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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 4. Hard IRQ Context</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="ch03s08.html" title="Locking Between Softirqs" /><link rel="next" href="ch04s02.html" title="Locking Between Two Hard IRQ Handlers" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 4. Hard IRQ Context</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch03s08.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="ch04s02.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 4. Hard IRQ Context"><div class="titlepage"><div><div><h2 class="title"><a id="hardirq-context"></a>Chapter 4. Hard IRQ Context</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="ch04.html#hardirq-softirq">Locking Between Hard IRQ and Softirqs/Tasklets</a></span></dt><dt><span class="sect1"><a href="ch04s02.html">Locking Between Two Hard IRQ Handlers</a></span></dt></dl></div><p>
Hardware interrupts usually communicate with a
tasklet or softirq. Frequently this involves putting work in a
queue, which the softirq will take out.
</p><div class="sect1" title="Locking Between Hard IRQ and Softirqs/Tasklets"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="hardirq-softirq"></a>Locking Between Hard IRQ and Softirqs/Tasklets</h2></div></div></div><p>
If a hardware irq handler shares data with a softirq, you have
two concerns. Firstly, the softirq processing can be
interrupted by a hardware interrupt, and secondly, the
critical region could be entered by a hardware interrupt on
another CPU. This is where <code class="function">spin_lock_irq()</code> is
used. It is defined to disable interrupts on that cpu, then grab
the lock. <code class="function">spin_unlock_irq()</code> does the reverse.
</p><p>
The irq handler does not to use
<code class="function">spin_lock_irq()</code>, because the softirq cannot
run while the irq handler is running: it can use
<code class="function">spin_lock()</code>, which is slightly faster. The
only exception would be if a different hardware irq handler uses
the same lock: <code class="function">spin_lock_irq()</code> will stop
that from interrupting us.
</p><p>
This works perfectly for UP as well: the spin lock vanishes,
and this macro simply becomes <code class="function">local_irq_disable()</code>
(<code class="filename">include/asm/smp.h</code>), which
protects you from the softirq/tasklet/BH being run.
</p><p>
<code class="function">spin_lock_irqsave()</code>
(<code class="filename">include/linux/spinlock.h</code>) is a variant
which saves whether interrupts were on or off in a flags word,
which is passed to <code class="function">spin_unlock_irqrestore()</code>. This
means that the same code can be used inside an hard irq handler (where
interrupts are already off) and in softirqs (where the irq
disabling is required).
</p><p>
Note that softirqs (and hence tasklets and timers) are run on
return from hardware interrupts, so
<code class="function">spin_lock_irq()</code> also stops these. In that
sense, <code class="function">spin_lock_irqsave()</code> is the most
general and powerful locking function.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch03s08.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="ch04s02.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Locking Between Softirqs </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Locking Between Two Hard IRQ Handlers</td></tr></table></div></body></html>
