</para>
<sect1 id="lock-intro">
- <title>Two Main Types of Kernel Locks: Spinlocks and Semaphores</title>
+ <title>Three Main Types of Kernel Locks: Spinlocks, Mutexes and Semaphores</title>
<para>
There are three main types of kernel locks. The fundamental type
<function>spin_lock_irqsave()</function>, which is a superset
of all other spinlock primitives.
</para>
+
<table>
<title>Table of Locking Requirements</title>
<tgroup cols="11">
<tbody>
+
<row>
<entry></entry>
<entry>IRQ Handler A</entry>
<row>
<entry>IRQ Handler B</entry>
-<entry>spin_lock_irqsave</entry>
+<entry>SLIS</entry>
<entry>None</entry>
</row>
<row>
<entry>Softirq A</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock</entry>
+<entry>SLI</entry>
+<entry>SLI</entry>
+<entry>SL</entry>
</row>
<row>
<entry>Softirq B</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
+<entry>SLI</entry>
+<entry>SLI</entry>
+<entry>SL</entry>
+<entry>SL</entry>
</row>
<row>
<entry>Tasklet A</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
+<entry>SLI</entry>
+<entry>SLI</entry>
+<entry>SL</entry>
+<entry>SL</entry>
<entry>None</entry>
</row>
<row>
<entry>Tasklet B</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
+<entry>SLI</entry>
+<entry>SLI</entry>
+<entry>SL</entry>
+<entry>SL</entry>
+<entry>SL</entry>
<entry>None</entry>
</row>
<row>
<entry>Timer A</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
+<entry>SLI</entry>
+<entry>SLI</entry>
+<entry>SL</entry>
+<entry>SL</entry>
+<entry>SL</entry>
+<entry>SL</entry>
<entry>None</entry>
</row>
<row>
<entry>Timer B</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
-<entry>spin_lock</entry>
+<entry>SLI</entry>
+<entry>SLI</entry>
+<entry>SL</entry>
+<entry>SL</entry>
+<entry>SL</entry>
+<entry>SL</entry>
+<entry>SL</entry>
<entry>None</entry>
</row>
<row>
<entry>User Context A</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock_bh</entry>
-<entry>spin_lock_bh</entry>
-<entry>spin_lock_bh</entry>
-<entry>spin_lock_bh</entry>
-<entry>spin_lock_bh</entry>
-<entry>spin_lock_bh</entry>
+<entry>SLI</entry>
+<entry>SLI</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
<entry>None</entry>
</row>
<row>
<entry>User Context B</entry>
+<entry>SLI</entry>
+<entry>SLI</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
+<entry>SLBH</entry>
+<entry>DI</entry>
+<entry>None</entry>
+</row>
+
+</tbody>
+</tgroup>
+</table>
+
+ <table>
+<title>Legend for Locking Requirements Table</title>
+<tgroup cols="2">
+<tbody>
+
+<row>
+<entry>SLIS</entry>
+<entry>spin_lock_irqsave</entry>
+</row>
+<row>
+<entry>SLI</entry>
<entry>spin_lock_irq</entry>
-<entry>spin_lock_irq</entry>
-<entry>spin_lock_bh</entry>
-<entry>spin_lock_bh</entry>
-<entry>spin_lock_bh</entry>
-<entry>spin_lock_bh</entry>
-<entry>spin_lock_bh</entry>
+</row>
+<row>
+<entry>SL</entry>
+<entry>spin_lock</entry>
+</row>
+<row>
+<entry>SLBH</entry>
<entry>spin_lock_bh</entry>
+</row>
+<row>
+<entry>DI</entry>
<entry>down_interruptible</entry>
-<entry>None</entry>
</row>
</tbody>
</tgroup>
</table>
+
</sect1>
</chapter>
<para>
For our first example, we assume that all operations are in user
context (ie. from system calls), so we can sleep. This means we can
-use a semaphore to protect the cache and all the objects within
+use a mutex to protect the cache and all the objects within
it. Here's the code:
</para>
#include <linux/list.h>
#include <linux/slab.h>
#include <linux/string.h>
-#include <asm/semaphore.h>
+#include <linux/mutex.h>
#include <asm/errno.h>
struct object
};
/* Protects the cache, cache_num, and the objects within it */
-static DECLARE_MUTEX(cache_lock);
+static DEFINE_MUTEX(cache_lock);
static LIST_HEAD(cache);
static unsigned int cache_num = 0;
#define MAX_CACHE_SIZE 10
obj->id = id;
obj->popularity = 0;
- down(&cache_lock);
+ mutex_lock(&cache_lock);
__cache_add(obj);
- up(&cache_lock);
+ mutex_unlock(&cache_lock);
return 0;
}
void cache_delete(int id)
{
- down(&cache_lock);
+ mutex_lock(&cache_lock);
__cache_delete(__cache_find(id));
- up(&cache_lock);
+ mutex_unlock(&cache_lock);
}
int cache_find(int id, char *name)
struct object *obj;
int ret = -ENOENT;
- down(&cache_lock);
+ mutex_lock(&cache_lock);
obj = __cache_find(id);
if (obj) {
ret = 0;
strcpy(name, obj->name);
}
- up(&cache_lock);
+ mutex_unlock(&cache_lock);
return ret;
}
</programlisting>
int popularity;
};
--static DECLARE_MUTEX(cache_lock);
+-static DEFINE_MUTEX(cache_lock);
+static spinlock_t cache_lock = SPIN_LOCK_UNLOCKED;
static LIST_HEAD(cache);
static unsigned int cache_num = 0;
obj->id = id;
obj->popularity = 0;
-- down(&cache_lock);
+- mutex_lock(&cache_lock);
+ spin_lock_irqsave(&cache_lock, flags);
__cache_add(obj);
-- up(&cache_lock);
+- mutex_unlock(&cache_lock);
+ spin_unlock_irqrestore(&cache_lock, flags);
return 0;
}
void cache_delete(int id)
{
-- down(&cache_lock);
+- mutex_lock(&cache_lock);
+ unsigned long flags;
+
+ spin_lock_irqsave(&cache_lock, flags);
__cache_delete(__cache_find(id));
-- up(&cache_lock);
+- mutex_unlock(&cache_lock);
+ spin_unlock_irqrestore(&cache_lock, flags);
}
int ret = -ENOENT;
+ unsigned long flags;
-- down(&cache_lock);
+- mutex_lock(&cache_lock);
+ spin_lock_irqsave(&cache_lock, flags);
obj = __cache_find(id);
if (obj) {
ret = 0;
strcpy(name, obj->name);
}
-- up(&cache_lock);
+- mutex_unlock(&cache_lock);
+ spin_unlock_irqrestore(&cache_lock, flags);
return ret;
}
<para>
Our final dilemma is this: when can we actually destroy the
removed element? Remember, a reader might be stepping through
- this element in the list right now: it we free this element and
+ this element in the list right now: if we free this element and
the <symbol>next</symbol> pointer changes, the reader will jump
off into garbage and crash. We need to wait until we know that
all the readers who were traversing the list when we deleted the