perf record: Fix segfault with --no-mmap-pages

[linux/fpc-iii.git] / fs / sysfs / file.c

/*
 * fs/sysfs/file.c - sysfs regular (text) file implementation
 *
 * Copyright (c) 2001-3 Patrick Mochel
 * Copyright (c) 2007 SUSE Linux Products GmbH
 * Copyright (c) 2007 Tejun Heo <teheo@suse.de>
 *
 * This file is released under the GPLv2.
 *
 * Please see Documentation/filesystems/sysfs.txt for more information.
 */

#include <linux/module.h>
#include <linux/kobject.h>
#include <linux/kallsyms.h>
#include <linux/slab.h>
#include <linux/fsnotify.h>
#include <linux/namei.h>
#include <linux/poll.h>
#include <linux/list.h>
#include <linux/mutex.h>
#include <linux/limits.h>
#include <linux/uaccess.h>

#include "sysfs.h"

/*
 * There's one sysfs_buffer for each open file and one
 * sysfs_open_dirent for each sysfs_dirent with one or more open
 * files.
 *
 * filp->private_data points to sysfs_buffer and
 * sysfs_dirent->s_attr.open points to sysfs_open_dirent.  s_attr.open
 * is protected by sysfs_open_dirent_lock.
 */
static DEFINE_SPINLOCK(sysfs_open_dirent_lock);

struct sysfs_open_dirent {
        atomic_t                refcnt;
        atomic_t                event;
        wait_queue_head_t       poll;
        struct list_head        buffers; /* goes through sysfs_buffer.list */
};

struct sysfs_buffer {
        size_t                  count;
        loff_t                  pos;
        char                    *page;
        const struct sysfs_ops  *ops;
        struct mutex            mutex;
        int                     needs_read_fill;
        int                     event;
        struct list_head        list;
};

/**
 *      fill_read_buffer - allocate and fill buffer from object.
 *      @dentry:        dentry pointer.
 *      @buffer:        data buffer for file.
 *
 *      Allocate @buffer->page, if it hasn't been already, then call the
 *      kobject's show() method to fill the buffer with this attribute's
 *      data.
 *      This is called only once, on the file's first read unless an error
 *      is returned.
 */
static int fill_read_buffer(struct dentry *dentry, struct sysfs_buffer *buffer)
{
        struct sysfs_dirent *attr_sd = dentry->d_fsdata;
        struct kobject *kobj = attr_sd->s_parent->s_dir.kobj;
        const struct sysfs_ops *ops = buffer->ops;
        int ret = 0;
        ssize_t count;

        if (!buffer->page)
                buffer->page = (char *) get_zeroed_page(GFP_KERNEL);
        if (!buffer->page)
                return -ENOMEM;

        /* need attr_sd for attr and ops, its parent for kobj */
        if (!sysfs_get_active(attr_sd))
                return -ENODEV;

        buffer->event = atomic_read(&attr_sd->s_attr.open->event);
        count = ops->show(kobj, attr_sd->s_attr.attr, buffer->page);

        sysfs_put_active(attr_sd);

        /*
         * The code works fine with PAGE_SIZE return but it's likely to
         * indicate truncated result or overflow in normal use cases.
         */
        if (count >= (ssize_t)PAGE_SIZE) {
                print_symbol("fill_read_buffer: %s returned bad count\n",
                        (unsigned long)ops->show);
                /* Try to struggle along */
                count = PAGE_SIZE - 1;
        }
        if (count >= 0) {
                buffer->needs_read_fill = 0;
                buffer->count = count;
        } else {
                ret = count;
        }
        return ret;
}

/**
 *      sysfs_read_file - read an attribute.
 *      @file:  file pointer.
 *      @buf:   buffer to fill.
 *      @count: number of bytes to read.
 *      @ppos:  starting offset in file.
 *
 *      Userspace wants to read an attribute file. The attribute descriptor
 *      is in the file's ->d_fsdata. The target object is in the directory's
 *      ->d_fsdata.
 *
 *      We call fill_read_buffer() to allocate and fill the buffer from the
 *      object's show() method exactly once (if the read is happening from
 *      the beginning of the file). That should fill the entire buffer with
 *      all the data the object has to offer for that attribute.
 *      We then call flush_read_buffer() to copy the buffer to userspace
 *      in the increments specified.
 */

static ssize_t
sysfs_read_file(struct file *file, char __user *buf, size_t count, loff_t *ppos)
{
        struct sysfs_buffer *buffer = file->private_data;
        ssize_t retval = 0;

        mutex_lock(&buffer->mutex);
        if (buffer->needs_read_fill || *ppos == 0) {
                retval = fill_read_buffer(file->f_path.dentry, buffer);
                if (retval)
                        goto out;
        }
        pr_debug("%s: count = %zd, ppos = %lld, buf = %s\n",
                 __func__, count, *ppos, buffer->page);
        retval = simple_read_from_buffer(buf, count, ppos, buffer->page,
                                         buffer->count);
out:
        mutex_unlock(&buffer->mutex);
        return retval;
}

/**
 *      fill_write_buffer - copy buffer from userspace.
 *      @buffer:        data buffer for file.
 *      @buf:           data from user.
 *      @count:         number of bytes in @userbuf.
 *
 *      Allocate @buffer->page if it hasn't been already, then
 *      copy the user-supplied buffer into it.
 */
static int fill_write_buffer(struct sysfs_buffer *buffer,
                             const char __user *buf, size_t count)
{
        int error;

        if (!buffer->page)
                buffer->page = (char *)get_zeroed_page(GFP_KERNEL);
        if (!buffer->page)
                return -ENOMEM;

        if (count >= PAGE_SIZE)
                count = PAGE_SIZE - 1;
        error = copy_from_user(buffer->page, buf, count);
        buffer->needs_read_fill = 1;
        /* if buf is assumed to contain a string, terminate it by \0,
           so e.g. sscanf() can scan the string easily */
        buffer->page[count] = 0;
        return error ? -EFAULT : count;
}


/**
 *      flush_write_buffer - push buffer to kobject.
 *      @dentry:        dentry to the attribute
 *      @buffer:        data buffer for file.
 *      @count:         number of bytes
 *
 *      Get the correct pointers for the kobject and the attribute we're
 *      dealing with, then call the store() method for the attribute,
 *      passing the buffer that we acquired in fill_write_buffer().
 */
static int flush_write_buffer(struct dentry *dentry,
                              struct sysfs_buffer *buffer, size_t count)
{
        struct sysfs_dirent *attr_sd = dentry->d_fsdata;
        struct kobject *kobj = attr_sd->s_parent->s_dir.kobj;
        const struct sysfs_ops *ops = buffer->ops;
        int rc;

        /* need attr_sd for attr and ops, its parent for kobj */
        if (!sysfs_get_active(attr_sd))
                return -ENODEV;

        rc = ops->store(kobj, attr_sd->s_attr.attr, buffer->page, count);

        sysfs_put_active(attr_sd);

        return rc;
}


/**
 *      sysfs_write_file - write an attribute.
 *      @file:  file pointer
 *      @buf:   data to write
 *      @count: number of bytes
 *      @ppos:  starting offset
 *
 *      Similar to sysfs_read_file(), though working in the opposite direction.
 *      We allocate and fill the data from the user in fill_write_buffer(),
 *      then push it to the kobject in flush_write_buffer().
 *      There is no easy way for us to know if userspace is only doing a partial
 *      write, so we don't support them. We expect the entire buffer to come
 *      on the first write.
 *      Hint: if you're writing a value, first read the file, modify only the
 *      the value you're changing, then write entire buffer back.
 */
static ssize_t sysfs_write_file(struct file *file, const char __user *buf,
                                size_t count, loff_t *ppos)
{
        struct sysfs_buffer *buffer = file->private_data;
        ssize_t len;

        mutex_lock(&buffer->mutex);
        len = fill_write_buffer(buffer, buf, count);
        if (len > 0)
                len = flush_write_buffer(file->f_path.dentry, buffer, len);
        if (len > 0)
                *ppos += len;
        mutex_unlock(&buffer->mutex);
        return len;
}

/**
 *      sysfs_get_open_dirent - get or create sysfs_open_dirent
 *      @sd: target sysfs_dirent
 *      @buffer: sysfs_buffer for this instance of open
 *
 *      If @sd->s_attr.open exists, increment its reference count;
 *      otherwise, create one.  @buffer is chained to the buffers
 *      list.
 *
 *      LOCKING:
 *      Kernel thread context (may sleep).
 *
 *      RETURNS:
 *      0 on success, -errno on failure.
 */
static int sysfs_get_open_dirent(struct sysfs_dirent *sd,
                                 struct sysfs_buffer *buffer)
{
        struct sysfs_open_dirent *od, *new_od = NULL;

 retry:
        spin_lock_irq(&sysfs_open_dirent_lock);

        if (!sd->s_attr.open && new_od) {
                sd->s_attr.open = new_od;
                new_od = NULL;
        }

        od = sd->s_attr.open;
        if (od) {
                atomic_inc(&od->refcnt);
                list_add_tail(&buffer->list, &od->buffers);
        }

        spin_unlock_irq(&sysfs_open_dirent_lock);

        if (od) {
                kfree(new_od);
                return 0;
        }

        /* not there, initialize a new one and retry */
        new_od = kmalloc(sizeof(*new_od), GFP_KERNEL);
        if (!new_od)
                return -ENOMEM;

        atomic_set(&new_od->refcnt, 0);
        atomic_set(&new_od->event, 1);
        init_waitqueue_head(&new_od->poll);
        INIT_LIST_HEAD(&new_od->buffers);
        goto retry;
}

/**
 *      sysfs_put_open_dirent - put sysfs_open_dirent
 *      @sd: target sysfs_dirent
 *      @buffer: associated sysfs_buffer
 *
 *      Put @sd->s_attr.open and unlink @buffer from the buffers list.
 *      If reference count reaches zero, disassociate and free it.
 *
 *      LOCKING:
 *      None.
 */
static void sysfs_put_open_dirent(struct sysfs_dirent *sd,
                                  struct sysfs_buffer *buffer)
{
        struct sysfs_open_dirent *od = sd->s_attr.open;
        unsigned long flags;

        spin_lock_irqsave(&sysfs_open_dirent_lock, flags);

        list_del(&buffer->list);
        if (atomic_dec_and_test(&od->refcnt))
                sd->s_attr.open = NULL;
        else
                od = NULL;

        spin_unlock_irqrestore(&sysfs_open_dirent_lock, flags);

        kfree(od);
}

static int sysfs_open_file(struct inode *inode, struct file *file)
{
        struct sysfs_dirent *attr_sd = file->f_path.dentry->d_fsdata;
        struct kobject *kobj = attr_sd->s_parent->s_dir.kobj;
        struct sysfs_buffer *buffer;
        const struct sysfs_ops *ops;
        int error = -EACCES;

        /* need attr_sd for attr and ops, its parent for kobj */
        if (!sysfs_get_active(attr_sd))
                return -ENODEV;

        /* every kobject with an attribute needs a ktype assigned */
        if (kobj->ktype && kobj->ktype->sysfs_ops)
                ops = kobj->ktype->sysfs_ops;
        else {
                WARN(1, KERN_ERR
                     "missing sysfs attribute operations for kobject: %s\n",
                     kobject_name(kobj));
                goto err_out;
        }

        /* File needs write support.
         * The inode's perms must say it's ok,
         * and we must have a store method.
         */
        if (file->f_mode & FMODE_WRITE) {
                if (!(inode->i_mode & S_IWUGO) || !ops->store)
                        goto err_out;
        }

        /* File needs read support.
         * The inode's perms must say it's ok, and we there
         * must be a show method for it.
         */
        if (file->f_mode & FMODE_READ) {
                if (!(inode->i_mode & S_IRUGO) || !ops->show)
                        goto err_out;
        }

        /* No error? Great, allocate a buffer for the file, and store it
         * it in file->private_data for easy access.
         */
        error = -ENOMEM;
        buffer = kzalloc(sizeof(struct sysfs_buffer), GFP_KERNEL);
        if (!buffer)
                goto err_out;

        mutex_init(&buffer->mutex);
        buffer->needs_read_fill = 1;
        buffer->ops = ops;
        file->private_data = buffer;

        /* make sure we have open dirent struct */
        error = sysfs_get_open_dirent(attr_sd, buffer);
        if (error)
                goto err_free;

        /* open succeeded, put active references */
        sysfs_put_active(attr_sd);
        return 0;

 err_free:
        kfree(buffer);
 err_out:
        sysfs_put_active(attr_sd);
        return error;
}

static int sysfs_release(struct inode *inode, struct file *filp)
{
        struct sysfs_dirent *sd = filp->f_path.dentry->d_fsdata;
        struct sysfs_buffer *buffer = filp->private_data;

        sysfs_put_open_dirent(sd, buffer);

        if (buffer->page)
                free_page((unsigned long)buffer->page);
        kfree(buffer);

        return 0;
}

/* Sysfs attribute files are pollable.  The idea is that you read
 * the content and then you use 'poll' or 'select' to wait for
 * the content to change.  When the content changes (assuming the
 * manager for the kobject supports notification), poll will
 * return POLLERR|POLLPRI, and select will return the fd whether
 * it is waiting for read, write, or exceptions.
 * Once poll/select indicates that the value has changed, you
 * need to close and re-open the file, or seek to 0 and read again.
 * Reminder: this only works for attributes which actively support
 * it, and it is not possible to test an attribute from userspace
 * to see if it supports poll (Neither 'poll' nor 'select' return
 * an appropriate error code).  When in doubt, set a suitable timeout value.
 */
static unsigned int sysfs_poll(struct file *filp, poll_table *wait)
{
        struct sysfs_buffer *buffer = filp->private_data;
        struct sysfs_dirent *attr_sd = filp->f_path.dentry->d_fsdata;
        struct sysfs_open_dirent *od = attr_sd->s_attr.open;

        /* need parent for the kobj, grab both */
        if (!sysfs_get_active(attr_sd))
                goto trigger;

        poll_wait(filp, &od->poll, wait);

        sysfs_put_active(attr_sd);

        if (buffer->event != atomic_read(&od->event))
                goto trigger;

        return DEFAULT_POLLMASK;

 trigger:
        buffer->needs_read_fill = 1;
        return DEFAULT_POLLMASK|POLLERR|POLLPRI;
}

void sysfs_notify_dirent(struct sysfs_dirent *sd)
{
        struct sysfs_open_dirent *od;
        unsigned long flags;

        spin_lock_irqsave(&sysfs_open_dirent_lock, flags);

        if (!WARN_ON(sysfs_type(sd) != SYSFS_KOBJ_ATTR)) {
                od = sd->s_attr.open;
                if (od) {
                        atomic_inc(&od->event);
                        wake_up_interruptible(&od->poll);
                }
        }

        spin_unlock_irqrestore(&sysfs_open_dirent_lock, flags);
}
EXPORT_SYMBOL_GPL(sysfs_notify_dirent);

void sysfs_notify(struct kobject *k, const char *dir, const char *attr)
{
        struct sysfs_dirent *sd = k->sd;

        mutex_lock(&sysfs_mutex);

        if (sd && dir)
                sd = sysfs_find_dirent(sd, NULL, dir);
        if (sd && attr)
                sd = sysfs_find_dirent(sd, NULL, attr);
        if (sd)
                sysfs_notify_dirent(sd);

        mutex_unlock(&sysfs_mutex);
}
EXPORT_SYMBOL_GPL(sysfs_notify);

const struct file_operations sysfs_file_operations = {
        .read           = sysfs_read_file,
        .write          = sysfs_write_file,
        .llseek         = generic_file_llseek,
        .open           = sysfs_open_file,
        .release        = sysfs_release,
        .poll           = sysfs_poll,
};

static int sysfs_attr_ns(struct kobject *kobj, const struct attribute *attr,
                         const void **pns)
{
        struct sysfs_dirent *dir_sd = kobj->sd;
        const struct sysfs_ops *ops;
        const void *ns = NULL;
        int err;

        if (!dir_sd) {
                WARN(1, KERN_ERR "sysfs: kobject %s without dirent\n",
                        kobject_name(kobj));
                return -ENOENT;
        }

        err = 0;
        if (!sysfs_ns_type(dir_sd))
                goto out;

        err = -EINVAL;
        if (!kobj->ktype)
                goto out;
        ops = kobj->ktype->sysfs_ops;
        if (!ops)
                goto out;
        if (!ops->namespace)
                goto out;

        err = 0;
        ns = ops->namespace(kobj, attr);
out:
        if (err) {
                WARN(1, KERN_ERR
                     "missing sysfs namespace attribute operation for kobject: %s\n",
                     kobject_name(kobj));
        }
        *pns = ns;
        return err;
}

int sysfs_add_file_mode(struct sysfs_dirent *dir_sd,
                        const struct attribute *attr, int type, umode_t amode)
{
        umode_t mode = (amode & S_IALLUGO) | S_IFREG;
        struct sysfs_addrm_cxt acxt;
        struct sysfs_dirent *sd;
        const void *ns;
        int rc;

        rc = sysfs_attr_ns(dir_sd->s_dir.kobj, attr, &ns);
        if (rc)
                return rc;

        sd = sysfs_new_dirent(attr->name, mode, type);
        if (!sd)
                return -ENOMEM;

        sd->s_ns = ns;
        sd->s_attr.attr = (void *)attr;
        sysfs_dirent_init_lockdep(sd);

        sysfs_addrm_start(&acxt, dir_sd);
        rc = sysfs_add_one(&acxt, sd);
        sysfs_addrm_finish(&acxt);

        if (rc)
                sysfs_put(sd);

        return rc;
}


int sysfs_add_file(struct sysfs_dirent *dir_sd, const struct attribute *attr,
                   int type)
{
        return sysfs_add_file_mode(dir_sd, attr, type, attr->mode);
}


/**
 *      sysfs_create_file - create an attribute file for an object.
 *      @kobj:  object we're creating for.
 *      @attr:  attribute descriptor.
 */
int sysfs_create_file(struct kobject *kobj, const struct attribute *attr)
{
        BUG_ON(!kobj || !kobj->sd || !attr);

        return sysfs_add_file(kobj->sd, attr, SYSFS_KOBJ_ATTR);

}
EXPORT_SYMBOL_GPL(sysfs_create_file);

int sysfs_create_files(struct kobject *kobj, const struct attribute **ptr)
{
        int err = 0;
        int i;

        for (i = 0; ptr[i] && !err; i++)
                err = sysfs_create_file(kobj, ptr[i]);
        if (err)
                while (--i >= 0)
                        sysfs_remove_file(kobj, ptr[i]);
        return err;
}
EXPORT_SYMBOL_GPL(sysfs_create_files);

/**
 * sysfs_add_file_to_group - add an attribute file to a pre-existing group.
 * @kobj: object we're acting for.
 * @attr: attribute descriptor.
 * @group: group name.
 */
int sysfs_add_file_to_group(struct kobject *kobj,
                const struct attribute *attr, const char *group)
{
        struct sysfs_dirent *dir_sd;
        int error;

        if (group)
                dir_sd = sysfs_get_dirent(kobj->sd, NULL, group);
        else
                dir_sd = sysfs_get(kobj->sd);

        if (!dir_sd)
                return -ENOENT;

        error = sysfs_add_file(dir_sd, attr, SYSFS_KOBJ_ATTR);
        sysfs_put(dir_sd);

        return error;
}
EXPORT_SYMBOL_GPL(sysfs_add_file_to_group);

/**
 * sysfs_chmod_file - update the modified mode value on an object attribute.
 * @kobj: object we're acting for.
 * @attr: attribute descriptor.
 * @mode: file permissions.
 *
 */
int sysfs_chmod_file(struct kobject *kobj, const struct attribute *attr,
                     umode_t mode)
{
        struct sysfs_dirent *sd;
        struct iattr newattrs;
        const void *ns;
        int rc;

        rc = sysfs_attr_ns(kobj, attr, &ns);
        if (rc)
                return rc;

        mutex_lock(&sysfs_mutex);

        rc = -ENOENT;
        sd = sysfs_find_dirent(kobj->sd, ns, attr->name);
        if (!sd)
                goto out;

        newattrs.ia_mode = (mode & S_IALLUGO) | (sd->s_mode & ~S_IALLUGO);
        newattrs.ia_valid = ATTR_MODE;
        rc = sysfs_sd_setattr(sd, &newattrs);

 out:
        mutex_unlock(&sysfs_mutex);
        return rc;
}
EXPORT_SYMBOL_GPL(sysfs_chmod_file);

/**
 *      sysfs_remove_file - remove an object attribute.
 *      @kobj:  object we're acting for.
 *      @attr:  attribute descriptor.
 *
 *      Hash the attribute name and kill the victim.
 */
void sysfs_remove_file(struct kobject *kobj, const struct attribute *attr)
{
        const void *ns;

        if (sysfs_attr_ns(kobj, attr, &ns))
                return;

        sysfs_hash_and_remove(kobj->sd, ns, attr->name);
}
EXPORT_SYMBOL_GPL(sysfs_remove_file);

void sysfs_remove_files(struct kobject *kobj, const struct attribute **ptr)
{
        int i;
        for (i = 0; ptr[i]; i++)
                sysfs_remove_file(kobj, ptr[i]);
}
EXPORT_SYMBOL_GPL(sysfs_remove_files);

/**
 * sysfs_remove_file_from_group - remove an attribute file from a group.
 * @kobj: object we're acting for.
 * @attr: attribute descriptor.
 * @group: group name.
 */
void sysfs_remove_file_from_group(struct kobject *kobj,
                const struct attribute *attr, const char *group)
{
        struct sysfs_dirent *dir_sd;

        if (group)
                dir_sd = sysfs_get_dirent(kobj->sd, NULL, group);
        else
                dir_sd = sysfs_get(kobj->sd);
        if (dir_sd) {
                sysfs_hash_and_remove(dir_sd, NULL, attr->name);
                sysfs_put(dir_sd);
        }
}
EXPORT_SYMBOL_GPL(sysfs_remove_file_from_group);

struct sysfs_schedule_callback_struct {
        struct list_head        workq_list;
        struct kobject          *kobj;
        void                    (*func)(void *);
        void                    *data;
        struct module           *owner;
        struct work_struct      work;
};

static struct workqueue_struct *sysfs_workqueue;
static DEFINE_MUTEX(sysfs_workq_mutex);
static LIST_HEAD(sysfs_workq);
static void sysfs_schedule_callback_work(struct work_struct *work)
{
        struct sysfs_schedule_callback_struct *ss = container_of(work,
                        struct sysfs_schedule_callback_struct, work);

        (ss->func)(ss->data);
        kobject_put(ss->kobj);
        module_put(ss->owner);
        mutex_lock(&sysfs_workq_mutex);
        list_del(&ss->workq_list);
        mutex_unlock(&sysfs_workq_mutex);
        kfree(ss);
}

/**
 * sysfs_schedule_callback - helper to schedule a callback for a kobject
 * @kobj: object we're acting for.
 * @func: callback function to invoke later.
 * @data: argument to pass to @func.
 * @owner: module owning the callback code
 *
 * sysfs attribute methods must not unregister themselves or their parent
 * kobject (which would amount to the same thing).  Attempts to do so will
 * deadlock, since unregistration is mutually exclusive with driver
 * callbacks.
 *
 * Instead methods can call this routine, which will attempt to allocate
 * and schedule a workqueue request to call back @func with @data as its
 * argument in the workqueue's process context.  @kobj will be pinned
 * until @func returns.
 *
 * Returns 0 if the request was submitted, -ENOMEM if storage could not
 * be allocated, -ENODEV if a reference to @owner isn't available,
 * -EAGAIN if a callback has already been scheduled for @kobj.
 */
int sysfs_schedule_callback(struct kobject *kobj, void (*func)(void *),
                void *data, struct module *owner)
{
        struct sysfs_schedule_callback_struct *ss, *tmp;

        if (!try_module_get(owner))
                return -ENODEV;

        mutex_lock(&sysfs_workq_mutex);
        list_for_each_entry_safe(ss, tmp, &sysfs_workq, workq_list)
                if (ss->kobj == kobj) {
                        module_put(owner);
                        mutex_unlock(&sysfs_workq_mutex);
                        return -EAGAIN;
                }
        mutex_unlock(&sysfs_workq_mutex);

        if (sysfs_workqueue == NULL) {
                sysfs_workqueue = create_singlethread_workqueue("sysfsd");
                if (sysfs_workqueue == NULL) {
                        module_put(owner);
                        return -ENOMEM;
                }
        }

        ss = kmalloc(sizeof(*ss), GFP_KERNEL);
        if (!ss) {
                module_put(owner);
                return -ENOMEM;
        }
        kobject_get(kobj);
        ss->kobj = kobj;
        ss->func = func;
        ss->data = data;
        ss->owner = owner;
        INIT_WORK(&ss->work, sysfs_schedule_callback_work);
        INIT_LIST_HEAD(&ss->workq_list);
        mutex_lock(&sysfs_workq_mutex);
        list_add_tail(&ss->workq_list, &sysfs_workq);
        mutex_unlock(&sysfs_workq_mutex);
        queue_work(sysfs_workqueue, &ss->work);
        return 0;
}
EXPORT_SYMBOL_GPL(sysfs_schedule_callback);