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If data is always accessed from within the same IRQ handler, you
don't need a lock at all: the kernel already guarantees that the
irq handler will not run simultaneously on multiple CPUs.
</p><p>
Manfred Spraul points out that you can still do this, even if
the data is very occasionally accessed in user context or
softirqs/tasklets. The irq handler doesn't use a lock, and
all other accesses are done as so:
</p><pre class="programlisting">
spin_lock(&lock);
disable_irq(irq);
...
enable_irq(irq);
spin_unlock(&lock);
</pre><p>
The <code class="function">disable_irq()</code> prevents the irq handler
from running (and waits for it to finish if it's currently
running on other CPUs). The spinlock prevents any other
accesses happening at the same time. Naturally, this is slower
than just a <code class="function">spin_lock_irq()</code> call, so it
only makes sense if this type of access happens extremely
rarely.
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