spinlock 学习记录

</pre>2. spinlock</h1><p>spinlock 具有阻塞性。</p><p>spin_lock(&lock);</p><p>spin_unlock(&lock);</p><p></p><p>if( !spin_trylock(&lock) )</p><p><span style="white-space:pre">	</span>return EBUSY;</p><p></p><p>/* 修改函数：<span style="background-color: rgb(240, 240, 240);">foo_show </span>*/</p><p><pre name="code" class="html">/*
* Sample kobject implementation
*
* Copyright (C) 2004-2007 Greg Kroah-Hartman <greg@kroah.com>
* Copyright (C) 2007 Novell Inc.
*
* Released under the GPL version 2 only.
*
*/
#include <linux/kobject.h>
#include <linux/string.h>
#include <linux/sysfs.h>
#include <linux/module.h>
#include <linux/init.h>
#include <asm/uaccess.h>
#include <linux/delay.h>
#include <linux/spinlock.h>

/*
* This module shows how to create a simple subdirectory in sysfs called
* /sys/kernel/kobject-example  In that directory, 3 files are created:
* "foo", "baz", and "bar".  If an integer is written to these files, it can be
* later read out of it.
*/

static atomic_t atomic_lock=ATOMIC_INIT(1);
DEFINE_SPINLOCK(lock);
static int foo;
static int baz;
static int bar;

/*
* The "foo" file where a static variable is read from and written to.
*/
static ssize_t foo_show(struct kobject *kobj, struct kobj_attribute *attr,
char *buf)
{

if( !spin_trylock(&lock)){
return -EBUSY;
}
mdelay(5000);

return sprintf(buf, "%d\n", foo);
}

static ssize_t foo_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buf, size_t count)
{
spin_unlock(&lock);
//printk("222222222\n");
sscanf(buf, "%du", &foo);
return count;
}

static struct kobj_attribute foo_attribute =
__ATTR(foo, 0666, foo_show, foo_store);

/*
* More complex function where we determine which variable is being accessed by
* looking at the attribute for the "baz" and "bar" files.
*/
static ssize_t b_show(struct kobject *kobj, struct kobj_attribute *attr,
char *buf)
{
int var;

if (strcmp(attr->attr.name, "baz") == 0)
var = baz;
else
var = bar;
return sprintf(buf, "%d\n", var);
}

static ssize_t b_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buf, size_t count)
{
int var;

sscanf(buf, "%du", &var);
if (strcmp(attr->attr.name, "baz") == 0)
baz = var;
else
bar = var;
return count;
}

static struct kobj_attribute baz_attribute =
__ATTR(baz, 0666, b_show, b_store);
static struct kobj_attribute bar_attribute =
__ATTR(bar, 0666, b_show, b_store);

/*
* Create a group of attributes so that we can create and destroy them all
* at once.
*/
static struct attribute *attrs[] = {
&foo_attribute.attr,
&baz_attribute.attr,
&bar_attribute.attr,
NULL,	/* need to NULL terminate the list of attributes */
};

/*
* An unnamed attribute group will put all of the attributes directly in
* the kobject directory.  If we specify a name, a subdirectory will be
* created for the attributes with the directory being the name of the
* attribute group.
*/
static struct attribute_group attr_group = {
.attrs = attrs,
};

static struct kobject *example_kobj;

static int __init example_init(void)
{
int retval;

/*
* Create a simple kobject with the name of "kobject_example",
* located under /sys/kernel/
*
* As this is a simple directory, no uevent will be sent to
* userspace.  That is why this function should not be used for
* any type of dynamic kobjects, where the name and number are
* not known ahead of time.
*/
example_kobj = kobject_create_and_add("kobject_example", kernel_kobj);
if (!example_kobj)
return -ENOMEM;

/* Create the files associated with this kobject */
retval = sysfs_create_group(example_kobj, &attr_group);
if (retval)
kobject_put(example_kobj);

return retval;
}

static void __exit example_exit(void)
{
kobject_put(example_kobj);
}

module_init(example_init);
module_exit(example_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Greg Kroah-Hartman <greg@kroah.com>");

Test way:

Terminal :

cat /sys/kernel/kobject_example/foo

cat /sys/kernel/kobject_example/foo #提示设备或资源忙

只有解除限制 echo 2 > /sys/kernel/kobject_example/foo

才可以进行 cat /sys/kernel/kobject_example/foo

2. 遇到的问题

int spin_trylock(&lock)

lock 处于上锁时，返回0 .

可以参考x86 atomic_dec_and_test()实现，理解会深刻一些.