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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>Chapter 8. Common Problems</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Unreliable Guide To Locking" /><link rel="up" href="index.html" title="Unreliable Guide To Locking" /><link rel="prev" href="ch07s04.html" title="Protecting The Objects Themselves" /><link rel="next" href="ch08s02.html" title="Preventing Deadlock" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 8. Common Problems</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch07s04.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="ch08s02.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 8. Common Problems"><div class="titlepage"><div><div><h2 class="title"><a id="common-problems"></a>Chapter 8. Common Problems</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="ch08.html#deadlock">Deadlock: Simple and Advanced</a></span></dt><dt><span class="sect1"><a href="ch08s02.html">Preventing Deadlock</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch08s02.html#techs-deadlock-overprevent">Overzealous Prevention Of Deadlocks</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch08s03.html">Racing Timers: A Kernel Pastime</a></span></dt></dl></div><div class="sect1" title="Deadlock: Simple and Advanced"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="deadlock"></a>Deadlock: Simple and Advanced</h2></div></div></div><p>
There is a coding bug where a piece of code tries to grab a
spinlock twice: it will spin forever, waiting for the lock to
be released (spinlocks, rwlocks and mutexes are not
recursive in Linux). This is trivial to diagnose: not a
stay-up-five-nights-talk-to-fluffy-code-bunnies kind of
problem.
</p><p>
For a slightly more complex case, imagine you have a region
shared by a softirq and user context. If you use a
<code class="function">spin_lock()</code> call to protect it, it is
possible that the user context will be interrupted by the softirq
while it holds the lock, and the softirq will then spin
forever trying to get the same lock.
</p><p>
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
<span class="symbol">CONFIG_SMP</span>=n. You'll still get data corruption
in the second example).
</p><p>
This complete lockup is easy to diagnose: on SMP boxes the
watchdog timer or compiling with <span class="symbol">DEBUG_SPINLOCK</span> set
(<code class="filename">include/linux/spinlock.h</code>) will show this up
immediately when it happens.
</p><p>
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.
</p><p>
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:
</p><div class="table"><a id="id3051530"></a><p class="title"><b>Table 8.1. Consequences</b></p><div class="table-contents"><table summary="Consequences" border="1"><colgroup><col /><col /></colgroup><thead><tr><th align="left">CPU 1</th><th align="left">CPU 2</th></tr></thead><tbody><tr><td align="left">Grab lock A -> OK</td><td align="left">Grab lock B -> OK</td></tr><tr><td align="left">Grab lock B -> spin</td><td align="left">Grab lock A -> spin</td></tr></tbody></table></div></div><br class="table-break" /><p>
The two CPUs will spin forever, waiting for the other to give up
their lock. It will look, smell, and feel like a crash.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch07s04.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="ch08s02.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Protecting The Objects Themselves </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Preventing Deadlock</td></tr></table></div></body></html>
