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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>Locks and Uniprocessor Kernels</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="ch03.html" title="Chapter 3. Locking in the Linux Kernel" /><link rel="prev" href="ch03.html" title="Chapter 3. Locking in the Linux Kernel" /><link rel="next" href="ch03s03.html" title="Locking Only In User Context" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Locks and Uniprocessor Kernels</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch03.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Locking in the Linux Kernel</th><td width="20%" align="right"> <a accesskey="n" href="ch03s03.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Locks and Uniprocessor Kernels"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="uniprocessor"></a>Locks and Uniprocessor Kernels</h2></div></div></div><p>
For kernels compiled without <span class="symbol">CONFIG_SMP</span>, and
without <span class="symbol">CONFIG_PREEMPT</span> 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.
</p><p>
If the kernel is compiled without <span class="symbol">CONFIG_SMP</span>,
but <span class="symbol">CONFIG_PREEMPT</span> 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.
</p><p>
You should always test your locking code with <span class="symbol">CONFIG_SMP</span>
and <span class="symbol">CONFIG_PREEMPT</span> enabled, even if you don't have an SMP test box, because it
will still catch some kinds of locking bugs.
</p><p>
Mutexes still exist, because they are required for
synchronization between <a class="firstterm" href="go01.html#gloss-usercontext"><em class="firstterm">user
contexts</em></a>, as we will see below.
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