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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>Accessing From Interrupt 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="ch07.html" title="Chapter 7. Common Examples" /><link rel="prev" href="ch07.html" title="Chapter 7. Common Examples" /><link rel="next" href="ch07s03.html" title="Exposing Objects Outside This File" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Accessing From Interrupt Context</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch07.html">Prev</a> </td><th width="60%" align="center">Chapter 7. Common Examples</th><td width="20%" align="right"> <a accesskey="n" href="ch07s03.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Accessing From Interrupt Context"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="examples-interrupt"></a>Accessing From Interrupt Context</h2></div></div></div><p>
Now consider the case where <code class="function">cache_find</code> can be
called from interrupt context: either a hardware interrupt or a
softirq. An example would be a timer which deletes object from the
cache.
</p><p>
The change is shown below, in standard patch format: the
<span class="symbol">-</span> are lines which are taken away, and the
<span class="symbol">+</span> are lines which are added.
</p><pre class="programlisting">
--- cache.c.usercontext 2003-12-09 13:58:54.000000000 +1100
+++ cache.c.interrupt 2003-12-09 14:07:49.000000000 +1100
@@ -12,7 +12,7 @@
int popularity;
};
-static DEFINE_MUTEX(cache_lock);
+static DEFINE_SPINLOCK(cache_lock);
static LIST_HEAD(cache);
static unsigned int cache_num = 0;
#define MAX_CACHE_SIZE 10
@@ -55,6 +55,7 @@
int cache_add(int id, const char *name)
{
struct object *obj;
+ unsigned long flags;
if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL)
return -ENOMEM;
@@ -63,30 +64,33 @@
obj->id = id;
obj->popularity = 0;
- mutex_lock(&cache_lock);
+ spin_lock_irqsave(&cache_lock, flags);
__cache_add(obj);
- mutex_unlock(&cache_lock);
+ spin_unlock_irqrestore(&cache_lock, flags);
return 0;
}
void cache_delete(int id)
{
- mutex_lock(&cache_lock);
+ unsigned long flags;
+
+ spin_lock_irqsave(&cache_lock, flags);
__cache_delete(__cache_find(id));
- mutex_unlock(&cache_lock);
+ spin_unlock_irqrestore(&cache_lock, flags);
}
int cache_find(int id, char *name)
{
struct object *obj;
int ret = -ENOENT;
+ unsigned long flags;
- mutex_lock(&cache_lock);
+ spin_lock_irqsave(&cache_lock, flags);
obj = __cache_find(id);
if (obj) {
ret = 0;
strcpy(name, obj->name);
}
- mutex_unlock(&cache_lock);
+ spin_unlock_irqrestore(&cache_lock, flags);
return ret;
}
</pre><p>
Note that the <code class="function">spin_lock_irqsave</code> will turn off
interrupts if they are on, otherwise does nothing (if we are already
in an interrupt handler), hence these functions are safe to call from
any context.
</p><p>
Unfortunately, <code class="function">cache_add</code> calls
<code class="function">kmalloc</code> with the <span class="symbol">GFP_KERNEL</span>
flag, which is only legal in user context. I have assumed that
<code class="function">cache_add</code> is still only called in user context,
otherwise this should become a parameter to
<code class="function">cache_add</code>.
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