Linux内核kobject结构体分析

1、前言

Linux内核中有大量的驱动，而这些驱动往往具有类似的结构，根据面向对象的思想，可以将共同的部分提取为父类，而这个父类就是kobject，kobject结构体中包含了大量设备的必须信息，而三大类设备驱动都需要包含这个kobject结构，运用面向对象的思想来看问题，也就是继承来自kobject，一个kobject对象往往就对应sysfs中的一个目录，kobject是组成设备模型的基本结构，kobject需要处理的基本任务有如下：

（1）对象的引用计数

当一个内核对象被创建时，不可能知道对象的存活时间，跟踪该对象的的生命周期的一个方法就是使用引用计数，当内核中没有代码持有该对象的引用时，说明该对象可以被销毁了；

（2）sysfs表述

一个kobject对象往往对应sysfs中的一个目录，kobject被用来与内核交互并创建它的sysfs可见表述；

（3）数据结构关联

从整体上看，设备模型是一个友好而复杂的数据结构，通过kobject对象实现了大量的连接而构成了一个多层次的体系结构；

（4）热拔插事件处理

当系统中的硬件被热插拔时，在kobject子系统的控制下，将产生事件以通知用户空间。

2、kobject以及相关结构体

Linux内核源码中对kobject结构体的定义在include/linux/kobject.h文件中，实现在lib/kobjet.c，struct kobject结构体的定义如下所示：

struct kobject {
    const char        *name;
    struct list_head    entry;
    struct kobject        *parent;
    struct kset        *kset;
    struct kobj_type    *ktype;
    struct kernfs_node    *sd; /* sysfs directory entry */
    struct kref        kref;
#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
    struct delayed_work    release;
#endif
    unsigned int state_initialized:;
    unsigned int state_in_sysfs:;
    unsigned int state_add_uevent_sent:;
    unsigned int state_remove_uevent_sent:;
    unsigned int uevent_suppress:;
};

结构体常用成员解释：

name：表示kobject对象的名字，对应sysfs下的一个目录；

entry：在kobject中嵌入双向链表list_head结构；

parent：指向当前kobject父对象的指针；

kset：表示当前kobject对象所属的集合；

ktype：表示当前kobject对象所属类型；

sd：用于VFS文件系统的目录项，是设备与文件之间的桥梁，sysfs中的符号链接就是通过kernfs_node内的联合体实现的；

kref：是对kobject的引用计数，当引用计数为0时，就回调之前注册的release函数释放该对象；

state_initialized:1：初始化的标志位，在对象被初始化时置位，用于表示对象已被初始化；

state_in_sysfs:1：表示kobject对象在sysfs中的状态，在对应目录中被创建则置1，否则为0；

state_add_uevent_sent:1：添加设备的uevent事件是否发送标志，添加设备时会向用户空间发送uevent事件，请求新增设备；

state_remove_uevent_sent:1：删除设备的uevent事件是否发送标志，删除设备时会向用户空间发送uevent事件，请求卸载设备。

struct kobj_type定义了kobject的类型，该结构体既有操作的函数，也有默认的公共属性，该结构体定义如下：

struct kobj_type {
    void (*release)(struct kobject *kobj);
    const struct sysfs_ops *sysfs_ops;
    struct attribute **default_attrs;
    const struct kobj_ns_type_operations *(*child_ns_type)(struct kobject *kobj);
    const void *(*namespace)(struct kobject *kobj);
    void (*get_ownership)(struct kobject *kobj, kuid_t *uid, kgid_t *gid);
};

结构体常用成员解释：

release：函数指针，当引用计数为0时，在kobject释放时调用；

sysfs_ops：定义了读写属性文件时调用的函数；

default_attrs：定义了这类kobject默认属性，指向attribute数组的指针。

struct sysfs_ops结构体是用于实现属性的的函数操作集，其定义如下所示：

struct sysfs_ops {
    ssize_t    (*show)(struct kobject *, struct attribute *, char *);
    ssize_t    (*store)(struct kobject *, struct attribute *, const char *, size_t);
};

show：函数用于读取一个属性到用户空间，函数的第一个参数是要读取的kobject指针，它对应要读的目录，第二个参数是要读的属性，第三个参数存放读到的属性的缓冲区，当函数调用成功后，返回实际读取的数据长度；

store：函数用于将属性写入到内核，函数第一个参数是与写相关的kobject指针，它对应要写的目录，第二个参数是要写的属性，第三个参数是要写入的数据，第四个参数是要写入的参数长度，这个长度不能超过PAGESIZE个字节大小，只有当拥有属性写权限时，才能调用store()函数。

struct attribute定义了kobject的属性，该结构体定义如下：

struct attribute {
    const char        *name;
    umode_t            mode;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
    bool            ignore_lockdep:;
    struct lock_class_key    *key;
    struct lock_class_key    skey;
#endif
};

常用结构体成员解释：

name：属性文件的名称，属性文件将在kobject的sysfs目录中显示；

mode：属性文件的读写权限。

struct kset可以看成是在kobject上的拓展，它包含了一个kobject的链表，可以方便地表示sysfs中目录与子目录的关系，该结构体定义如下：

/**
 * struct kset - a set of kobjects of a specific type, belonging to a specific subsystem.
 *
 * A kset defines a group of kobjects.  They can be individually
 * different "types" but overall these kobjects all want to be grouped
 * together and operated on in the same manner.  ksets are used to
 * define the attribute callbacks and other common events that happen to
 * a kobject.
 *
 * @list: the list of all kobjects for this kset
 * @list_lock: a lock for iterating over the kobjects
 * @kobj: the embedded kobject for this kset (recursion, isn't it fun...)
 * @uevent_ops: the set of uevent operations for this kset.  These are
 * called whenever a kobject has something happen to it so that the kset
 * can add new environment variables, or filter out the uevents if so
 * desired.
 */
struct kset {
    struct list_head list;
    spinlock_t list_lock;
    struct kobject kobj;
    const struct kset_uevent_ops *uevent_ops;
};

结构体常用成员解释：

list：用来挂在链表上的结构，包含在一个kset的所有kobject构成了一个双向循坏链表，list_head是这个链表的头部，这个链表用来连接第一个和最后一个kobject对象，第一个kobject使用entry连接kset集合以及第二个kobject对象，第二个kobject对象使用entry连接第一个kobject对象和第三个kobject对象，最终形成一个kobject对象的链表；

list_lock：用于迭代kobjects的锁；

kobj：归属于该kset的所有的kobject的共有parent，这个parent就是体现内核设备组织结构的关键，同时，kset的引用计数就是内嵌的kobject对象的引用计数；

uevent_ops：此kset的uevent操作函数集。

struct kobj_attribute是kobject在attribute上的拓展，添加了专门读写kobject_attribute属性的函数，结构体定义如下：

struct kobj_attribute {
    struct attribute attr;
    ssize_t (*show)(struct kobject *kobj, struct kobj_attribute *attr,
            char *buf);
    ssize_t (*store)(struct kobject *kobj, struct kobj_attribute *attr,
             const char *buf, size_t count);
};

结构体成员解释：

attr：kobject的公共属性，包括名称和读写权限；

show：函数指针，用于读取属性到用户空间；

store：函数指针，用于用户空间写属性到内核。

注意：无论是kobjec还是kset（说到底是kset内部的kobject），都提供了使用kobj_attribute的快速创建方法。

3、kobject实现操作

在Linux内核中提供了一系列kobject的实现方法，接下来，进行简要分析：

/*
 * populate_dir - populate directory with attributes.
 * @kobj: object we're working on.
 *
 * Most subsystems have a set of default attributes that are associated
 * with an object that registers with them.  This is a helper called during
 * object registration that loops through the default attributes of the
 * subsystem and creates attributes files for them in sysfs.
 */
static int populate_dir(struct kobject *kobj)
{
    struct kobj_type *t = get_ktype(kobj);
    struct attribute *attr;
    int error = ;
    int i;

    if (t && t->default_attrs) {
        for (i = ; (attr = t->default_attrs[i]) != NULL; i++) {
            error = sysfs_create_file(kobj, attr);//在sysfs目录创建属性文件
            if (error)
                break;
        }
    }
    return error;
}

static int create_dir(struct kobject *kobj)
{
    const struct kobj_ns_type_operations *ops;
    int error;

    error = sysfs_create_dir_ns(kobj, kobject_namespace(kobj));//创建sysfs目录
    if (error)
        return error;

    error = populate_dir(kobj);//在sysfs目录中填充属性文件
    if (error) {
        sysfs_remove_dir(kobj);
        return error;
    }

    /*
     * @kobj->sd may be deleted by an ancestor going away.  Hold an
     * extra reference so that it stays until @kobj is gone.
     */
    sysfs_get(kobj->sd);//引用计数加1

    /*
     * If @kobj has ns_ops, its children need to be filtered based on
     * their namespace tags.  Enable namespace support on @kobj->sd.
     */
    ops = kobj_child_ns_ops(kobj);
    if (ops) {
        BUG_ON(ops->type <= KOBJ_NS_TYPE_NONE);
        BUG_ON(ops->type >= KOBJ_NS_TYPES);
        BUG_ON(!kobj_ns_type_registered(ops->type));

        sysfs_enable_ns(kobj->sd);
    }

    return ;
}

上面两个函数中，create_dir()用于在sysfs中创建kobj相应的目录，populate_dir()创建kobj中默认属性对应的文件，create_dir()函数就是调用populate_dir()实现的。

static int get_kobj_path_length(struct kobject *kobj)
{
    int length = ;
    struct kobject *parent = kobj;

    /* walk up the ancestors until we hit the one pointing to the
     * root.
     * Add 1 to strlen for leading '/' of each level.
     */
    do {
        if (kobject_name(parent) == NULL)
            return ;
        length += strlen(kobject_name(parent)) + ;
        parent = parent->parent;
    } while (parent);
    return length;
}

static void fill_kobj_path(struct kobject *kobj, char *path, int length)
{
    struct kobject *parent;

    --length;
    for (parent = kobj; parent; parent = parent->parent) {
        int cur = strlen(kobject_name(parent));
        /* back up enough to print this name with '/' */
        length -= cur;
        strncpy(path + length, kobject_name(parent), cur);
        *(path + --length) = '/';
    }

    pr_debug("kobject: '%s' (%p): %s: path = '%s'\n", kobject_name(kobj),
         kobj, __func__, path);
}

/**
 * kobject_get_path - generate and return the path associated with a given kobj and kset pair.
 *
 * @kobj:    kobject in question, with which to build the path
 * @gfp_mask:    the allocation type used to allocate the path
 *
 * The result must be freed by the caller with kfree().
 */
char *kobject_get_path(struct kobject *kobj, gfp_t gfp_mask)
{
    char *path;
    int len;

    len = get_kobj_path_length(kobj);
    if (len == )
        return NULL;
    path = kzalloc(len, gfp_mask);
    if (!path)
        return NULL;
    fill_kobj_path(kobj, path, len);

    return path;
}

在上面的三个函数中，前面两个函数是内部函数，get_kobj_path_length()函数用于获得kobj路径名的长度，fill_kobj_path()函数用于将kobj路径名填充到path缓冲区，然后将指针返回，kobject_get_path()函数就是调用这两个函数进行实现的。

/* add the kobject to its kset's list */
static void kobj_kset_join(struct kobject *kobj)
{
    if (!kobj->kset)
        return;

    kset_get(kobj->kset);//引用计数加1
    spin_lock(&kobj->kset->list_lock);
    list_add_tail(&kobj->entry, &kobj->kset->list);//在链表尾部插入节点
    spin_unlock(&kobj->kset->list_lock);
}

/* remove the kobject from its kset's list */
static void kobj_kset_leave(struct kobject *kobj)
{
    if (!kobj->kset)
        return;

    spin_lock(&kobj->kset->list_lock);
    list_del_init(&kobj->entry);//链表删除节点
    spin_unlock(&kobj->kset->list_lock);
    kset_put(kobj->kset);//引用计数减1
}

上面两个函数的功能是相对的，kobj_kset_join()用于将kobj加入到kobj->kset的链表中，kobj_kset_leave()用于将kobj从kobj->kset的链表中删除。

static void kobject_init_internal(struct kobject *kobj)
{
    if (!kobj)
        return;
    kref_init(&kobj->kref);//引用计数初始化
    INIT_LIST_HEAD(&kobj->entry);//链表初始化
    kobj->state_in_sysfs = ;
    kobj->state_add_uevent_sent = ;
    kobj->state_remove_uevent_sent = ;
    kobj->state_initialized = ;
}

static int kobject_add_internal(struct kobject *kobj)
{
    int error = ;
    struct kobject *parent;

    if (!kobj)
        return -ENOENT;

    if (!kobj->name || !kobj->name[]) {
        WARN(, "kobject: (%p): attempted to be registered with empty "
             "name!\n", kobj);
        return -EINVAL;
    }

    parent = kobject_get(kobj->parent);//父kobject引用计数加1

    /* join kset if set, use it as parent if we do not already have one */
    if (kobj->kset) {
        if (!parent)
            parent = kobject_get(&kobj->kset->kobj);
        kobj_kset_join(kobj);
        kobj->parent = parent;//设置kobject的parent指针
    }

    pr_debug("kobject: '%s' (%p): %s: parent: '%s', set: '%s'\n",
         kobject_name(kobj), kobj, __func__,
         parent ? kobject_name(parent) : "<NULL>",
         kobj->kset ? kobject_name(&kobj->kset->kobj) : "<NULL>");

    error = create_dir(kobj);//在sysfs中创建目录
    if (error) {
        kobj_kset_leave(kobj);
        kobject_put(parent);
        kobj->parent = NULL;

        /* be noisy on error issues */
        if (error == -EEXIST)
            pr_err("%s failed for %s with -EEXIST, don't try to register things with the same name in the same directory.\n",
                   __func__, kobject_name(kobj));
        else
            pr_err("%s failed for %s (error: %d parent: %s)\n",
                   __func__, kobject_name(kobj), error,
                   parent ? kobject_name(parent) : "'none'");
    } else
        kobj->state_in_sysfs = ;

    return error;
}

kobject_init_internal()函数用来初始化kobj，kobject_add_internal()函数用来将kobj加入已有的结构中去，这两个函数有密切的联系，在kobject结构体中有很多变量，但是重要的只有两个，一个是kset，一个是parent，这两个变量表示了kobject在整个体系中的位置，决不能自行决定，需要外部参与设置，kobject的创建过程分为init和add两个阶段。kobject_init_internal()把一些结构体变量进行初始化后，等外界设置了parent和kset后，再调用kobject_add_internal()把kobject添加到适当的位置，并创建相应的sysfs目录及文件。

/**
 * kobject_set_name_vargs - Set the name of an kobject
 * @kobj: struct kobject to set the name of
 * @fmt: format string used to build the name
 * @vargs: vargs to format the string.
 */
int kobject_set_name_vargs(struct kobject *kobj, const char *fmt,
                  va_list vargs)
{
    const char *s;

    if (kobj->name && !fmt)
        return ;

    s = kvasprintf_const(GFP_KERNEL, fmt, vargs);
    if (!s)
        return -ENOMEM;

    /*
     * ewww... some of these buggers have '/' in the name ... If
     * that's the case, we need to make sure we have an actual
     * allocated copy to modify, since kvasprintf_const may have
     * returned something from .rodata.
     */
    if (strchr(s, '/')) {
        char *t;

        t = kstrdup(s, GFP_KERNEL);
        kfree_const(s);
        if (!t)
            return -ENOMEM;
        strreplace(t, '/', '!');
        s = t;
    }
    kfree_const(kobj->name);
    kobj->name = s;

    return ;
}

/**
 * kobject_set_name - Set the name of a kobject
 * @kobj: struct kobject to set the name of
 * @fmt: format string used to build the name
 *
 * This sets the name of the kobject.  If you have already added the
 * kobject to the system, you must call kobject_rename() in order to
 * change the name of the kobject.
 */
int kobject_set_name(struct kobject *kobj, const char *fmt, ...)
{
    va_list vargs;
    int retval;

    va_start(vargs, fmt);
    retval = kobject_set_name_vargs(kobj, fmt, vargs);
    va_end(vargs);

    return retval;
}

kobject_set_name()函数的作用是设置kobj的名称，它通过调用kobject_set_name_vargs()来实现，需要注意的时，kobject_set_name()仅限于kobject添加到体系之前使用，因为该函数只是修改了名字，并未通知到用户空间。

/**
 * kobject_init - initialize a kobject structure
 * @kobj: pointer to the kobject to initialize
 * @ktype: pointer to the ktype for this kobject.
 *
 * This function will properly initialize a kobject such that it can then
 * be passed to the kobject_add() call.
 *
 * After this function is called, the kobject MUST be cleaned up by a call
 * to kobject_put(), not by a call to kfree directly to ensure that all of
 * the memory is cleaned up properly.
 */
void kobject_init(struct kobject *kobj, struct kobj_type *ktype)
{
    char *err_str;

    if (!kobj) {
        err_str = "invalid kobject pointer!";
        goto error;
    }
    if (!ktype) {
        err_str = "must have a ktype to be initialized properly!\n";
        goto error;
    }
    if (kobj->state_initialized) {
        /* do not error out as sometimes we can recover */
        printk(KERN_ERR "kobject (%p): tried to init an initialized "
               "object, something is seriously wrong.\n", kobj);
        dump_stack();
    }

    kobject_init_internal(kobj);
    kobj->ktype = ktype;
    return;

error:
    printk(KERN_ERR "kobject (%p): %s\n", kobj, err_str);
    dump_stack();
}

kobject_init()函数就是调用了kobject_init_internal()函数进行一些结构体变量初始化，然后又设置了ktype结构体成员，ktype的作用是管理一些默认属性。

static int kobject_add_varg(struct kobject *kobj,
                       struct kobject *parent,
                       const char *fmt, va_list vargs)
{
    int retval;

    retval = kobject_set_name_vargs(kobj, fmt, vargs);
    if (retval) {
        printk(KERN_ERR "kobject: can not set name properly!\n");
        return retval;
    }
    kobj->parent = parent;
    return kobject_add_internal(kobj);
}

/**
 * kobject_add - the main kobject add function
 * @kobj: the kobject to add
 * @parent: pointer to the parent of the kobject.
 * @fmt: format to name the kobject with.
 *
 * The kobject name is set and added to the kobject hierarchy in this
 * function.
 *
 * If @parent is set, then the parent of the @kobj will be set to it.
 * If @parent is NULL, then the parent of the @kobj will be set to the
 * kobject associated with the kset assigned to this kobject.  If no kset
 * is assigned to the kobject, then the kobject will be located in the
 * root of the sysfs tree.
 *
 * If this function returns an error, kobject_put() must be called to
 * properly clean up the memory associated with the object.
 * Under no instance should the kobject that is passed to this function
 * be directly freed with a call to kfree(), that can leak memory.
 *
 * Note, no "add" uevent will be created with this call, the caller should set
 * up all of the necessary sysfs files for the object and then call
 * kobject_uevent() with the UEVENT_ADD parameter to ensure that
 * userspace is properly notified of this kobject's creation.
 */
int kobject_add(struct kobject *kobj, struct kobject *parent,
        const char *fmt, ...)
{
    va_list args;
    int retval;

    if (!kobj)
        return -EINVAL;

    if (!kobj->state_initialized) {
        printk(KERN_ERR "kobject '%s' (%p): tried to add an "
               "uninitialized object, something is seriously wrong.\n",
               kobject_name(kobj), kobj);
        dump_stack();
        return -EINVAL;
    }
    va_start(args, fmt);
    retval = kobject_add_varg(kobj, parent, fmt, args);
    va_end(args);

    return retval;
}

kobject_add()函数用于将kobj添加到体系中去，但是该函数还有一个附加的功能，设置kobj的名字，parent作为参数被传进来。

/**
 * kobject_init_and_add - initialize a kobject structure and add it to the kobject hierarchy
 * @kobj: pointer to the kobject to initialize
 * @ktype: pointer to the ktype for this kobject.
 * @parent: pointer to the parent of this kobject.
 * @fmt: the name of the kobject.
 *
 * This function combines the call to kobject_init() and
 * kobject_add().  The same type of error handling after a call to
 * kobject_add() and kobject lifetime rules are the same here.
 */
int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype,
             struct kobject *parent, const char *fmt, ...)
{
    va_list args;
    int retval;

    kobject_init(kobj, ktype);

    va_start(args, fmt);
    retval = kobject_add_varg(kobj, parent, fmt, args);
    va_end(args);

    return retval;
}

kobject_init_and_add()其实是kobjec_init()和kobject_add()的合并，将kobject初始化后并添加到kobject的层次结构中去。

/**
 * kobject_rename - change the name of an object
 * @kobj: object in question.
 * @new_name: object's new name
 *
 * It is the responsibility of the caller to provide mutual
 * exclusion between two different calls of kobject_rename
 * on the same kobject and to ensure that new_name is valid and
 * won't conflict with other kobjects.
 */
int kobject_rename(struct kobject *kobj, const char *new_name)
{
    int error = ;
    const char *devpath = NULL;
    const char *dup_name = NULL, *name;
    char *devpath_string = NULL;
    char *envp[];

    kobj = kobject_get(kobj);
    if (!kobj)
        return -EINVAL;
    if (!kobj->parent)
        return -EINVAL;

    devpath = kobject_get_path(kobj, GFP_KERNEL);
    if (!devpath) {
        error = -ENOMEM;
        goto out;
    }
    devpath_string = kmalloc(strlen(devpath) + , GFP_KERNEL);
    if (!devpath_string) {
        error = -ENOMEM;
        goto out;
    }
    sprintf(devpath_string, "DEVPATH_OLD=%s", devpath);
    envp[] = devpath_string;
    envp[] = NULL;

    name = dup_name = kstrdup_const(new_name, GFP_KERNEL);
    if (!name) {
        error = -ENOMEM;
        goto out;
    }

    error = sysfs_rename_dir_ns(kobj, new_name, kobject_namespace(kobj));
    if (error)
        goto out;

    /* Install the new kobject name */
    dup_name = kobj->name;
    kobj->name = name;

    /* This function is mostly/only used for network interface.
     * Some hotplug package track interfaces by their name and
     * therefore want to know when the name is changed by the user. */
    kobject_uevent_env(kobj, KOBJ_MOVE, envp);

out:
    kfree_const(dup_name);
    kfree(devpath_string);
    kfree(devpath);
    kobject_put(kobj);

    return error;
}

kobject_rename()函数实现的功能为，当kobj已经添加到系统后，可以使用此函数调用实现更改kobject的名字，它除了能完成kobject_set_name()的功能以外，还向用户空间通知这一消息。

/**
 * kobject_move - move object to another parent
 * @kobj: object in question.
 * @new_parent: object's new parent (can be NULL)
 */
int kobject_move(struct kobject *kobj, struct kobject *new_parent)
{
    int error;
    struct kobject *old_parent;
    const char *devpath = NULL;
    char *devpath_string = NULL;
    char *envp[];

    kobj = kobject_get(kobj);
    if (!kobj)
        return -EINVAL;
    new_parent = kobject_get(new_parent);
    if (!new_parent) {
        if (kobj->kset)
            new_parent = kobject_get(&kobj->kset->kobj);
    }

    /* old object path */
    devpath = kobject_get_path(kobj, GFP_KERNEL);
    if (!devpath) {
        error = -ENOMEM;
        goto out;
    }
    devpath_string = kmalloc(strlen(devpath) + , GFP_KERNEL);
    if (!devpath_string) {
        error = -ENOMEM;
        goto out;
    }
    sprintf(devpath_string, "DEVPATH_OLD=%s", devpath);
    envp[] = devpath_string;
    envp[] = NULL;
    error = sysfs_move_dir_ns(kobj, new_parent, kobject_namespace(kobj));
    if (error)
        goto out;
    old_parent = kobj->parent;
    kobj->parent = new_parent;
    new_parent = NULL;
    kobject_put(old_parent);
    kobject_uevent_env(kobj, KOBJ_MOVE, envp);
out:
    kobject_put(new_parent);
    kobject_put(kobj);
    kfree(devpath_string);
    kfree(devpath);
    return error;
}

kobjec_move()函数实现的功能是在kobj添加到系统后，调用此函数能将kobj移动到新的parent下，同时会通知用户空间。

/**
 * kobject_del - unlink kobject from hierarchy.
 * @kobj: object.
 */
void kobject_del(struct kobject *kobj)
{
    struct kernfs_node *sd;

    if (!kobj)
        return;

    sd = kobj->sd;
    sysfs_remove_dir(kobj);
    sysfs_put(sd);

    kobj->state_in_sysfs = ;
    kobj_kset_leave(kobj);
    kobject_put(kobj->parent);
    kobj->parent = NULL;
}

kobject_del()函数用于将kobj从系统中删除，和kobject_add()是相对的操作。

/**
 * kobject_get - increment refcount for object.
 * @kobj: object.
 */
struct kobject *kobject_get(struct kobject *kobj)
{
    if (kobj) {
        if (!kobj->state_initialized)
            WARN(, 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;
}
/*
 * kobject_cleanup - free kobject resources.
 * @kobj: object to cleanup
 */
static void kobject_cleanup(struct kobject *kobj)
{
    struct kobj_type *t = get_ktype(kobj);
    const char *name = kobj->name;

    pr_debug("kobject: '%s' (%p): %s, parent %p\n",
         kobject_name(kobj), kobj, __func__, kobj->parent);

    if (t && !t->release)
        pr_debug("kobject: '%s' (%p): does not have a release() "
             "function, it is broken and must be fixed.\n",
             kobject_name(kobj), kobj);

    /* send "remove" if the caller did not do it but sent "add" */
    if (kobj->state_add_uevent_sent && !kobj->state_remove_uevent_sent) {
        pr_debug("kobject: '%s' (%p): auto cleanup 'remove' event\n",
             kobject_name(kobj), kobj);
        kobject_uevent(kobj, KOBJ_REMOVE);
    }

    /* remove from sysfs if the caller did not do it */
    if (kobj->state_in_sysfs) {
        pr_debug("kobject: '%s' (%p): auto cleanup kobject_del\n",
             kobject_name(kobj), kobj);
        kobject_del(kobj);
    }

    if (t && t->release) {
        pr_debug("kobject: '%s' (%p): calling ktype release\n",
             kobject_name(kobj), kobj);
        t->release(kobj);
    }

    /* free name if we allocated it */
    if (name) {
        pr_debug("kobject: '%s': free name\n", name);
        kfree_const(name);
    }
}

#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
static void kobject_delayed_cleanup(struct work_struct *work)
{
    kobject_cleanup(container_of(to_delayed_work(work),
                     struct kobject, release));
}
#endif

static void kobject_release(struct kref *kref)
{
    struct kobject *kobj = container_of(kref, struct kobject, kref);
#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
    unsigned long delay = HZ + HZ * (get_random_int() & 0x3);
    pr_info("kobject: '%s' (%p): %s, parent %p (delayed %ld)\n",
         kobject_name(kobj), kobj, __func__, kobj->parent, delay);
    INIT_DELAYED_WORK(&kobj->release, kobject_delayed_cleanup);

    schedule_delayed_work(&kobj->release, delay);
#else
    kobject_cleanup(kobj);
#endif
}

/**
 * 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(, 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);
    }
}

kobject_get()和kobject_put()函数完成的是kobject的引用计数功能，kobject_put()函数在当引用计数为0时，撤销整个kobject的存在：向用户空间发送REMOVE信息，从sysfs中删除相应的目录，并调用ktype中定义的release函数，并释放name所占用的空间。kobject_cleanup()函数用来释放kobject创建时所分配的资源。

上述提到到API接口，基本上概括了kobject从创建到删除，包括kobject的名字修改、位置修改以及引用计数的变动等功能。但是，kobject的创建仍然比较麻烦，因为ktype需要自己去实现，下面是Linux内核中为kobject提供的一种快速创建的方法：

首先是kobject的属性文件的读写函数定义：

/* default kobject attribute operations */
static ssize_t kobj_attr_show(struct kobject *kobj, struct attribute *attr,
                  char *buf)
{
    struct kobj_attribute *kattr;
    ssize_t ret = -EIO;

    kattr = container_of(attr, struct kobj_attribute, attr);
    if (kattr->show)
        ret = kattr->show(kobj, kattr, buf);
    return ret;
}

static ssize_t kobj_attr_store(struct kobject *kobj, struct attribute *attr,
                   const char *buf, size_t count)
{
    struct kobj_attribute *kattr;
    ssize_t ret = -EIO;

    kattr = container_of(attr, struct kobj_attribute, attr);
    if (kattr->store)
        ret = kattr->store(kobj, kattr, buf, count);
    return ret;
}

const struct sysfs_ops kobj_sysfs_ops = {
    .show    = kobj_attr_show,
    .store    = kobj_attr_store,
};

然后是，kobject的释放函数，以及kobj_type结构体变量的定义，如下：

static void dynamic_kobj_release(struct kobject *kobj)
{
    pr_debug("kobject: (%p): %s\n", kobj, __func__);
    kfree(kobj);
}

static struct kobj_type dynamic_kobj_ktype = {
    .release    = dynamic_kobj_release,
    .sysfs_ops    = &kobj_sysfs_ops,
};

这是Linux内核里面为kobject自身提供的一种kobj_type，名称叫做dynamic_kobj_type，它并没有提供默认的属性，但是提供了release函数和访问属性的方法。

/**
 * kobject_create - create a struct kobject dynamically
 *
 * This function creates a kobject structure dynamically and sets it up
 * to be a "dynamic" kobject with a default release function set up.
 *
 * If the kobject was not able to be created, NULL will be returned.
 * The kobject structure returned from here must be cleaned up with a
 * call to kobject_put() and not kfree(), as kobject_init() has
 * already been called on this structure.
 */
struct kobject *kobject_create(void)
{
    struct kobject *kobj;

    kobj = kzalloc(sizeof(*kobj), GFP_KERNEL);
    if (!kobj)
        return NULL;

    kobject_init(kobj, &dynamic_kobj_ktype);
    return kobj;
}

/**
 * kobject_create_and_add - create a struct kobject dynamically and register it with sysfs
 *
 * @name: the name for the kobject
 * @parent: the parent kobject of this kobject, if any.
 *
 * This function creates a kobject structure dynamically and registers it
 * with sysfs.  When you are finished with this structure, call
 * kobject_put() and the structure will be dynamically freed when
 * it is no longer being used.
 *
 * If the kobject was not able to be created, NULL will be returned.
 */
struct kobject *kobject_create_and_add(const char *name, struct kobject *parent)
{
    struct kobject *kobj;
    int retval;

    kobj = kobject_create();
    if (!kobj)
        return NULL;

    retval = kobject_add(kobj, parent, "%s", name);
    if (retval) {
        printk(KERN_WARNING "%s: kobject_add error: %d\n",
               __func__, retval);
        kobject_put(kobj);
        kobj = NULL;
    }
    return kobj;
}

在kobject_create()和kobject_create_add()函数中，使用了dynamic_kobj_ktype这个结构体变量，这是一种很好的偷懒方式，因为release()函数会释放掉kobj，所以这里的kobj必须是动态创建的，这里的kobject_create()和kobject_init()相对，kobject_create_and_add()和kobject_init_and_add()相对。需要注意的是，这里创建的kobject无法嵌入到其它结构，是独立存在的，所以用到的地方很少。

/**
 * kset_init - initialize a kset for use
 * @k: kset
 */
void kset_init(struct kset *k)
{
    kobject_init_internal(&k->kobj);
    INIT_LIST_HEAD(&k->list);
    spin_lock_init(&k->list_lock);
}

kset_init()函数用于将kset结构体初始化，和kobject初始化类似。

/**
 * kset_register - initialize and add a kset.
 * @k: kset.
 */
int kset_register(struct kset *k)
{
    int err;

    if (!k)
        return -EINVAL;

    kset_init(k);
    err = kobject_add_internal(&k->kobj);
    if (err)
        return err;
    kobject_uevent(&k->kobj, KOBJ_ADD);
    return ;
}

kset_register()函数用来初始化并添加到kset，函数比较简单，它负责将kset中的kobject添加到系统，并向用户空间发布KOBJ_ADD消息，所以在调用之前，需要先设置好k->kobj.name、k->kobj.parent和k->kobj.kset这些成员。

/**
 * kset_unregister - remove a kset.
 * @k: kset.
 */
void kset_unregister(struct kset *k)
{
    if (!k)
        return;
    kobject_del(&k->kobj);
    kobject_put(&k->kobj);
}

kset_unregister()和kset_register()函数功能是相对的，用于移除kset，并减少引用计数。

/**
 * kset_find_obj - search for object in kset.
 * @kset: kset we're looking in.
 * @name: object's name.
 *
 * Lock kset via @kset->subsys, and iterate over @kset->list,
 * looking for a matching kobject. If matching object is found
 * take a reference and return the object.
 */
struct kobject *kset_find_obj(struct kset *kset, const char *name)
{
    struct kobject *k;
    struct kobject *ret = NULL;

    spin_lock(&kset->list_lock);

    list_for_each_entry(k, &kset->list, entry) {
        if (kobject_name(k) && !strcmp(kobject_name(k), name)) {
            ret = kobject_get_unless_zero(k);
            break;
        }
    }

    spin_unlock(&kset->list_lock);
    return ret;
}

kset_find_obj()函数功能为通过从kset的链表中寻找名为name的kobject，实现了链表的遍历。

与kobject类似，kset也提供看一种kobj_type，名称为kset_ktype，实现如下：

static void kset_release(struct kobject *kobj)
{
    struct kset *kset = container_of(kobj, struct kset, kobj);
    pr_debug("kobject: '%s' (%p): %s\n",
         kobject_name(kobj), kobj, __func__);
    kfree(kset);
}

void kset_get_ownership(struct kobject *kobj, kuid_t *uid, kgid_t *gid)
{
    if (kobj->parent)
        kobject_get_ownership(kobj->parent, uid, gid);
}

static struct kobj_type kset_ktype = {
    .sysfs_ops    = &kobj_sysfs_ops,
    .release    = kset_release,
    .get_ownership    = kset_get_ownership,
};

/**
 * kset_create - create a struct kset dynamically
 *
 * @name: the name for the kset
 * @uevent_ops: a struct kset_uevent_ops for the kset
 * @parent_kobj: the parent kobject of this kset, if any.
 *
 * This function creates a kset structure dynamically.  This structure can
 * then be registered with the system and show up in sysfs with a call to
 * kset_register().  When you are finished with this structure, if
 * kset_register() has been called, call kset_unregister() and the
 * structure will be dynamically freed when it is no longer being used.
 *
 * If the kset was not able to be created, NULL will be returned.
 */
static struct kset *kset_create(const char *name,
                const struct kset_uevent_ops *uevent_ops,
                struct kobject *parent_kobj)
{
    struct kset *kset;
    int retval;

    kset = kzalloc(sizeof(*kset), GFP_KERNEL);
    if (!kset)
        return NULL;
    retval = kobject_set_name(&kset->kobj, "%s", name);
    if (retval) {
        kfree(kset);
        return NULL;
    }
    kset->uevent_ops = uevent_ops;
    kset->kobj.parent = parent_kobj;

    /*
     * The kobject of this kset will have a type of kset_ktype and belong to
     * no kset itself.  That way we can properly free it when it is
     * finished being used.
     */
    kset->kobj.ktype = &kset_ktype;
    kset->kobj.kset = NULL;

    return kset;
}

/**
 * kset_create_and_add - create a struct kset dynamically and add it to sysfs
 *
 * @name: the name for the kset
 * @uevent_ops: a struct kset_uevent_ops for the kset
 * @parent_kobj: the parent kobject of this kset, if any.
 *
 * This function creates a kset structure dynamically and registers it
 * with sysfs.  When you are finished with this structure, call
 * kset_unregister() and the structure will be dynamically freed when it
 * is no longer being used.
 *
 * If the kset was not able to be created, NULL will be returned.
 */
struct kset *kset_create_and_add(const char *name,
                 const struct kset_uevent_ops *uevent_ops,
                 struct kobject *parent_kobj)
{
    struct kset *kset;
    int error;

    kset = kset_create(name, uevent_ops, parent_kobj);
    if (!kset)
        return NULL;
    error = kset_register(kset);
    if (error) {
        kfree(kset);
        return NULL;
    }
    return kset;
}

kset_create()和kset_create_and_add()就是使用kset_type来快速创建kset的函数，当需要在sysfs中创建单纯的目录时，可以使用这两个函数进行创建。

static inline struct kset *to_kset(struct kobject *kobj)
{
    return kobj ? container_of(kobj, struct kset, kobj) : NULL;
}

static inline struct kset *kset_get(struct kset *k)
{
    return k ? to_kset(kobject_get(&k->kobj)) : NULL;
}

static inline void kset_put(struct kset *k)
{
    kobject_put(&k->kobj);
}

static inline struct kobj_type *get_ktype(struct kobject *kobj)
{
    return kobj->ktype;
}

上面这些函数是在kobject.h文件里面的内联函数，to_kset()函数通过kobj的指针获取kset结构体的首地址，kset_get()和kset_put()为引用计数操作，get_ktype()函数用于获取kobject的ktype类型。

4、 kobject的层次结构

内核用kobject结构将各个对象连接起来组成一个分层的结构体系，从而与模型化的子系统相匹配。kobject结构体中的parent指针成员、kset指针成员和链表list_head实现了kobject的层次结构，parent指针最重要的用途是在sysfs分层结构中定位对象，下图是一个简单的kobject、kset的分层结构图：

5、小节

本文主要对kobject结构体以及相关的结构体以及Linux内核提供的相关API做了简要介绍，kobject结构体和设备驱动模型和sysfs的实现密切相关，必须深刻理解。

参考：

https://blog.csdn.net/yuanmengliang/article/details/52700529

https://blog.csdn.net/qb_2008/article/details/6846779

https://www.cnblogs.com/xiaojiang1025/p/6193959.html

《LINUX设备驱动程序（第三版）》