1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>Chapter&#160;2.&#160;The Players</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="index.html" title="Unreliable Guide To Hacking The Linux Kernel"><link rel="prev" href="introduction.html" title="Chapter&#160;1.&#160;Introduction"><link rel="next" href="basics-hardirqs.html" title="Hardware Interrupts (Hard IRQs)"></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">Chapter&#160;2.&#160;The Players</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="introduction.html">Prev</a>&#160;</td><th width="60%" align="center">&#160;</th><td width="20%" align="right">&#160;<a accesskey="n" href="basics-hardirqs.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="basic-players"></a>Chapter&#160;2.&#160;The Players</h1></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl class="toc"><dt><span class="sect1"><a href="basic-players.html#basics-usercontext">User Context</a></span></dt><dt><span class="sect1"><a href="basics-hardirqs.html">Hardware Interrupts (Hard IRQs)</a></span></dt><dt><span class="sect1"><a href="basics-softirqs.html">Software Interrupt Context: Softirqs and Tasklets</a></span></dt></dl></div><p>
2   At any time each of the CPUs in a system can be:
3  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
4     not associated with any process, serving a hardware interrupt;
5    </p></li><li class="listitem"><p>
6     not associated with any process, serving a softirq or tasklet;
7    </p></li><li class="listitem"><p>
8     running in kernel space, associated with a process (user context);
9    </p></li><li class="listitem"><p>
10     running a process in user space.
11    </p></li></ul></div><p>
12   There is an ordering between these.  The bottom two can preempt
13   each other, but above that is a strict hierarchy: each can only be
14   preempted by the ones above it.  For example, while a softirq is
15   running on a CPU, no other softirq will preempt it, but a hardware
16   interrupt can.  However, any other CPUs in the system execute
17   independently.
18  </p><p>
19   We'll see a number of ways that the user context can block
20   interrupts, to become truly non-preemptable.
21  </p><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="basics-usercontext"></a>User Context</h2></div></div></div><p>
22    User context is when you are coming in from a system call or other
23    trap: like userspace, you can be preempted by more important tasks
24    and by interrupts.  You can sleep, by calling
25    <code class="function">schedule()</code>.
26   </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
27     You are always in user context on module load and unload,
28     and on operations on the block device layer.
29    </p></div><p>
30    In user context, the <code class="varname">current</code> pointer (indicating 
31    the task we are currently executing) is valid, and
32    <code class="function">in_interrupt()</code>
33    (<code class="filename">include/linux/interrupt.h</code>) is <span class="returnvalue">false
34    </span>.  
35   </p><div class="caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p>
36     Beware that if you have preemption or softirqs disabled
37     (see below), <code class="function">in_interrupt()</code> will return a 
38     false positive.
39    </p></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="introduction.html">Prev</a>&#160;</td><td width="20%" align="center">&#160;</td><td width="40%" align="right">&#160;<a accesskey="n" href="basics-hardirqs.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter&#160;1.&#160;Introduction&#160;</td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top">&#160;Hardware Interrupts (Hard IRQs)</td></tr></table></div></body></html>
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