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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>Atomicity</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Writing an ALSA Driver" /><link rel="up" href="ch05.html" title="Chapter 5. PCM Interface" /><link rel="prev" href="ch05s07.html" title="Interrupt Handler" /><link rel="next" href="ch05s09.html" title="Constraints" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Atomicity</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch05s07.html">Prev</a> </td><th width="60%" align="center">Chapter 5. PCM Interface</th><td width="20%" align="right"> <a accesskey="n" href="ch05s09.html">Next</a></td></tr></table><hr /></div><div class="section" title="Atomicity"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="pcm-interface-atomicity"></a>Atomicity</h2></div></div></div><p>
One of the most important (and thus difficult to debug) problems
in kernel programming are race conditions.
In the Linux kernel, they are usually avoided via spin-locks, mutexes
or semaphores. In general, if a race condition can happen
in an interrupt handler, it has to be managed atomically, and you
have to use a spinlock to protect the critical session. If the
critical section is not in interrupt handler code and
if taking a relatively long time to execute is acceptable, you
should use mutexes or semaphores instead.
</p><p>
As already seen, some pcm callbacks are atomic and some are
not. For example, the <em class="parameter"><code>hw_params</code></em> callback is
non-atomic, while <em class="parameter"><code>trigger</code></em> callback is
atomic. This means, the latter is called already in a spinlock
held by the PCM middle layer. Please take this atomicity into
account when you choose a locking scheme in the callbacks.
</p><p>
In the atomic callbacks, you cannot use functions which may call
<code class="function">schedule</code> or go to
<code class="function">sleep</code>. Semaphores and mutexes can sleep,
and hence they cannot be used inside the atomic callbacks
(e.g. <em class="parameter"><code>trigger</code></em> callback).
To implement some delay in such a callback, please use
<code class="function">udelay()</code> or <code class="function">mdelay()</code>.
</p><p>
All three atomic callbacks (trigger, pointer, and ack) are
called with local interrupts disabled.
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