/*
   * linux/fs/seq_file.c
   *
   * helper functions for making synthetic files from sequences of records.
   * initial implementation -- AV, Oct 2001.
   */
  
  #include <linux/fs.h>

  #include <linux/export.h>

  #include <linux/seq_file.h>
  #include <linux/slab.h>
  
  #include <asm/uaccess.h>
  #include <asm/page.h>

  
  /*
   * seq_files have a buffer which can may overflow. When this happens a larger
   * buffer is reallocated and all the data will be printed again.
   * The overflow state is true when m->count == m->size.
   */
  static bool seq_overflow(struct seq_file *m)
  {
  	return m->count == m->size;
  }
  
  static void seq_set_overflow(struct seq_file *m)
  {
  	m->count = m->size;
  }

  /**
   *	seq_open -	initialize sequential file
   *	@file: file we initialize
   *	@op: method table describing the sequence
   *
   *	seq_open() sets @file, associating it with a sequence described
   *	by @op.  @op->start() sets the iterator up and returns the first
   *	element of sequence. @op->stop() shuts it down.  @op->next()
   *	returns the next element of sequence.  @op->show() prints element
   *	into the buffer.  In case of error ->start() and ->next() return
   *	ERR_PTR(error).  In the end of sequence they return %NULL. ->show()
   *	returns 0 in case of success and negative number in case of error.

   *	Returning SEQ_SKIP means "discard this element and move on".

   */

  int seq_open(struct file *file, const struct seq_operations *op)

  {

  	struct seq_file *p = file->private_data;
  
  	if (!p) {
  		p = kmalloc(sizeof(*p), GFP_KERNEL);
  		if (!p)
  			return -ENOMEM;
  		file->private_data = p;
  	}

  	memset(p, 0, sizeof(*p));

  	mutex_init(&p->lock);

  	p->op = op;

  
  	/*
  	 * Wrappers around seq_open(e.g. swaps_open) need to be
  	 * aware of this. If they set f_version themselves, they
  	 * should call seq_open first and then set f_version.
  	 */
  	file->f_version = 0;

  	/*
  	 * seq_files support lseek() and pread().  They do not implement
  	 * write() at all, but we clear FMODE_PWRITE here for historical
  	 * reasons.
  	 *
  	 * If a client of seq_files a) implements file.write() and b) wishes to
  	 * support pwrite() then that client will need to implement its own
  	 * file.open() which calls seq_open() and then sets FMODE_PWRITE.
  	 */
  	file->f_mode &= ~FMODE_PWRITE;

  	return 0;
  }
  EXPORT_SYMBOL(seq_open);

  static int traverse(struct seq_file *m, loff_t offset)
  {
  	loff_t pos = 0, index;
  	int error = 0;
  	void *p;
  
  	m->version = 0;
  	index = 0;
  	m->count = m->from = 0;
  	if (!offset) {
  		m->index = index;
  		return 0;
  	}
  	if (!m->buf) {
  		m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);
  		if (!m->buf)
  			return -ENOMEM;
  	}
  	p = m->op->start(m, &index);
  	while (p) {
  		error = PTR_ERR(p);
  		if (IS_ERR(p))
  			break;
  		error = m->op->show(m, p);
  		if (error < 0)
  			break;
  		if (unlikely(error)) {
  			error = 0;
  			m->count = 0;
  		}

  		if (seq_overflow(m))

  			goto Eoverflow;
  		if (pos + m->count > offset) {
  			m->from = offset - pos;
  			m->count -= m->from;
  			m->index = index;
  			break;
  		}
  		pos += m->count;
  		m->count = 0;
  		if (pos == offset) {
  			index++;
  			m->index = index;
  			break;
  		}
  		p = m->op->next(m, p, &index);
  	}
  	m->op->stop(m, p);

  	m->index = index;

  	return error;
  
  Eoverflow:
  	m->op->stop(m, p);
  	kfree(m->buf);
  	m->buf = kmalloc(m->size <<= 1, GFP_KERNEL);
  	return !m->buf ? -ENOMEM : -EAGAIN;
  }

  /**
   *	seq_read -	->read() method for sequential files.

   *	@file: the file to read from
   *	@buf: the buffer to read to
   *	@size: the maximum number of bytes to read
   *	@ppos: the current position in the file

   *
   *	Ready-made ->f_op->read()
   */
  ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
  {

  	struct seq_file *m = file->private_data;

  	size_t copied = 0;
  	loff_t pos;
  	size_t n;
  	void *p;
  	int err = 0;

  	mutex_lock(&m->lock);

  	/*
  	 * seq_file->op->..m_start/m_stop/m_next may do special actions
  	 * or optimisations based on the file->f_version, so we want to
  	 * pass the file->f_version to those methods.
  	 *
  	 * seq_file->version is just copy of f_version, and seq_file
  	 * methods can treat it simply as file version.
  	 * It is copied in first and copied out after all operations.
  	 * It is convenient to have it as  part of structure to avoid the
  	 * need of passing another argument to all the seq_file methods.
  	 */
  	m->version = file->f_version;

  	/* Don't assume *ppos is where we left it */
  	if (unlikely(*ppos != m->read_pos)) {

  		while ((err = traverse(m, *ppos)) == -EAGAIN)
  			;
  		if (err) {
  			/* With prejudice... */
  			m->read_pos = 0;
  			m->version = 0;
  			m->index = 0;
  			m->count = 0;
  			goto Done;

  		} else {
  			m->read_pos = *ppos;

  		}
  	}

  	/* grab buffer if we didn't have one */
  	if (!m->buf) {
  		m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);
  		if (!m->buf)
  			goto Enomem;
  	}
  	/* if not empty - flush it first */
  	if (m->count) {
  		n = min(m->count, size);
  		err = copy_to_user(buf, m->buf + m->from, n);
  		if (err)
  			goto Efault;
  		m->count -= n;
  		m->from += n;
  		size -= n;
  		buf += n;
  		copied += n;
  		if (!m->count)
  			m->index++;
  		if (!size)
  			goto Done;
  	}
  	/* we need at least one record in buffer */

  	pos = m->index;
  	p = m->op->start(m, &pos);

  	while (1) {

  		err = PTR_ERR(p);
  		if (!p || IS_ERR(p))
  			break;
  		err = m->op->show(m, p);

  		if (err < 0)

  			break;

  		if (unlikely(err))
  			m->count = 0;

  		if (unlikely(!m->count)) {
  			p = m->op->next(m, p, &pos);
  			m->index = pos;
  			continue;
  		}

  		if (m->count < m->size)
  			goto Fill;
  		m->op->stop(m, p);
  		kfree(m->buf);
  		m->buf = kmalloc(m->size <<= 1, GFP_KERNEL);
  		if (!m->buf)
  			goto Enomem;
  		m->count = 0;
  		m->version = 0;

  		pos = m->index;
  		p = m->op->start(m, &pos);

  	}
  	m->op->stop(m, p);
  	m->count = 0;
  	goto Done;
  Fill:
  	/* they want more? let's try to get some more */
  	while (m->count < size) {
  		size_t offs = m->count;
  		loff_t next = pos;
  		p = m->op->next(m, p, &next);
  		if (!p || IS_ERR(p)) {
  			err = PTR_ERR(p);
  			break;
  		}
  		err = m->op->show(m, p);

  		if (seq_overflow(m) || err) {

  			m->count = offs;

  			if (likely(err <= 0))
  				break;

  		}
  		pos = next;
  	}
  	m->op->stop(m, p);
  	n = min(m->count, size);
  	err = copy_to_user(buf, m->buf, n);
  	if (err)
  		goto Efault;
  	copied += n;
  	m->count -= n;
  	if (m->count)
  		m->from = n;
  	else
  		pos++;
  	m->index = pos;
  Done:
  	if (!copied)
  		copied = err;

  	else {

  		*ppos += copied;

  		m->read_pos += copied;
  	}

  	file->f_version = m->version;

  	mutex_unlock(&m->lock);

  	return copied;
  Enomem:
  	err = -ENOMEM;
  	goto Done;
  Efault:
  	err = -EFAULT;
  	goto Done;
  }
  EXPORT_SYMBOL(seq_read);

  /**
   *	seq_lseek -	->llseek() method for sequential files.

   *	@file: the file in question
   *	@offset: new position
   *	@origin: 0 for absolute, 1 for relative position

   *
   *	Ready-made ->f_op->llseek()
   */
  loff_t seq_lseek(struct file *file, loff_t offset, int origin)
  {

  	struct seq_file *m = file->private_data;

  	loff_t retval = -EINVAL;

  	mutex_lock(&m->lock);

  	m->version = file->f_version;
  	switch (origin) {
  		case 1:
  			offset += file->f_pos;
  		case 0:
  			if (offset < 0)
  				break;
  			retval = offset;

  			if (offset != m->read_pos) {

  				while ((retval=traverse(m, offset)) == -EAGAIN)
  					;
  				if (retval) {
  					/* with extreme prejudice... */
  					file->f_pos = 0;

  					m->read_pos = 0;

  					m->version = 0;
  					m->index = 0;
  					m->count = 0;
  				} else {

  					m->read_pos = offset;

  					retval = file->f_pos = offset;
  				}
  			}
  	}

  	file->f_version = m->version;

  	mutex_unlock(&m->lock);

  	return retval;
  }
  EXPORT_SYMBOL(seq_lseek);
  
  /**
   *	seq_release -	free the structures associated with sequential file.
   *	@file: file in question

   *	@inode: file->f_path.dentry->d_inode

   *
   *	Frees the structures associated with sequential file; can be used
   *	as ->f_op->release() if you don't have private data to destroy.
   */
  int seq_release(struct inode *inode, struct file *file)
  {

  	struct seq_file *m = file->private_data;

  	kfree(m->buf);
  	kfree(m);
  	return 0;
  }
  EXPORT_SYMBOL(seq_release);
  
  /**
   *	seq_escape -	print string into buffer, escaping some characters
   *	@m:	target buffer
   *	@s:	string
   *	@esc:	set of characters that need escaping
   *
   *	Puts string into buffer, replacing each occurrence of character from
   *	@esc with usual octal escape.  Returns 0 in case of success, -1 - in
   *	case of overflow.
   */
  int seq_escape(struct seq_file *m, const char *s, const char *esc)
  {
  	char *end = m->buf + m->size;
          char *p;
  	char c;
  
          for (p = m->buf + m->count; (c = *s) != '\0' && p < end; s++) {
  		if (!strchr(esc, c)) {
  			*p++ = c;
  			continue;
  		}
  		if (p + 3 < end) {
  			*p++ = '\\';
  			*p++ = '0' + ((c & 0300) >> 6);
  			*p++ = '0' + ((c & 070) >> 3);
  			*p++ = '0' + (c & 07);
  			continue;
  		}

  		seq_set_overflow(m);

  		return -1;
          }
  	m->count = p - m->buf;
          return 0;
  }
  EXPORT_SYMBOL(seq_escape);

  int seq_vprintf(struct seq_file *m, const char *f, va_list args)

  {

  	int len;
  
  	if (m->count < m->size) {

  		len = vsnprintf(m->buf + m->count, m->size - m->count, f, args);

  		if (m->count + len < m->size) {
  			m->count += len;
  			return 0;
  		}
  	}

  	seq_set_overflow(m);

  	return -1;
  }

  EXPORT_SYMBOL(seq_vprintf);
  
  int seq_printf(struct seq_file *m, const char *f, ...)
  {
  	int ret;
  	va_list args;
  
  	va_start(args, f);
  	ret = seq_vprintf(m, f, args);
  	va_end(args);
  
  	return ret;
  }

  EXPORT_SYMBOL(seq_printf);

  /**

   *	mangle_path -	mangle and copy path to buffer beginning
   *	@s: buffer start
   *	@p: beginning of path in above buffer
   *	@esc: set of characters that need escaping

   *
   *      Copy the path from @p to @s, replacing each occurrence of character from
   *      @esc with usual octal escape.
   *      Returns pointer past last written character in @s, or NULL in case of
   *      failure.
   */

  char *mangle_path(char *s, const char *p, const char *esc)

  {
  	while (s <= p) {
  		char c = *p++;
  		if (!c) {
  			return s;
  		} else if (!strchr(esc, c)) {
  			*s++ = c;
  		} else if (s + 4 > p) {
  			break;
  		} else {
  			*s++ = '\\';
  			*s++ = '0' + ((c & 0300) >> 6);
  			*s++ = '0' + ((c & 070) >> 3);
  			*s++ = '0' + (c & 07);
  		}
  	}
  	return NULL;
  }

  EXPORT_SYMBOL(mangle_path);

  /**
   * seq_path - seq_file interface to print a pathname
   * @m: the seq_file handle
   * @path: the struct path to print
   * @esc: set of characters to escape in the output
   *
   * return the absolute path of 'path', as represented by the
   * dentry / mnt pair in the path parameter.

   */

  int seq_path(struct seq_file *m, const struct path *path, const char *esc)

  {

  	char *buf;
  	size_t size = seq_get_buf(m, &buf);
  	int res = -1;
  
  	if (size) {
  		char *p = d_path(path, buf, size);

  		if (!IS_ERR(p)) {

  			char *end = mangle_path(buf, p, esc);
  			if (end)
  				res = end - buf;

  		}
  	}

  	seq_commit(m, res);
  
  	return res;

  }
  EXPORT_SYMBOL(seq_path);

  /*

   * Same as seq_path, but relative to supplied root.

   */

  int seq_path_root(struct seq_file *m, const struct path *path,
  		  const struct path *root, const char *esc)

  {

  	char *buf;
  	size_t size = seq_get_buf(m, &buf);
  	int res = -ENAMETOOLONG;
  
  	if (size) {

  		char *p;

  		p = __d_path(path, root, buf, size);

  		if (!p)
  			return SEQ_SKIP;

  		res = PTR_ERR(p);

  		if (!IS_ERR(p)) {

  			char *end = mangle_path(buf, p, esc);
  			if (end)
  				res = end - buf;
  			else
  				res = -ENAMETOOLONG;

  		}
  	}

  	seq_commit(m, res);

  	return res < 0 && res != -ENAMETOOLONG ? res : 0;

  }
  
  /*

   * returns the path of the 'dentry' from the root of its filesystem.
   */

  int seq_dentry(struct seq_file *m, struct dentry *dentry, const char *esc)

  {

  	char *buf;
  	size_t size = seq_get_buf(m, &buf);
  	int res = -1;
  
  	if (size) {
  		char *p = dentry_path(dentry, buf, size);

  		if (!IS_ERR(p)) {

  			char *end = mangle_path(buf, p, esc);
  			if (end)
  				res = end - buf;

  		}
  	}

  	seq_commit(m, res);
  
  	return res;

  }

  int seq_bitmap(struct seq_file *m, const unsigned long *bits,
  				   unsigned int nr_bits)

  {

  	if (m->count < m->size) {
  		int len = bitmap_scnprintf(m->buf + m->count,
  				m->size - m->count, bits, nr_bits);
  		if (m->count + len < m->size) {
  			m->count += len;
  			return 0;
  		}

  	}

  	seq_set_overflow(m);

  	return -1;
  }

  EXPORT_SYMBOL(seq_bitmap);

  int seq_bitmap_list(struct seq_file *m, const unsigned long *bits,

  		unsigned int nr_bits)
  {
  	if (m->count < m->size) {
  		int len = bitmap_scnlistprintf(m->buf + m->count,
  				m->size - m->count, bits, nr_bits);
  		if (m->count + len < m->size) {
  			m->count += len;
  			return 0;
  		}
  	}

  	seq_set_overflow(m);

  	return -1;
  }
  EXPORT_SYMBOL(seq_bitmap_list);

  static void *single_start(struct seq_file *p, loff_t *pos)
  {
  	return NULL + (*pos == 0);
  }
  
  static void *single_next(struct seq_file *p, void *v, loff_t *pos)
  {
  	++*pos;
  	return NULL;
  }
  
  static void single_stop(struct seq_file *p, void *v)
  {
  }
  
  int single_open(struct file *file, int (*show)(struct seq_file *, void *),
  		void *data)
  {
  	struct seq_operations *op = kmalloc(sizeof(*op), GFP_KERNEL);
  	int res = -ENOMEM;
  
  	if (op) {
  		op->start = single_start;
  		op->next = single_next;
  		op->stop = single_stop;
  		op->show = show;
  		res = seq_open(file, op);
  		if (!res)
  			((struct seq_file *)file->private_data)->private = data;
  		else
  			kfree(op);
  	}
  	return res;
  }
  EXPORT_SYMBOL(single_open);
  
  int single_release(struct inode *inode, struct file *file)
  {

  	const struct seq_operations *op = ((struct seq_file *)file->private_data)->op;

  	int res = seq_release(inode, file);
  	kfree(op);
  	return res;
  }
  EXPORT_SYMBOL(single_release);
  
  int seq_release_private(struct inode *inode, struct file *file)
  {
  	struct seq_file *seq = file->private_data;
  
  	kfree(seq->private);
  	seq->private = NULL;
  	return seq_release(inode, file);
  }
  EXPORT_SYMBOL(seq_release_private);

  void *__seq_open_private(struct file *f, const struct seq_operations *ops,
  		int psize)
  {
  	int rc;
  	void *private;
  	struct seq_file *seq;
  
  	private = kzalloc(psize, GFP_KERNEL);
  	if (private == NULL)
  		goto out;
  
  	rc = seq_open(f, ops);
  	if (rc < 0)
  		goto out_free;
  
  	seq = f->private_data;
  	seq->private = private;
  	return private;
  
  out_free:
  	kfree(private);
  out:
  	return NULL;
  }
  EXPORT_SYMBOL(__seq_open_private);
  
  int seq_open_private(struct file *filp, const struct seq_operations *ops,
  		int psize)
  {
  	return __seq_open_private(filp, ops, psize) ? 0 : -ENOMEM;
  }
  EXPORT_SYMBOL(seq_open_private);

  int seq_putc(struct seq_file *m, char c)
  {
  	if (m->count < m->size) {
  		m->buf[m->count++] = c;
  		return 0;
  	}
  	return -1;
  }
  EXPORT_SYMBOL(seq_putc);
  
  int seq_puts(struct seq_file *m, const char *s)
  {
  	int len = strlen(s);
  	if (m->count + len < m->size) {
  		memcpy(m->buf + m->count, s, len);
  		m->count += len;
  		return 0;
  	}

  	seq_set_overflow(m);

  	return -1;
  }
  EXPORT_SYMBOL(seq_puts);

  /*
   * A helper routine for putting decimal numbers without rich format of printf().
   * only 'unsigned long long' is supported.
   * This routine will put one byte delimiter + number into seq_file.
   * This routine is very quick when you show lots of numbers.
   * In usual cases, it will be better to use seq_printf(). It's easier to read.
   */
  int seq_put_decimal_ull(struct seq_file *m, char delimiter,
  			unsigned long long num)
  {
  	int len;
  
  	if (m->count + 2 >= m->size) /* we'll write 2 bytes at least */
  		goto overflow;

  	if (delimiter)
  		m->buf[m->count++] = delimiter;

  
  	if (num < 10) {
  		m->buf[m->count++] = num + '0';
  		return 0;
  	}
  
  	len = num_to_str(m->buf + m->count, m->size - m->count, num);
  	if (!len)
  		goto overflow;
  	m->count += len;
  	return 0;
  overflow:

  	seq_set_overflow(m);

  	return -1;
  }
  EXPORT_SYMBOL(seq_put_decimal_ull);

  int seq_put_decimal_ll(struct seq_file *m, char delimiter,
  			long long num)
  {
  	if (num < 0) {
  		if (m->count + 3 >= m->size) {

  			seq_set_overflow(m);

  			return -1;
  		}
  		if (delimiter)
  			m->buf[m->count++] = delimiter;
  		num = -num;
  		delimiter = '-';
  	}
  	return seq_put_decimal_ull(m, delimiter, num);
  
  }
  EXPORT_SYMBOL(seq_put_decimal_ll);

  /**
   * seq_write - write arbitrary data to buffer
   * @seq: seq_file identifying the buffer to which data should be written
   * @data: data address
   * @len: number of bytes
   *
   * Return 0 on success, non-zero otherwise.
   */
  int seq_write(struct seq_file *seq, const void *data, size_t len)
  {
  	if (seq->count + len < seq->size) {
  		memcpy(seq->buf + seq->count, data, len);
  		seq->count += len;
  		return 0;
  	}

  	seq_set_overflow(seq);

  	return -1;
  }
  EXPORT_SYMBOL(seq_write);

  struct list_head *seq_list_start(struct list_head *head, loff_t pos)
  {
  	struct list_head *lh;
  
  	list_for_each(lh, head)
  		if (pos-- == 0)
  			return lh;
  
  	return NULL;
  }

  EXPORT_SYMBOL(seq_list_start);
  
  struct list_head *seq_list_start_head(struct list_head *head, loff_t pos)
  {
  	if (!pos)
  		return head;
  
  	return seq_list_start(head, pos - 1);
  }

  EXPORT_SYMBOL(seq_list_start_head);
  
  struct list_head *seq_list_next(void *v, struct list_head *head, loff_t *ppos)
  {
  	struct list_head *lh;
  
  	lh = ((struct list_head *)v)->next;
  	++*ppos;
  	return lh == head ? NULL : lh;
  }

  EXPORT_SYMBOL(seq_list_next);

  
  /**
   * seq_hlist_start - start an iteration of a hlist
   * @head: the head of the hlist
   * @pos:  the start position of the sequence
   *
   * Called at seq_file->op->start().
   */
  struct hlist_node *seq_hlist_start(struct hlist_head *head, loff_t pos)
  {
  	struct hlist_node *node;
  
  	hlist_for_each(node, head)
  		if (pos-- == 0)
  			return node;
  	return NULL;
  }
  EXPORT_SYMBOL(seq_hlist_start);
  
  /**
   * seq_hlist_start_head - start an iteration of a hlist
   * @head: the head of the hlist
   * @pos:  the start position of the sequence
   *
   * Called at seq_file->op->start(). Call this function if you want to
   * print a header at the top of the output.
   */
  struct hlist_node *seq_hlist_start_head(struct hlist_head *head, loff_t pos)
  {
  	if (!pos)
  		return SEQ_START_TOKEN;
  
  	return seq_hlist_start(head, pos - 1);
  }
  EXPORT_SYMBOL(seq_hlist_start_head);
  
  /**
   * seq_hlist_next - move to the next position of the hlist
   * @v:    the current iterator
   * @head: the head of the hlist

   * @ppos: the current position

   *
   * Called at seq_file->op->next().
   */
  struct hlist_node *seq_hlist_next(void *v, struct hlist_head *head,
  				  loff_t *ppos)
  {
  	struct hlist_node *node = v;
  
  	++*ppos;
  	if (v == SEQ_START_TOKEN)
  		return head->first;
  	else
  		return node->next;
  }
  EXPORT_SYMBOL(seq_hlist_next);

  
  /**
   * seq_hlist_start_rcu - start an iteration of a hlist protected by RCU
   * @head: the head of the hlist
   * @pos:  the start position of the sequence
   *
   * Called at seq_file->op->start().
   *
   * This list-traversal primitive may safely run concurrently with
   * the _rcu list-mutation primitives such as hlist_add_head_rcu()
   * as long as the traversal is guarded by rcu_read_lock().
   */
  struct hlist_node *seq_hlist_start_rcu(struct hlist_head *head,
  				       loff_t pos)
  {
  	struct hlist_node *node;
  
  	__hlist_for_each_rcu(node, head)
  		if (pos-- == 0)
  			return node;
  	return NULL;
  }
  EXPORT_SYMBOL(seq_hlist_start_rcu);
  
  /**
   * seq_hlist_start_head_rcu - start an iteration of a hlist protected by RCU
   * @head: the head of the hlist
   * @pos:  the start position of the sequence
   *
   * Called at seq_file->op->start(). Call this function if you want to
   * print a header at the top of the output.
   *
   * This list-traversal primitive may safely run concurrently with
   * the _rcu list-mutation primitives such as hlist_add_head_rcu()
   * as long as the traversal is guarded by rcu_read_lock().
   */
  struct hlist_node *seq_hlist_start_head_rcu(struct hlist_head *head,
  					    loff_t pos)
  {
  	if (!pos)
  		return SEQ_START_TOKEN;
  
  	return seq_hlist_start_rcu(head, pos - 1);
  }
  EXPORT_SYMBOL(seq_hlist_start_head_rcu);
  
  /**
   * seq_hlist_next_rcu - move to the next position of the hlist protected by RCU
   * @v:    the current iterator
   * @head: the head of the hlist

   * @ppos: the current position

   *
   * Called at seq_file->op->next().
   *
   * This list-traversal primitive may safely run concurrently with
   * the _rcu list-mutation primitives such as hlist_add_head_rcu()
   * as long as the traversal is guarded by rcu_read_lock().
   */
  struct hlist_node *seq_hlist_next_rcu(void *v,
  				      struct hlist_head *head,
  				      loff_t *ppos)
  {
  	struct hlist_node *node = v;
  
  	++*ppos;
  	if (v == SEQ_START_TOKEN)
  		return rcu_dereference(head->first);
  	else
  		return rcu_dereference(node->next);
  }
  EXPORT_SYMBOL(seq_hlist_next_rcu);