UBUNTU: Ubuntu-4.13.0-45.50

[mirror_ubuntu-artful-kernel.git] / fs / seq_file.c

/*
 * 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/vmalloc.h>
#include <linux/slab.h>
#include <linux/cred.h>
#include <linux/mm.h>
#include <linux/printk.h>
#include <linux/string_helpers.h>

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

static void seq_set_overflow(struct seq_file *m)
{
        m->count = m->size;
}

static void *seq_buf_alloc(unsigned long size)
{
        return kvmalloc(size, GFP_KERNEL);
}

/**
 *      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".
 *      Note: seq_open() will allocate a struct seq_file and store its
 *      pointer in @file->private_data. This pointer should not be modified.
 */
int seq_open(struct file *file, const struct seq_operations *op)
{
        struct seq_file *p;

        WARN_ON(file->private_data);

        p = kzalloc(sizeof(*p), GFP_KERNEL);
        if (!p)
                return -ENOMEM;

        file->private_data = p;

        mutex_init(&p->lock);
        p->op = op;

        // No refcounting: the lifetime of 'p' is constrained
        // to the lifetime of the file.
        p->file = file;

        /*
         * 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 = seq_buf_alloc(m->size = PAGE_SIZE);
                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_has_overflowed(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);
        kvfree(m->buf);
        m->count = 0;
        m->buf = seq_buf_alloc(m->size <<= 1);
        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;

        /*
         * if request is to read from zero offset, reset iterator to first
         * record as it might have been already advanced by previous requests
         */
        if (*ppos == 0)
                m->index = 0;

        /* 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 = seq_buf_alloc(m->size = PAGE_SIZE);
                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->from = 0;
                        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);
                kvfree(m->buf);
                m->count = 0;
                m->buf = seq_buf_alloc(m->size <<= 1);
                if (!m->buf)
                        goto Enomem;
                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_has_overflowed(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
 *      @whence: 0 for absolute, 1 for relative position
 *
 *      Ready-made ->f_op->llseek()
 */
loff_t seq_lseek(struct file *file, loff_t offset, int whence)
{
        struct seq_file *m = file->private_data;
        loff_t retval = -EINVAL;

        mutex_lock(&m->lock);
        m->version = file->f_version;
        switch (whence) {
        case SEEK_CUR:
                offset += file->f_pos;
        case SEEK_SET:
                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;
                        }
                } else {
                        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: its 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;
        kvfree(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.
 *      Use seq_has_overflowed() to check for errors.
 */
void seq_escape(struct seq_file *m, const char *s, const char *esc)
{
        char *buf;
        size_t size = seq_get_buf(m, &buf);
        int ret;

        ret = string_escape_str(s, buf, size, ESCAPE_OCTAL, esc);
        seq_commit(m, ret < size ? ret : -1);
}
EXPORT_SYMBOL(seq_escape);

void 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;
                }
        }
        seq_set_overflow(m);
}
EXPORT_SYMBOL(seq_vprintf);

void seq_printf(struct seq_file *m, const char *f, ...)
{
        va_list args;

        va_start(args, f);
        seq_vprintf(m, f, args);
        va_end(args);
}
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);

/**
 * seq_file_path - seq_file interface to print a pathname of a file
 * @m: the seq_file handle
 * @file: the struct file to print
 * @esc: set of characters to escape in the output
 *
 * return the absolute path to the file.
 */
int seq_file_path(struct seq_file *m, struct file *file, const char *esc)
{
        return seq_path(m, &file->f_path, esc);
}
EXPORT_SYMBOL(seq_file_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;
}
EXPORT_SYMBOL(seq_dentry);

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_open_size(struct file *file, int (*show)(struct seq_file *, void *),
                void *data, size_t size)
{
        char *buf = seq_buf_alloc(size);
        int ret;
        if (!buf)
                return -ENOMEM;
        ret = single_open(file, show, data);
        if (ret) {
                kvfree(buf);
                return ret;
        }
        ((struct seq_file *)file->private_data)->buf = buf;
        ((struct seq_file *)file->private_data)->size = size;
        return 0;
}
EXPORT_SYMBOL(single_open_size);

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);

void seq_putc(struct seq_file *m, char c)
{
        if (m->count >= m->size)
                return;

        m->buf[m->count++] = c;
}
EXPORT_SYMBOL(seq_putc);

void seq_puts(struct seq_file *m, const char *s)
{
        int len = strlen(s);

        if (m->count + len >= m->size) {
                seq_set_overflow(m);
                return;
        }
        memcpy(m->buf + m->count, s, len);
        m->count += len;
}
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 strlen(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.
 */
void seq_put_decimal_ull(struct seq_file *m, const char *delimiter,
                         unsigned long long num)
{
        int len;

        if (m->count + 2 >= m->size) /* we'll write 2 bytes at least */
                goto overflow;

        len = strlen(delimiter);
        if (m->count + len >= m->size)
                goto overflow;

        memcpy(m->buf + m->count, delimiter, len);
        m->count += len;

        if (m->count + 1 >= m->size)
                goto overflow;

        if (num < 10) {
                m->buf[m->count++] = num + '0';
                return;
        }

        len = num_to_str(m->buf + m->count, m->size - m->count, num);
        if (!len)
                goto overflow;

        m->count += len;
        return;

overflow:
        seq_set_overflow(m);
}
EXPORT_SYMBOL(seq_put_decimal_ull);

void seq_put_decimal_ll(struct seq_file *m, const char *delimiter, long long num)
{
        int len;

        if (m->count + 3 >= m->size) /* we'll write 2 bytes at least */
                goto overflow;

        len = strlen(delimiter);
        if (m->count + len >= m->size)
                goto overflow;

        memcpy(m->buf + m->count, delimiter, len);
        m->count += len;

        if (m->count + 2 >= m->size)
                goto overflow;

        if (num < 0) {
                m->buf[m->count++] = '-';
                num = -num;
        }

        if (num < 10) {
                m->buf[m->count++] = num + '0';
                return;
        }

        len = num_to_str(m->buf + m->count, m->size - m->count, num);
        if (!len)
                goto overflow;

        m->count += len;
        return;

overflow:
        seq_set_overflow(m);
}
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);

/**
 * seq_pad - write padding spaces to buffer
 * @m: seq_file identifying the buffer to which data should be written
 * @c: the byte to append after padding if non-zero
 */
void seq_pad(struct seq_file *m, char c)
{
        int size = m->pad_until - m->count;
        if (size > 0)
                seq_printf(m, "%*s", size, "");
        if (c)
                seq_putc(m, c);
}
EXPORT_SYMBOL(seq_pad);

/* A complete analogue of print_hex_dump() */
void seq_hex_dump(struct seq_file *m, const char *prefix_str, int prefix_type,
                  int rowsize, int groupsize, const void *buf, size_t len,
                  bool ascii)
{
        const u8 *ptr = buf;
        int i, linelen, remaining = len;
        char *buffer;
        size_t size;
        int ret;

        if (rowsize != 16 && rowsize != 32)
                rowsize = 16;

        for (i = 0; i < len && !seq_has_overflowed(m); i += rowsize) {
                linelen = min(remaining, rowsize);
                remaining -= rowsize;

                switch (prefix_type) {
                case DUMP_PREFIX_ADDRESS:
                        seq_printf(m, "%s%p: ", prefix_str, ptr + i);
                        break;
                case DUMP_PREFIX_OFFSET:
                        seq_printf(m, "%s%.8x: ", prefix_str, i);
                        break;
                default:
                        seq_printf(m, "%s", prefix_str);
                        break;
                }

                size = seq_get_buf(m, &buffer);
                ret = hex_dump_to_buffer(ptr + i, linelen, rowsize, groupsize,
                                         buffer, size, ascii);
                seq_commit(m, ret < size ? ret : -1);

                seq_putc(m, '\n');
        }
}
EXPORT_SYMBOL(seq_hex_dump);

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);

/**
 * seq_hlist_start_precpu - start an iteration of a percpu hlist array
 * @head: pointer to percpu array of struct hlist_heads
 * @cpu:  pointer to cpu "cursor"
 * @pos:  start position of sequence
 *
 * Called at seq_file->op->start().
 */
struct hlist_node *
seq_hlist_start_percpu(struct hlist_head __percpu *head, int *cpu, loff_t pos)
{
        struct hlist_node *node;

        for_each_possible_cpu(*cpu) {
                hlist_for_each(node, per_cpu_ptr(head, *cpu)) {
                        if (pos-- == 0)
                                return node;
                }
        }
        return NULL;
}
EXPORT_SYMBOL(seq_hlist_start_percpu);

/**
 * seq_hlist_next_percpu - move to the next position of the percpu hlist array
 * @v:    pointer to current hlist_node
 * @head: pointer to percpu array of struct hlist_heads
 * @cpu:  pointer to cpu "cursor"
 * @pos:  start position of sequence
 *
 * Called at seq_file->op->next().
 */
struct hlist_node *
seq_hlist_next_percpu(void *v, struct hlist_head __percpu *head,
                        int *cpu, loff_t *pos)
{
        struct hlist_node *node = v;

        ++*pos;

        if (node->next)
                return node->next;

        for (*cpu = cpumask_next(*cpu, cpu_possible_mask); *cpu < nr_cpu_ids;
             *cpu = cpumask_next(*cpu, cpu_possible_mask)) {
                struct hlist_head *bucket = per_cpu_ptr(head, *cpu);

                if (!hlist_empty(bucket))
                        return bucket->first;
        }
        return NULL;
}
EXPORT_SYMBOL(seq_hlist_next_percpu);