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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>Locking Between User Context and Softirqs</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="ch03s03.html" title="Locking Only In User Context" /><link rel="next" href="ch03s05.html" title="Locking Between User Context and Tasklets" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Locking Between User Context and Softirqs</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch03s03.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="ch03s05.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Locking Between User Context and Softirqs"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="lock-user-bh"></a>Locking Between User Context and Softirqs</h2></div></div></div><p>
If a <a class="firstterm" href="go01.html#gloss-softirq"><em class="firstterm">softirq</em></a> shares
data with user context, you have two problems. Firstly, the current
user context can be interrupted by a softirq, and secondly, the
critical region could be entered from another CPU. This is where
<code class="function">spin_lock_bh()</code>
(<code class="filename">include/linux/spinlock.h</code>) is
used. It disables softirqs on that CPU, then grabs the lock.
<code class="function">spin_unlock_bh()</code> does the reverse. (The
'_bh' suffix is a historical reference to "Bottom Halves", the
old name for software interrupts. It should really be
called spin_lock_softirq()' in a perfect world).
</p><p>
Note that you can also use <code class="function">spin_lock_irq()</code>
or <code class="function">spin_lock_irqsave()</code> here, which stop
hardware interrupts as well: see <a class="xref" href="ch04.html" title="Chapter 4. Hard IRQ Context">Chapter 4, <i>Hard IRQ Context</i></a>.
</p><p>
This works perfectly for <a class="firstterm" href="go01.html#gloss-up"><em class="firstterm"><acronym class="acronym">UP
</acronym></em></a> as well: the spin lock vanishes, and this macro
simply becomes <code class="function">local_bh_disable()</code>
(<code class="filename">include/linux/interrupt.h</code>), which
protects you from the softirq being run.
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