searching, adding or deleting an object.
+ 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>
- The purpose of this lock is not to protect the individual
- objects: you might have a separate lock inside each one for
- that. It is to protect the <emphasis>data structure
- containing the objects</emphasis> from race conditions. Often
- the same lock is used to protect the contents of all the
- objects as well, for simplicity, but they are inherently
- orthogonal (and many other big words designed to confuse).
+ 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>
- Changing this to a read-write lock will often help markedly if
- reads are far more common that writes. If not, there is
- another approach you can use to reduce the time the lock is
- held: reference counts.
+ 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>
- In this approach, an object has an owner, who sets the
- reference count to one. Whenever you get a pointer to the
- object, you increment the reference count (a `get' operation).
- Whenever you relinquish a pointer, you decrement the reference
- count (a `put' operation). When the owner wants to destroy
- it, they mark it dead, and do a put.
+ 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>
- Whoever drops the reference count to zero (usually implemented
- with <function>atomic_dec_and_test()</function>) actually cleans
- up and frees the object.
+ 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>
- This means that you are guaranteed that the object won't
- vanish underneath you, even though you no longer have a lock
- for the collection.
+ 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>
- Here's some skeleton code:
+ 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>
- <programlisting>
- void create_foo(struct foo *x)
- {
- atomic_set(&x->use, 1);
- spin_lock_bh(&list_lock);
- ... insert in list ...
- spin_unlock_bh(&list_lock);
- }
-
- struct foo *get_foo(int desc)
- {
- struct foo *ret;
-
- spin_lock_bh(&list_lock);
- ... find in list ...
- if (ret) atomic_inc(&ret->use);
- spin_unlock_bh(&list_lock);
-
- return ret;
- }
-
- void put_foo(struct foo *x)
- {
- if (atomic_dec_and_test(&x->use))
- kfree(foo);
- }
+ <chapter id="sleeping-things">
+ <title>What Functions Are Safe To Call From Interrupts?</title>
- void destroy_foo(struct foo *x)
- {
- spin_lock_bh(&list_lock);
- ... remove from list ...
- spin_unlock_bh(&list_lock);
-
- put_foo(x);
- }
- </programlisting>
+ <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>
- <sect2 id="helpful-macros">
- <title>Macros To Help You</title>
- <para>
- There are a set of debugging macros tucked inside
- <filename class=headerfile>include/linux/netfilter_ipv4/lockhelp.h</filename>
- and <filename class=headerfile>listhelp.h</filename>: these are very
- useful for ensuring that locks are held in the right places to protect
- infrastructure.
- </para>
- </sect2>
- </sect1>
-
- <sect1 id="sleeping-things">
- <title>Things Which Sleep</title>
+ <sect1 id="sleeping">
+ <title>Some Functions Which Sleep</title>
<para>
- You can never call the following routines while holding a
- spinlock, as they may sleep. This also means you need to be in
- user context.
+ 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>
</itemizedlist>
- <para>
- <function>printk()</function> can be called in
- <emphasis>any</emphasis> context, interestingly enough.
- </para>
- </sect1>
-
- <sect1 id="sparc">
- <title>The Fucked Up Sparc</title>
+ <sect1 id="dont-sleep">
+ <title>Some Functions Which Don't Sleep</title>
<para>
- Alan Cox says <quote>the irq disable/enable is in the register
- window on a sparc</quote>. Andi Kleen says <quote>when you do
- restore_flags in a different function you mess up all the
- register windows</quote>.
+ Some functions are safe to call from any context, or holding
+ almost any lock.
</para>
- <para>
- So never pass the flags word set by
- <function>spin_lock_irqsave()</function> and brethren to another
- function (unless it's declared <type>inline</type>. Usually no-one
- does this, but now you've been warned. Dave Miller can never do
- anything in a straightforward manner (I can say that, because I have
- pictures of him and a certain PowerPC maintainer in a compromising
- position).
- </para>
- </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>
+ <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>
<para>
Thanks to Martin Pool, Philipp Rumpf, Stephen Rothwell, Paul
- Mackerras, Ruedi Aschwanden, Alan Cox, Manfred Spraul and Tim
- Waugh for proofreading, correcting, flaming, commenting.
+ 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>
<glossary id="glossary">
<title>Glossary</title>
+ <glossentry id="gloss-preemption">
+ <glossterm>preemption</glossterm>
+ <glossdef>
+ <para>
+ Prior to 2.5, or when <symbol>CONFIG_PREEMPT</symbol> is
+ unset, processes in user context inside the kernel would not
+ preempt each other (ie. you had that CPU until you have it up,
+ except for interrupts). With the addition of
+ <symbol>CONFIG_PREEMPT</symbol> in 2.5.4, this changed: when
+ in user context, higher priority tasks can "cut in": spinlocks
+ were changed to disable preemption, even on UP.
+ </para>
+ </glossdef>
+ </glossentry>
+
<glossentry id="gloss-bh">
<glossterm>bh</glossterm>
<glossdef>
<para>
Bottom Half: for historical reasons, functions with
- `_bh' in them often now refer to any software interrupt, e.g.
+ '_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
<glossdef>
<para>
Hardware interrupt request. <function>in_irq()</function> returns
- <returnvalue>true</returnvalue> in a hardware interrupt handler (it
- also returns true when interrupts are blocked).
+ <returnvalue>true</returnvalue> in a hardware interrupt handler.
</para>
</glossdef>
</glossentry>
<para>
Not user context: processing a hardware irq or software irq.
Indicated by the <function>in_interrupt()</function> macro
- returning <returnvalue>true</returnvalue> (although it also
- returns true when interrupts or BHs are blocked).
+ returning <returnvalue>true</returnvalue>.
</para>
</glossdef>
</glossentry>
</glossentry>
<glossentry id="gloss-softirq">
- <glossterm>softirq</glossterm>
+ <glossterm>Software Interrupt / softirq</glossterm>
<glossdef>
<para>
- Strictly speaking, one of up to 32 enumerated software
+ 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 and bottom halves as
+ Sometimes used to refer to tasklets as
well (ie. all software interrupts).
</para>
</glossdef>
</glossentry>
- <glossentry id="gloss-swinterrupt">
- <glossterm>Software Interrupt / Software IRQ</glossterm>
+ <glossentry id="gloss-tasklet">
+ <glossterm>tasklet</glossterm>
<glossdef>
<para>
- Software interrupt handler. <function>in_irq()</function> returns
- <returnvalue>false</returnvalue>; <function>in_softirq()</function>
- returns <returnvalue>true</returnvalue>. Tasklets, softirqs and
- bottom halves all fall into the category of `software interrupts'.
+ A dynamically-registrable software interrupt,
+ which is guaranteed to only run on one CPU at a time.
</para>
</glossdef>
</glossentry>
- <glossentry id="gloss-tasklet">
- <glossterm>tasklet</glossterm>
+ <glossentry id="gloss-timers">
+ <glossterm>timer</glossterm>
<glossdef>
<para>
- A dynamically-registrable software interrupt,
- which is guaranteed to only run on one CPU at a time.
+ 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>
<glossterm>User Context</glossterm>
<glossdef>
<para>
- The kernel executing on behalf of a particular
- process or kernel thread (given by the <function>current()</function>
- macro.) Not to be confused with userspace. Can be interrupted by
- software or hardware interrupts.
+ 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>
projects / linux-flexiantxendom0-3.2.10.git / blobdiff