get_device()和put_device()函数

在linux内核中，get_device()和put_device()是一对相反的操作函数。
get_device()是对设备引用数量加一，put_device()是对设备引用减一

以下就是源码

/**
 * get_device - increment reference count for device.
 * @dev: device.
 *
 * This simply forwards the call to kobject_get(), though
 * we do take care to provide for the case that we get a NULL
 * pointer passed in.
 */
struct device *get_device(struct device *dev)
{
	return dev ? kobj_to_dev(kobject_get(&dev->kobj)) : NULL;
}

/**
 * kobject_get - increment refcount for object.
 * @kobj: object.
 */
struct kobject *kobject_get(struct kobject *kobj)
{
	if (kobj) {
		if (!kobj->state_initialized)
			WARN(1, KERN_WARNING "kobject: '%s' (%p): is not "
			       "initialized, yet kobject_get() is being "
			       "called.\n", kobject_name(kobj), kobj);
		kref_get(&kobj->kref);
	}
	return kobj;
}

/**
 * kref_get - increment refcount for object.
 * @kref: object.
 */
static inline void kref_get(struct kref *kref)
{
	refcount_inc(&kref->refcount);
}

/**
 * put_device - decrement reference count.
 * @dev: device in question.
 */
void put_device(struct device *dev)
{
	/* might_sleep(); */
	if (dev)
		kobject_put(&dev->kobj);
}

/**
 * kobject_put - decrement refcount for object.
 * @kobj: object.
 *
 * Decrement the refcount, and if 0, call kobject_cleanup().
 */
void kobject_put(struct kobject *kobj)
{
	if (kobj) {
		if (!kobj->state_initialized)
			WARN(1, KERN_WARNING "kobject: '%s' (%p): is not "
			       "initialized, yet kobject_put() is being "
			       "called.\n", kobject_name(kobj), kobj);
		kref_put(&kobj->kref, kobject_release);
	}
}

/**
 * kref_put - decrement refcount for object.
 * @kref: object.
 * @release: pointer to the function that will clean up the object when the
 *	     last reference to the object is released.
 *	     This pointer is required, and it is not acceptable to pass kfree
 *	     in as this function.  If the caller does pass kfree to this
 *	     function, you will be publicly mocked mercilessly by the kref
 *	     maintainer, and anyone else who happens to notice it.  You have
 *	     been warned.
 *
 * Decrement the refcount, and if 0, call release().
 * Return 1 if the object was removed, otherwise return 0.  Beware, if this
 * function returns 0, you still can not count on the kref from remaining in
 * memory.  Only use the return value if you want to see if the kref is now
 * gone, not present.
 */
static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
{
	if (refcount_dec_and_test(&kref->refcount)) {
		release(kref);
		return 1;
	}
	return 0;
}