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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>Hardware Interrupts (Hard IRQs)</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Unreliable Guide To Hacking The Linux Kernel" /><link rel="up" href="ch02.html" title="Chapter 2. The Players" /><link rel="prev" href="ch02.html" title="Chapter 2. The Players" /><link rel="next" href="ch02s03.html" title="Software Interrupt Context: Softirqs and Tasklets" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Hardware Interrupts (Hard IRQs)</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch02.html">Prev</a> </td><th width="60%" align="center">Chapter 2. The Players</th><td width="20%" align="right"> <a accesskey="n" href="ch02s03.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Hardware Interrupts (Hard IRQs)"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="basics-hardirqs"></a>Hardware Interrupts (Hard IRQs)</h2></div></div></div><p>
Timer ticks, <span class="hardware">network cards</span> and
<span class="hardware">keyboard</span> are examples of real
hardware which produce interrupts at any time. The kernel runs
interrupt handlers, which services the hardware. The kernel
guarantees that this handler is never re-entered: if the same
interrupt arrives, it is queued (or dropped). Because it
disables interrupts, this handler has to be fast: frequently it
simply acknowledges the interrupt, marks a 'software interrupt'
for execution and exits.
</p><p>
You can tell you are in a hardware interrupt, because
<code class="function">in_irq()</code> returns <span class="returnvalue">true</span>.
</p><div class="caution" title="Caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p>
Beware that this will return a false positive if interrupts are disabled
(see below).
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