1<html><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 V1.78.1"><link rel="home" href="index.html" title="Unreliable Guide To Hacking The Linux Kernel"><link rel="up" href="basic-players.html" title="Chapter&#160;2.&#160;The Players"><link rel="prev" href="basic-players.html" title="Chapter&#160;2.&#160;The Players"><link rel="next" href="basics-softirqs.html" title="Software Interrupt Context: Softirqs and Tasklets"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><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="basic-players.html">Prev</a>&#160;</td><th width="60%" align="center">Chapter&#160;2.&#160;The Players</th><td width="20%" align="right">&#160;<a accesskey="n" href="basics-softirqs.html">Next</a></td></tr></table><hr></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="basics-hardirqs"></a>Hardware Interrupts (Hard IRQs)</h2></div></div></div><p>
2    Timer ticks, <span class="hardware">network cards</span> and 
3    <span class="hardware">keyboard</span> are examples of real
4    hardware which produce interrupts at any time.  The kernel runs
5    interrupt handlers, which services the hardware.  The kernel
6    guarantees that this handler is never re-entered: if the same
7    interrupt arrives, it is queued (or dropped).  Because it
8    disables interrupts, this handler has to be fast: frequently it
9    simply acknowledges the interrupt, marks a 'software interrupt'
10    for execution and exits.
11   </p><p>
12    You can tell you are in a hardware interrupt, because 
13    <code class="function">in_irq()</code> returns <span class="returnvalue">true</span>.  
14   </p><div class="caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p>
15     Beware that this will return a false positive if interrupts are disabled 
16     (see below).
17    </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="basic-players.html">Prev</a>&#160;</td><td width="20%" align="center"><a accesskey="u" href="basic-players.html">Up</a></td><td width="40%" align="right">&#160;<a accesskey="n" href="basics-softirqs.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter&#160;2.&#160;The Players&#160;</td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top">&#160;Software Interrupt Context: Softirqs and Tasklets</td></tr></table></div></body></html>
18