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<book id="LKLockingGuide">
 <bookinfo>
  <title>Unreliable Guide To Locking</title>
  
  <authorgroup>
   <author>
    <firstname>Paul</firstname>
    <othername>Rusty</othername>
    <surname>Russell</surname>
    <affiliation>
     <address>
      <email>rusty@rustcorp.com.au</email>
     </address>
    </affiliation>
   </author>
  </authorgroup>

  <copyright>
   <year>2000</year>
   <holder>Paul 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 as we approach 2.4.
   </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.
   </para>

   <sect1 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>

    <para>
      This overlap, where what actually happens 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.
    </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>Two Main Types of Kernel Locks: Spinlocks and Semaphores</title>

   <para>
     There are two main types of kernel locks.  The fundamental type
     is the spinlock 
     (<filename class=headerfile>include/asm/spinlock.h</filename>), 
     which is a very simple single-holder lock: if you can't get the 
     spinlock, you keep trying (spinning) until you can.  Spinlocks are 
     very small and fast, and can be used anywhere.
   </para>
   <para>
     The second type is a semaphore
     (<filename class=headerfile>include/asm/semaphore.h</filename>): it
     can have more than one holder at any time (the number decided at
     initialization time), although it is most commonly used as a
     single-holder lock (a mutex).  If you can't get a semaphore,
     your task will put itself on the queue, and be woken up when the
     semaphore is released.  This means the CPU will do something
     else while you are waiting, but there are many cases when you
     simply can't sleep (see <xref linkend="sleeping-things">), and so
     have to use a spinlock instead.
   </para>
   <para>
     Neither type of lock is recursive: see
     <xref linkend="techniques-deadlocks">.
   </para>
 
   <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.
    </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.
    </para>

    <para>
      Semaphores still exist, because they are required for
      synchronization between <firstterm linkend="gloss-usercontext">user 
      contexts</firstterm>, as we will see below.
    </para>
   </sect1>

   <sect1 id="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>

     <para>
       If you have a data structure which is only ever accessed from
       user context, then you can use a simple semaphore
       (<filename>linux/asm/semaphore.h</filename>) to protect it.  This 
       is the most trivial case: you initialize the semaphore to the number 
       of resources available (usually 1), and call
       <function>down_interruptible()</function> to grab the semaphore, and 
       <function>up()</function> to release it.  There is also a 
       <function>down()</function>, which should be avoided, because it 
       will not return if a signal is received.
     </para>

     <para>
       Example: <filename>linux/net/core/netfilter.c</filename> allows 
       registration of new <function>setsockopt()</function> and 
       <function>getsockopt()</function> calls, with
       <function>nf_register_sockopt()</function>.  Registration and 
       de-registration are only done on module load and unload (and boot 
       time, where there is no concurrency), and the list of registrations 
       is only consulted for an unknown <function>setsockopt()</function>
       or <function>getsockopt()</function> system call.  The 
       <varname>nf_sockopt_mutex</varname> is perfect to protect this,
       especially since the setsockopt and getsockopt calls may well
       sleep.
     </para>
   </sect1>

   <sect1 id="lock-user-bh">
    <title>Locking Between User Context and BHs</title>

    <para>
      If a <firstterm linkend="gloss-bh">bottom half</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 
      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.
    </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/asm/softirq.h</filename>), which 
      protects you from the bottom half being run.
    </para>
   </sect1>

   <sect1 id="lock-user-tasklet">
    <title>Locking Between User Context and Tasklets/Soft IRQs</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.
    </para>
   </sect1>

   <sect1 id="lock-bh">
    <title>Locking Between Bottom Halves</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).
    </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>

    <para>
      Sometimes a tasklet might want to share data with another
      tasklet, or a bottom half.
    </para>

    <sect2 id="lock-tasklets-same">
     <title>The Same Tasklet</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</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
       <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 <firstterm linkend="gloss-softirq">softirq</firstterm> might 
      want to share data with itself, a tasklet, or a bottom half.
    </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 (which can be running on a different CPU), bottom half, 
       tasklet or the same or another softirq.
     </para>
    </sect2>
   </sect1>
  </chapter>

  <chapter id="hardirq-context">
   <title>Hard IRQ Context</title>

   <para>
     Hardware interrupts usually communicate with a bottom half,
     tasklet or softirq.  Frequently this involves putting work in a
     queue, which the BH/softirq will take out.
   </para>

   <sect1 id="hardirq-softirq">
    <title>Locking Between Hard IRQ and Softirqs/Tasklets/BHs</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>
      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_lock_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>
   </sect1>
  </chapter>

  <chapter id="common-techniques">
   <title>Common Techniques</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.
   </para>

   <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>
     Lock data, not code.
   </para>

   <para>
     Be reluctant to introduce new locks.
   </para>

   <para>
     Strangely enough, this is the exact reverse of my advice when
     you <emphasis>have</emphasis> slept with someone crazier than yourself.
   </para>

   <sect1 id="techniques-no-writers">
    <title>No Writers in Interrupt Context</title>

    <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>.
    </para>

    <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>.
    </para>
   </sect1>

   <sect1 id="techniques-deadlocks">
    <title>Deadlock: Simple and Advanced</title>

    <para>
      There is a coding bug where a piece of code tries to grab a
      spinlock twice: it will spin forever, waiting for the lock to
      be released (spinlocks, rwlocks and semaphores are not
      recursive in Linux).  This is trivial to diagnose: not a
      stay-up-five-nights-talk-to-fluffy-code-bunnies kind of
      problem.
    </para>

    <para>
      For a slightly more complex case, imagine you have a region
      shared by a bottom half 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 
      forever trying to get the same lock.
    </para>

    <para>
      Both of these are called deadlock, and as shown above, it can
      occur even with a single CPU (although not on UP compiles,
      since spinlocks vanish on kernel compiles with 
      <symbol>CONFIG_SMP</symbol>=n. You'll still get data corruption 
      in the second example).
    </para>

    <para>
      This complete lockup is easy to diagnose: on SMP boxes the
      watchdog timer or compiling with <symbol>DEBUG_SPINLOCKS</symbol> set
      (<filename>include/linux/spinlock.h</filename>) will show this up 
      immediately when it happens.
    </para>

    <para>
      A more complex problem is the so-called `deadly embrace',
      involving two or more locks.  Say you have a hash table: each
      entry in the table is a spinlock, and a chain of hashed
      objects.  Inside a softirq handler, you sometimes want to
      alter an object from one place in the hash to another: you
      grab the spinlock of the old hash chain and the spinlock of
      the new hash chain, and delete the object from the old one,
      and insert it in the new one.
    </para>

    <para>
      There are two problems here.  First, if your code ever
      tries to move the object to the same chain, it will deadlock
      with itself as it tries to lock it twice.  Secondly, if the
      same softirq on another CPU is trying to move another object
      in the reverse direction, the following could happen:
    </para>

    <table>
     <title>Consequences</title>

     <tgroup cols=2 align=left>

      <thead>
       <row>
        <entry>CPU 1</entry>
        <entry>CPU 2</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>Grab lock A -&gt; OK</entry>
        <entry>Grab lock B -&gt; OK</entry>
       </row>
       <row>
        <entry>Grab lock B -&gt; spin</entry>
        <entry>Grab lock A -&gt; spin</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <para>
      The two CPUs will spin forever, waiting for the other to give up
      their lock.  It will look, smell, and feel like a crash.
    </para>

    <sect2 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>

    <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="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.
    </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?]
    </para>

    <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.
    </para>
   </sect1>

   <sect1 id="brlock">
    <title>Big Reader Locks</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.
    </para>

    <para>
      Fortunately, there are a limited number of these locks
      available: you have to go through a strict interview process
      to get one.
    </para>
   </sect1>

   <sect1 id="lock-avoidance-rw">
    <title>Avoiding Locks: Read And Write Ordering</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:
    </para>

    <programlisting>
        new-&gt;next = i-&gt;next;
        i-&gt;next = new;
    </programlisting>

    <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.
    </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:
    </para>
      
    <programlisting>
        new-&gt;next = i-&gt;next;
        wmb();
        i-&gt;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>.
    </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.
    </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>.
    </para>

    <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 -->
    </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).
    </para>

    <para>
      Note that the atomic operations are defined to act as both
      read and write barriers on all platforms.
    </para>
   </sect1>

   <sect1 id="ref-counts">
    <title>Protecting A Collection of Objects: Reference Counts</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.
    </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).
    </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.
    </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.
    </para>

    <para>
      Whoever drops the reference count to zero (usually implemented
      with <function>atomic_dec_and_test()</function>) actually cleans 
      up and frees the object.
    </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.
    </para>

    <para>
      Here's some skeleton code:
    </para>

    <programlisting>
        void create_foo(struct foo *x)
        {
                atomic_set(&amp;x-&gt;use, 1);
                spin_lock_bh(&amp;list_lock);
                ... insert in list ...
                spin_unlock_bh(&amp;list_lock);
        }

        struct foo *get_foo(int desc)
        {
                struct foo *ret;

                spin_lock_bh(&amp;list_lock);
                ... find in list ...
                if (ret) atomic_inc(&amp;ret-&gt;use);
                spin_unlock_bh(&amp;list_lock);

                return ret;
        }

        void put_foo(struct foo *x)
        {
                if (atomic_dec_and_test(&amp;x-&gt;use))
                        kfree(foo);
        }

        void destroy_foo(struct foo *x)
        {
                spin_lock_bh(&amp;list_lock);
                ... remove from list ...
                spin_unlock_bh(&amp;list_lock);

                put_foo(x);
        }
    </programlisting>

    <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>

    <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.
    </para>

    <itemizedlist>
     <listitem>
      <para>
        Accesses to 
        <firstterm linkend="gloss-userspace">userspace</firstterm>:
      </para>
      <itemizedlist>
       <listitem>
        <para>
          <function>copy_from_user()</function>
        </para>
       </listitem>
       <listitem>
        <para>
          <function>copy_to_user()</function>
        </para>
       </listitem>
       <listitem>
        <para>
          <function>get_user()</function>
        </para>
       </listitem>
       <listitem>
        <para>
          <function> put_user()</function>
        </para>
       </listitem>
      </itemizedlist>
     </listitem>

     <listitem>
      <para>
        <function>kmalloc(GFP_KERNEL)</function>
      </para>
     </listitem>

     <listitem>
      <para>
      <function>down_interruptible()</function> and
      <function>down()</function>
      </para>
      <para>
       There is a <function>down_trylock()</function> which can be
       used inside interrupt context, as it will not sleep.
       <function>up()</function> will also never sleep.
      </para>
     </listitem>
    </itemizedlist>

    <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>

    <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>.
    </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(&amp;list_lock);

        while (list) {
                struct foo *next = list-&gt;next;
                del_timer(&amp;list-&gt;timer);
                kfree(list);
                list = next;
        }

        spin_unlock_bh(&amp;list_lock);
    </programlisting>

    <para>
      Sooner or later, this will crash on SMP, because a timer can
      have just gone off before the <function>spin_lock_bh()</function>, 
      and it will only get the lock after we 
      <function>spin_unlock_bh()</function>, and then try to free
      the element (which has already been freed!).
    </para>

    <para>
      This can be avoided by checking the result of 
      <function>del_timer()</function>: if it returns
      <returnvalue>1</returnvalue>, the timer has been deleted.  
      If <returnvalue>0</returnvalue>, it means (in this
      case) that it is currently running, so we can do:
    </para>

    <programlisting>
        retry:  
                spin_lock_bh(&amp;list_lock);

                while (list) {
                        struct foo *next = list-&gt;next;
                        if (!del_timer(&amp;list-&gt;timer)) {
                                /* Give timer a chance to delete this */
                                spin_unlock_bh(&amp;list_lock);
                                goto retry;
                        }
                        kfree(list);
                        list = next;
                }

                spin_unlock_bh(&amp;list_lock);
    </programlisting>

    <para>
      Another common problem is deleting timers which restart
      themselves (by calling <function>add_timer()</function> at the end 
      of their timer function).  Because this is a fairly common case 
      which is prone to races, you can put a call to
      <function>timer_exit()</function> at the very end of your timer function,
      and user <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="references">
   <title>Further reading</title>

   <itemizedlist>
    <listitem>
     <para>
       <filename>Documentation/spinlocks.txt</filename>: 
       Linus Torvalds' spinlocking tutorial in the kernel sources.
     </para>
    </listitem>

    <listitem>
     <para>
       Unix Systems for Modern Architectures: Symmetric
       Multiprocessing and Caching for Kernel Programmers:
     </para>

     <para>
       Curt Schimmel's very good introduction to kernel level
       locking (not written for Linux, but nearly everything
       applies).  The book is expensive, but really worth every
       penny to understand SMP locking. [ISBN: 0201633388]
     </para>
    </listitem>
   </itemizedlist>
  </chapter>

  <chapter id="thanks">
    <title>Thanks</title>

    <para>
      Thanks to Telsa Gwynne for DocBooking, neatening and adding
      style.
    </para>

    <para>
      Thanks to Martin Pool, Philipp Rumpf, Stephen Rothwell, Paul
      Mackerras, Ruedi Aschwanden, Alan Cox, Manfred Spraul and Tim
      Waugh 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-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 (it 
       also returns true when interrupts are blocked).
     </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> (although it also
       returns true when interrupts or BHs are blocked).
     </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>softirq</glossterm>
    <glossdef>
     <para>
       Strictly speaking, 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
       well (ie. all software interrupts).
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-swinterrupt">
    <glossterm>Software Interrupt / Software IRQ</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'.
     </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-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 or kernel thread (given by the <function>current()</function>
       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>