Linux存储IO栈（1）-- 内核对象与对象集

Linux内核对象和对象集

内核对象作为Linux设备驱动模型的基础，主要是抽象和封装总线、设备、驱动、类和接口之间的关系具体实现的相关代码，并在sysfs中呈现。主要抽象成kobject和kset结构：

struct kobject {
    const char      *name;   //在sysfs中显示的名称
    struct list_head    entry;   //链入kset的kobj链表
    struct kobject      *parent; //指向父kobject，用于表示树形结构
    struct kset     *kset;   //指向链入的kset
    struct kobj_type    *ktype;  //抽象kobject的通用方法和属性
    struct kernfs_node  *sd;     //sysfs directory entry 
    struct kref     kref;    //引用计数 
#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
    struct delayed_work release; 
#endif
    unsigned int state_initialized:1;  //是否被初始化
    unsigned int state_in_sysfs:1;     //是否被添加到sysfs
    unsigned int state_add_uevent_sent:1; //是否发送ADD事件到用户空间
    unsigned int state_remove_uevent_sent:1; //是否发送REMOVE事件到用户空间
    unsigned int uevent_suppress:1; //事件是否被抑制
};

在kobject结构中ktype域是对kobject一些通用方法和属性进行封装：

struct kobj_type {
    void (*release)(struct kobject *kobj); //释放kobject结构时回调
    const struct sysfs_ops *sysfs_ops; //sysfs的操作函数
    struct attribute **default_attrs;  //默认属性
        //命名空间相关操作
    const struct kobj_ns_type_operations *(*child_ns_type)(struct kobject *kobj);
    const void *(*namespace)(struct kobject *kobj);
};

kset是一组kobject的集合，通过kset可以遍历这组kobject，如SCSI子系统中，设备是一种kobject，通过设备集kset，可以遍历所有的设备。

/**
 * 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; //链入kset的kobject链表
    spinlock_t list_lock;  //遍历链表是的自旋锁
    struct kobject kobj;   //本身可以当做kobject对待
    const struct kset_uevent_ops *uevent_ops; //发送uevent事件的回调函数
};

在发送事件到用户空间时，可以回调kset_uevent_ops中的3个回调函数

struct kset_uevent_ops {
    int (* const filter)(struct kset *kset, struct kobject *kobj);
    const char *(* const name)(struct kset *kset, struct kobject *kobj);
    int (* const uevent)(struct kset *kset, struct kobject *kobj,
              struct kobj_uevent_env *env);
};

• filter：在发送事件之前的过滤某些事件。
• name: 获取名称。
• uevent：设置uevent需要的环境变量。

内核对象相关操作

void kobject_init(struct kobject *kobj, struct kobj_type *ktype);
int kobject_add(struct kobject *kobj, struct kobject *parent, const char *fmt, ...);
int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype, struct kobject *parent, const char *fmt, ...);
void kobject_del(struct kobject *kobj);
struct kobject *  kobject_create(void);
struct kobject * kobject_create_and_add(const char *name, struct kobject *parent);
int kobject_rename(struct kobject *, const char *new_name);
int kobject_move(struct kobject *, struct kobject *);
struct kobject *kobject_get(struct kobject *kobj);
void kobject_put(struct kobject *kobj);
const void *kobject_namespace(struct kobject *kobj);
char *kobject_get_path(struct kobject *kobj, gfp_t flag);

内核对象创建及初始化

初始化流程主要在kobject_init:

/**
 * 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) {  //避免重复初始化
        /* 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); //完成初始化的主要函数
    kobj->ktype = ktype;
    return;

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

由上面函数可以看出由kobject_init_internal完成初始化：

static void kobject_init_internal(struct kobject *kobj)
{
    if (!kobj)
        return;
    kref_init(&kobj->kref);
    INIT_LIST_HEAD(&kobj->entry);
    kobj->state_in_sysfs = 0;
    kobj->state_add_uevent_sent = 0;
    kobj->state_remove_uevent_sent = 0;
    kobj->state_initialized = 1;
}

kobject_create函数仅仅是在调用kobject_init之前，先分配kobject空间。在kobject初始化之后，需要调用kobject_add将kobject添加到sysfs中。

/**
 * 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) { //add之前需要初始化
        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); //主要完成add操作
    va_end(args);

    return retval;
}
EXPORT_SYMBOL(kobject_add);

kobject_add_varg/kobject_add_internal主要完成将kobject添加到sysfs的操作：

static __printf(3, 0) int kobject_add_varg(struct kobject *kobj,
                       struct kobject *parent,
                       const char *fmt, va_list vargs)
{
    int retval;
        //设置kobject在sysfs中显示的名称
    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); //主要实现函数
}

static int kobject_add_internal(struct kobject *kobj)
{
    int error = 0;
    struct kobject *parent;

    if (!kobj)
        return -ENOENT;

    if (!kobj->name || !kobj->name[0]) {
        WARN(1, "kobject: (%p): attempted to be registered with empty "
             "name!\n", kobj);
        return -EINVAL;
    }

    parent = kobject_get(kobj->parent); //增加父对象的引用计数

    /* join kset if set, use it as parent if we do not already have one */
    if (kobj->kset) { //如果设置了kset，而没有设置parent，则把kset的kobject设置为parent
        if (!parent)
            parent = kobject_get(&kobj->kset->kobj);
        kobj_kset_join(kobj);
        kobj->parent = parent;
    }

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

    error = create_dir(kobj);  //创建sysfs对应的目录和属性文件
    if (error) {  //出错回滚
        kobj_kset_leave(kobj);
        kobject_put(parent);
        kobj->parent = NULL;

        /* be noisy on error issues */
        if (error == -EEXIST)
            WARN(1, "%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
            WARN(1, "%s failed for %s (error: %d parent: %s)\n",
                 __func__, kobject_name(kobj), error,
                 parent ? kobject_name(parent) : "'none'");
    } else
        kobj->state_in_sysfs = 1; //更新标志位

    return error;
}

由create_dir在sysfs创建真实的目录和文件，这点有下一篇sysfs详细描述。理解了kobject_init和kobject_add之后，由名字可以知道下面函数kobject_init_and_add和kobject_create_and_add

内核对象释放

调用kobject_del将对kobject释放：

/**
 * 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); //删除kobject在sysfs中的目录
    sysfs_put(sd);

    kobj->state_in_sysfs = 0; //设置标志位
    kobj_kset_leave(kobj);  //kobject脱离kset链表
    kobject_put(kobj->parent); //调用kobject_release释放
    kobj->parent = NULL;
}
EXPORT_SYMBOL(kobject_del);

/**
 * 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);  //调用kobject_release
    }
}
EXPORT_SYMBOL(kobject_put);

static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
{
    return kref_sub(kref, 1, release);
}

static inline int kref_sub(struct kref *kref, unsigned int count,
         void (*release)(struct kref *kref))
{
    WARN_ON(release == NULL);

    if (atomic_sub_and_test((int) count, &kref->refcount)) {
        release(kref); //调用kobject_release
        return 1;
    }
    return 0;
}

根据上面的代码追踪，得知kobject_release才是释放kobject的主角：

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);
    //延迟调用kobject_delayed_cleanup进行清理
    schedule_delayed_work(&kobj->release, delay);
#else
    kobject_cleanup(kobj);  //清理
#endif
}

如果在内核编译时指定CONFIG_DEBUG_KOBJECT_RELEASE，则使用延迟release方式调用kobject_delayed_cleanup，否则直接调用kobject_cleanup。

#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
static void kobject_delayed_cleanup(struct work_struct *work)
{
    kobject_cleanup(container_of(to_delayed_work(work), //最终还是调用
                     struct kobject, release));
}
#endif

/*
 * 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); //仅仅发送一次REMOVE消息
    }

    /* 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); //如果调用者没有清理sysfs，则清理
    }

    if (t && t->release) {
        pr_debug("kobject: '%s' (%p): calling ktype release\n",
             kobject_name(kobj), kobj);
        t->release(kobj); //调用kobj_type的release回调函数
    }

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

内核对象集相关操作

void kset_init(struct kset *kset);
struct kset *kset_create(const char *name, const struct kset_uevent_ops *uevent_ops, struct kobject *parent_kobj);
int kset_register(struct kset *kset);
void kset_unregister(struct kset *kset);
struct kset * kset_create_and_add(const char *name, const struct kset_uevent_ops *u, struct kobject *parent_kobj);

内核对象集创建及初始化

内核对象集由kset_create创建

/**
 * 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);  //分配空间
    if (!kset)
        return NULL;
    retval = kobject_set_name(&kset->kobj, "%s", name); //设置kset在sysfs中的名字
    if (retval) {
        kfree(kset);
        return NULL;
    }
    kset->uevent_ops = uevent_ops;   //设置uevent_ops
    kset->kobj.parent = parent_kobj; //设置kset的父对象

    /*
     * 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;  //设置kobj_type
    kset->kobj.kset = NULL;

    return kset;
}

内核对象集由kset_init执行初始化：

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

static void kobject_init_internal(struct kobject *kobj)
{
    if (!kobj)
        return;
    kref_init(&kobj->kref);
    INIT_LIST_HEAD(&kobj->entry);
    kobj->state_in_sysfs = 0;        //设置对应标志位
    kobj->state_add_uevent_sent = 0;
    kobj->state_remove_uevent_sent = 0;
    kobj->state_initialized = 1;
}

初始化kset之后，调用kset_register，将kset添加到sysfs：

/**
 * 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); //完成register动作，前面已说明
    if (err)
        return err;
    kobject_uevent(&k->kobj, KOBJ_ADD); //发送ADD事件到用户空间
    return 0;
}
EXPORT_SYMBOL(kset_register);

经过kset_create, kset_init和kset_register之后，kset已初始化并添加完成。当然kset_create_and_add包含了这三个函数。

内核对象集释放

内核对象的释放过程与kobject的释放过程类似，由kset_unregister完成：

/**
 * kset_unregister - remove a kset.
 * @k: kset.
 */
void kset_unregister(struct kset *k)
{
    if (!k)
        return;
    kobject_del(&k->kobj);  //删除sysfs的目录和属性文件，前面已说明
    kobject_put(&k->kobj);  //与kobject释放过程一致
}
EXPORT_SYMBOL(kset_unregister);

发送事件到用户空间

由前面的代码可以看到无论kobject或是kset，都会向用户空间发送事件，由kobject_uevent函数通过设置环境变量的方式完成：

struct kobj_uevent_env {
    char *argv[3];                //user_helper使用的命令
    char *envp[UEVENT_NUM_ENVP];  //环境变量数组
    int envp_idx;                 //当前环境变量索引
    char buf[UEVENT_BUFFER_SIZE]; //环境变量数据缓冲区
    int buflen;
};

/**
 * kobject_uevent - notify userspace by sending an uevent
 *
 * @action: action that is happening
 * @kobj: struct kobject that the action is happening to
 *
 * Returns 0 if kobject_uevent() is completed with success or the
 * corresponding error when it fails.
 */
int kobject_uevent(struct kobject *kobj, enum kobject_action action)
{
    return kobject_uevent_env(kobj, action, NULL); //实际完成发送函数
}
EXPORT_SYMBOL_GPL(kobject_uevent);

/**
 * kobject_uevent_env - send an uevent with environmental data
 *
 * @action: action that is happening
 * @kobj: struct kobject that the action is happening to
 * @envp_ext: pointer to environmental data
 *
 * Returns 0 if kobject_uevent_env() is completed with success or the
 * corresponding error when it fails.
 */
int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
               char *envp_ext[])
{
    struct kobj_uevent_env *env;
    const char *action_string = kobject_actions[action];
    const char *devpath = NULL;
    const char *subsystem;
    struct kobject *top_kobj;
    struct kset *kset;
    const struct kset_uevent_ops *uevent_ops;
    int i = 0;
    int retval = 0;
#ifdef CONFIG_NET
    struct uevent_sock *ue_sk;
#endif

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

    /* search the kset we belong to */
    top_kobj = kobj;
    while (!top_kobj->kset && top_kobj->parent)  //寻找最近的kset，kset中有鍀event_ops
        top_kobj = top_kobj->parent;

    if (!top_kobj->kset) {
        pr_debug("kobject: '%s' (%p): %s: attempted to send uevent "
             "without kset!\n", kobject_name(kobj), kobj,
             __func__);
        return -EINVAL;
    }

    kset = top_kobj->kset;
    uevent_ops = kset->uevent_ops;  //使用kset中的uevent_ops执行发送操作

    /* skip the event, if uevent_suppress is set*/
    if (kobj->uevent_suppress) {  //跳过设置为uevent_suppress的kobject
        pr_debug("kobject: '%s' (%p): %s: uevent_suppress "
                 "caused the event to drop!\n",
                 kobject_name(kobj), kobj, __func__);
        return 0;
    }
    /* skip the event, if the filter returns zero. */
    if (uevent_ops && uevent_ops->filter)  //调用uevent_ops的filter函数
        if (!uevent_ops->filter(kset, kobj)) {
            pr_debug("kobject: '%s' (%p): %s: filter function "
                 "caused the event to drop!\n",
                 kobject_name(kobj), kobj, __func__);
            return 0;
        }

    /* originating subsystem */
    if (uevent_ops && uevent_ops->name)  //确定发送事件的kobject名字
        subsystem = uevent_ops->name(kset, kobj);
    else
        subsystem = kobject_name(&kset->kobj);
    if (!subsystem) {
        pr_debug("kobject: '%s' (%p): %s: unset subsystem caused the "
             "event to drop!\n", kobject_name(kobj), kobj,
             __func__);
        return 0;
    }

    /* environment buffer */
    env = kzalloc(sizeof(struct kobj_uevent_env), GFP_KERNEL); //分配kobj_uevent_env
    if (!env)
        return -ENOMEM;

    /* complete object path */
    devpath = kobject_get_path(kobj, GFP_KERNEL);
    if (!devpath) {
        retval = -ENOENT;
        goto exit;
    }

    /* default keys 添加环境变量 */
    retval = add_uevent_var(env, "ACTION=%s", action_string);
    if (retval)
        goto exit;
    retval = add_uevent_var(env, "DEVPATH=%s", devpath);
    if (retval)
        goto exit;
    retval = add_uevent_var(env, "SUBSYSTEM=%s", subsystem);
    if (retval)
        goto exit;

    /* keys passed in from the caller */
    if (envp_ext) {
        for (i = 0; envp_ext[i]; i++) {
            retval = add_uevent_var(env, "%s", envp_ext[i]);
            if (retval)
                goto exit;
        }
    }

    /* let the kset specific function add its stuff */
    if (uevent_ops && uevent_ops->uevent) { //调用uevent回调函数，添加子系统特定的环境变量
        retval = uevent_ops->uevent(kset, kobj, env);
        if (retval) {
            pr_debug("kobject: '%s' (%p): %s: uevent() returned "
                 "%d\n", kobject_name(kobj), kobj,
                 __func__, retval);
            goto exit;
        }
    }

    /*
     * Mark "add" and "remove" events in the object to ensure proper
     * events to userspace during automatic cleanup. If the object did
     * send an "add" event, "remove" will automatically generated by
     * the core, if not already done by the caller.
     */
    if (action == KOBJ_ADD)
        kobj->state_add_uevent_sent = 1;
    else if (action == KOBJ_REMOVE)
        kobj->state_remove_uevent_sent = 1;

    mutex_lock(&uevent_sock_mutex);
    /* we will send an event, so request a new sequence number */
    retval = add_uevent_var(env, "SEQNUM=%llu", (unsigned long long)++uevent_seqnum);
    if (retval) {
        mutex_unlock(&uevent_sock_mutex);
        goto exit;
    }

#if defined(CONFIG_NET)  //如果在编译时指定CONFIG_NET，使用netlink发送
    /* send netlink message */
    list_for_each_entry(ue_sk, &uevent_sock_list, list) {
        struct sock *uevent_sock = ue_sk->sk;
        struct sk_buff *skb;
        size_t len;

        if (!netlink_has_listeners(uevent_sock, 1))
            continue;

        /* allocate message with the maximum possible size */
        len = strlen(action_string) + strlen(devpath) + 2;
        skb = alloc_skb(len + env->buflen, GFP_KERNEL);
        if (skb) {
            char *scratch;

            /* add header */
            scratch = skb_put(skb, len);
            sprintf(scratch, "%s@%s", action_string, devpath);

            /* copy keys to our continuous event payload buffer */
            for (i = 0; i < env->envp_idx; i++) {
                len = strlen(env->envp[i]) + 1;
                scratch = skb_put(skb, len);
                strcpy(scratch, env->envp[i]);
            }

            NETLINK_CB(skb).dst_group = 1;
            retval = netlink_broadcast_filtered(uevent_sock, skb, //使用netlink多播发送
                                0, 1, GFP_KERNEL,
                                kobj_bcast_filter,
                                kobj);
            /* ENOBUFS should be handled in userspace */
            if (retval == -ENOBUFS || retval == -ESRCH)
                retval = 0;
        } else
            retval = -ENOMEM;
    }
#endif
    mutex_unlock(&uevent_sock_mutex);

#ifdef CONFIG_UEVENT_HELPER  //不能使用netlink时，使用user_helper发送
    /* call uevent_helper, usually only enabled during early boot */
    if (uevent_helper[0] && !kobj_usermode_filter(kobj)) {
        struct subprocess_info *info;

        retval = add_uevent_var(env, "HOME=/");
        if (retval)
            goto exit;
        retval = add_uevent_var(env,
                    "PATH=/sbin:/bin:/usr/sbin:/usr/bin");
        if (retval)
            goto exit;
        retval = init_uevent_argv(env, subsystem); //组装需要调用的用户空间命令和参数
        if (retval)
            goto exit;

        retval = -ENOMEM;
        info = call_usermodehelper_setup(env->argv[0], env->argv,  //调用用户空间程序/sbin/hotplug
                         env->envp, GFP_KERNEL,
                         NULL, cleanup_uevent_env, env);
        if (info) {
            retval = call_usermodehelper_exec(info, UMH_NO_WAIT);
            env = NULL; /* freed by cleanup_uevent_env */
        }
    }
#endif

exit:
    kfree(devpath);
    kfree(env);
    return retval;
}
EXPORT_SYMBOL_GPL(kobject_uevent_env);