linux/fs/seq_file.c
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   1/*
   2 * linux/fs/seq_file.c
   3 *
   4 * helper functions for making synthetic files from sequences of records.
   5 * initial implementation -- AV, Oct 2001.
   6 */
   7
   8#include <linux/fs.h>
   9#include <linux/module.h>
  10#include <linux/seq_file.h>
  11#include <linux/slab.h>
  12
  13#include <asm/uaccess.h>
  14#include <asm/page.h>
  15
  16/**
  17 *      seq_open -      initialize sequential file
  18 *      @file: file we initialize
  19 *      @op: method table describing the sequence
  20 *
  21 *      seq_open() sets @file, associating it with a sequence described
  22 *      by @op.  @op->start() sets the iterator up and returns the first
  23 *      element of sequence. @op->stop() shuts it down.  @op->next()
  24 *      returns the next element of sequence.  @op->show() prints element
  25 *      into the buffer.  In case of error ->start() and ->next() return
  26 *      ERR_PTR(error).  In the end of sequence they return %NULL. ->show()
  27 *      returns 0 in case of success and negative number in case of error.
  28 *      Returning SEQ_SKIP means "discard this element and move on".
  29 */
  30int seq_open(struct file *file, const struct seq_operations *op)
  31{
  32        struct seq_file *p = file->private_data;
  33
  34        if (!p) {
  35                p = kmalloc(sizeof(*p), GFP_KERNEL);
  36                if (!p)
  37                        return -ENOMEM;
  38                file->private_data = p;
  39        }
  40        memset(p, 0, sizeof(*p));
  41        mutex_init(&p->lock);
  42        p->op = op;
  43
  44        /*
  45         * Wrappers around seq_open(e.g. swaps_open) need to be
  46         * aware of this. If they set f_version themselves, they
  47         * should call seq_open first and then set f_version.
  48         */
  49        file->f_version = 0;
  50
  51        /*
  52         * seq_files support lseek() and pread().  They do not implement
  53         * write() at all, but we clear FMODE_PWRITE here for historical
  54         * reasons.
  55         *
  56         * If a client of seq_files a) implements file.write() and b) wishes to
  57         * support pwrite() then that client will need to implement its own
  58         * file.open() which calls seq_open() and then sets FMODE_PWRITE.
  59         */
  60        file->f_mode &= ~FMODE_PWRITE;
  61        return 0;
  62}
  63EXPORT_SYMBOL(seq_open);
  64
  65static int traverse(struct seq_file *m, loff_t offset)
  66{
  67        loff_t pos = 0, index;
  68        int error = 0;
  69        void *p;
  70
  71        m->version = 0;
  72        index = 0;
  73        m->count = m->from = 0;
  74        if (!offset) {
  75                m->index = index;
  76                return 0;
  77        }
  78        if (!m->buf) {
  79                m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);
  80                if (!m->buf)
  81                        return -ENOMEM;
  82        }
  83        p = m->op->start(m, &index);
  84        while (p) {
  85                error = PTR_ERR(p);
  86                if (IS_ERR(p))
  87                        break;
  88                error = m->op->show(m, p);
  89                if (error < 0)
  90                        break;
  91                if (unlikely(error)) {
  92                        error = 0;
  93                        m->count = 0;
  94                }
  95                if (m->count == m->size)
  96                        goto Eoverflow;
  97                if (pos + m->count > offset) {
  98                        m->from = offset - pos;
  99                        m->count -= m->from;
 100                        m->index = index;
 101                        break;
 102                }
 103                pos += m->count;
 104                m->count = 0;
 105                if (pos == offset) {
 106                        index++;
 107                        m->index = index;
 108                        break;
 109                }
 110                p = m->op->next(m, p, &index);
 111        }
 112        m->op->stop(m, p);
 113        m->index = index;
 114        return error;
 115
 116Eoverflow:
 117        m->op->stop(m, p);
 118        kfree(m->buf);
 119        m->buf = kmalloc(m->size <<= 1, GFP_KERNEL);
 120        return !m->buf ? -ENOMEM : -EAGAIN;
 121}
 122
 123/**
 124 *      seq_read -      ->read() method for sequential files.
 125 *      @file: the file to read from
 126 *      @buf: the buffer to read to
 127 *      @size: the maximum number of bytes to read
 128 *      @ppos: the current position in the file
 129 *
 130 *      Ready-made ->f_op->read()
 131 */
 132ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 133{
 134        struct seq_file *m = file->private_data;
 135        size_t copied = 0;
 136        loff_t pos;
 137        size_t n;
 138        void *p;
 139        int err = 0;
 140
 141        mutex_lock(&m->lock);
 142
 143        /* Don't assume *ppos is where we left it */
 144        if (unlikely(*ppos != m->read_pos)) {
 145                m->read_pos = *ppos;
 146                while ((err = traverse(m, *ppos)) == -EAGAIN)
 147                        ;
 148                if (err) {
 149                        /* With prejudice... */
 150                        m->read_pos = 0;
 151                        m->version = 0;
 152                        m->index = 0;
 153                        m->count = 0;
 154                        goto Done;
 155                }
 156        }
 157
 158        /*
 159         * seq_file->op->..m_start/m_stop/m_next may do special actions
 160         * or optimisations based on the file->f_version, so we want to
 161         * pass the file->f_version to those methods.
 162         *
 163         * seq_file->version is just copy of f_version, and seq_file
 164         * methods can treat it simply as file version.
 165         * It is copied in first and copied out after all operations.
 166         * It is convenient to have it as  part of structure to avoid the
 167         * need of passing another argument to all the seq_file methods.
 168         */
 169        m->version = file->f_version;
 170        /* grab buffer if we didn't have one */
 171        if (!m->buf) {
 172                m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);
 173                if (!m->buf)
 174                        goto Enomem;
 175        }
 176        /* if not empty - flush it first */
 177        if (m->count) {
 178                n = min(m->count, size);
 179                err = copy_to_user(buf, m->buf + m->from, n);
 180                if (err)
 181                        goto Efault;
 182                m->count -= n;
 183                m->from += n;
 184                size -= n;
 185                buf += n;
 186                copied += n;
 187                if (!m->count)
 188                        m->index++;
 189                if (!size)
 190                        goto Done;
 191        }
 192        /* we need at least one record in buffer */
 193        pos = m->index;
 194        p = m->op->start(m, &pos);
 195        while (1) {
 196                err = PTR_ERR(p);
 197                if (!p || IS_ERR(p))
 198                        break;
 199                err = m->op->show(m, p);
 200                if (err < 0)
 201                        break;
 202                if (unlikely(err))
 203                        m->count = 0;
 204                if (unlikely(!m->count)) {
 205                        p = m->op->next(m, p, &pos);
 206                        m->index = pos;
 207                        continue;
 208                }
 209                if (m->count < m->size)
 210                        goto Fill;
 211                m->op->stop(m, p);
 212                kfree(m->buf);
 213                m->buf = kmalloc(m->size <<= 1, GFP_KERNEL);
 214                if (!m->buf)
 215                        goto Enomem;
 216                m->count = 0;
 217                m->version = 0;
 218                pos = m->index;
 219                p = m->op->start(m, &pos);
 220        }
 221        m->op->stop(m, p);
 222        m->count = 0;
 223        goto Done;
 224Fill:
 225        /* they want more? let's try to get some more */
 226        while (m->count < size) {
 227                size_t offs = m->count;
 228                loff_t next = pos;
 229                p = m->op->next(m, p, &next);
 230                if (!p || IS_ERR(p)) {
 231                        err = PTR_ERR(p);
 232                        break;
 233                }
 234                err = m->op->show(m, p);
 235                if (m->count == m->size || err) {
 236                        m->count = offs;
 237                        if (likely(err <= 0))
 238                                break;
 239                }
 240                pos = next;
 241        }
 242        m->op->stop(m, p);
 243        n = min(m->count, size);
 244        err = copy_to_user(buf, m->buf, n);
 245        if (err)
 246                goto Efault;
 247        copied += n;
 248        m->count -= n;
 249        if (m->count)
 250                m->from = n;
 251        else
 252                pos++;
 253        m->index = pos;
 254Done:
 255        if (!copied)
 256                copied = err;
 257        else {
 258                *ppos += copied;
 259                m->read_pos += copied;
 260        }
 261        file->f_version = m->version;
 262        mutex_unlock(&m->lock);
 263        return copied;
 264Enomem:
 265        err = -ENOMEM;
 266        goto Done;
 267Efault:
 268        err = -EFAULT;
 269        goto Done;
 270}
 271EXPORT_SYMBOL(seq_read);
 272
 273/**
 274 *      seq_lseek -     ->llseek() method for sequential files.
 275 *      @file: the file in question
 276 *      @offset: new position
 277 *      @origin: 0 for absolute, 1 for relative position
 278 *
 279 *      Ready-made ->f_op->llseek()
 280 */
 281loff_t seq_lseek(struct file *file, loff_t offset, int origin)
 282{
 283        struct seq_file *m = file->private_data;
 284        loff_t retval = -EINVAL;
 285
 286        mutex_lock(&m->lock);
 287        m->version = file->f_version;
 288        switch (origin) {
 289                case 1:
 290                        offset += file->f_pos;
 291                case 0:
 292                        if (offset < 0)
 293                                break;
 294                        retval = offset;
 295                        if (offset != m->read_pos) {
 296                                while ((retval=traverse(m, offset)) == -EAGAIN)
 297                                        ;
 298                                if (retval) {
 299                                        /* with extreme prejudice... */
 300                                        file->f_pos = 0;
 301                                        m->read_pos = 0;
 302                                        m->version = 0;
 303                                        m->index = 0;
 304                                        m->count = 0;
 305                                } else {
 306                                        m->read_pos = offset;
 307                                        retval = file->f_pos = offset;
 308                                }
 309                        }
 310        }
 311        file->f_version = m->version;
 312        mutex_unlock(&m->lock);
 313        return retval;
 314}
 315EXPORT_SYMBOL(seq_lseek);
 316
 317/**
 318 *      seq_release -   free the structures associated with sequential file.
 319 *      @file: file in question
 320 *      @inode: file->f_path.dentry->d_inode
 321 *
 322 *      Frees the structures associated with sequential file; can be used
 323 *      as ->f_op->release() if you don't have private data to destroy.
 324 */
 325int seq_release(struct inode *inode, struct file *file)
 326{
 327        struct seq_file *m = file->private_data;
 328        kfree(m->buf);
 329        kfree(m);
 330        return 0;
 331}
 332EXPORT_SYMBOL(seq_release);
 333
 334/**
 335 *      seq_escape -    print string into buffer, escaping some characters
 336 *      @m:     target buffer
 337 *      @s:     string
 338 *      @esc:   set of characters that need escaping
 339 *
 340 *      Puts string into buffer, replacing each occurrence of character from
 341 *      @esc with usual octal escape.  Returns 0 in case of success, -1 - in
 342 *      case of overflow.
 343 */
 344int seq_escape(struct seq_file *m, const char *s, const char *esc)
 345{
 346        char *end = m->buf + m->size;
 347        char *p;
 348        char c;
 349
 350        for (p = m->buf + m->count; (c = *s) != '\0' && p < end; s++) {
 351                if (!strchr(esc, c)) {
 352                        *p++ = c;
 353                        continue;
 354                }
 355                if (p + 3 < end) {
 356                        *p++ = '\\';
 357                        *p++ = '0' + ((c & 0300) >> 6);
 358                        *p++ = '0' + ((c & 070) >> 3);
 359                        *p++ = '0' + (c & 07);
 360                        continue;
 361                }
 362                m->count = m->size;
 363                return -1;
 364        }
 365        m->count = p - m->buf;
 366        return 0;
 367}
 368EXPORT_SYMBOL(seq_escape);
 369
 370int seq_printf(struct seq_file *m, const char *f, ...)
 371{
 372        va_list args;
 373        int len;
 374
 375        if (m->count < m->size) {
 376                va_start(args, f);
 377                len = vsnprintf(m->buf + m->count, m->size - m->count, f, args);
 378                va_end(args);
 379                if (m->count + len < m->size) {
 380                        m->count += len;
 381                        return 0;
 382                }
 383        }
 384        m->count = m->size;
 385        return -1;
 386}
 387EXPORT_SYMBOL(seq_printf);
 388
 389/**
 390 *      mangle_path -   mangle and copy path to buffer beginning
 391 *      @s: buffer start
 392 *      @p: beginning of path in above buffer
 393 *      @esc: set of characters that need escaping
 394 *
 395 *      Copy the path from @p to @s, replacing each occurrence of character from
 396 *      @esc with usual octal escape.
 397 *      Returns pointer past last written character in @s, or NULL in case of
 398 *      failure.
 399 */
 400char *mangle_path(char *s, char *p, char *esc)
 401{
 402        while (s <= p) {
 403                char c = *p++;
 404                if (!c) {
 405                        return s;
 406                } else if (!strchr(esc, c)) {
 407                        *s++ = c;
 408                } else if (s + 4 > p) {
 409                        break;
 410                } else {
 411                        *s++ = '\\';
 412                        *s++ = '0' + ((c & 0300) >> 6);
 413                        *s++ = '0' + ((c & 070) >> 3);
 414                        *s++ = '0' + (c & 07);
 415                }
 416        }
 417        return NULL;
 418}
 419EXPORT_SYMBOL(mangle_path);
 420
 421/**
 422 * seq_path - seq_file interface to print a pathname
 423 * @m: the seq_file handle
 424 * @path: the struct path to print
 425 * @esc: set of characters to escape in the output
 426 *
 427 * return the absolute path of 'path', as represented by the
 428 * dentry / mnt pair in the path parameter.
 429 */
 430int seq_path(struct seq_file *m, struct path *path, char *esc)
 431{
 432        char *buf;
 433        size_t size = seq_get_buf(m, &buf);
 434        int res = -1;
 435
 436        if (size) {
 437                char *p = d_path(path, buf, size);
 438                if (!IS_ERR(p)) {
 439                        char *end = mangle_path(buf, p, esc);
 440                        if (end)
 441                                res = end - buf;
 442                }
 443        }
 444        seq_commit(m, res);
 445
 446        return res;
 447}
 448EXPORT_SYMBOL(seq_path);
 449
 450/*
 451 * Same as seq_path, but relative to supplied root.
 452 */
 453int seq_path_root(struct seq_file *m, struct path *path, struct path *root,
 454                  char *esc)
 455{
 456        char *buf;
 457        size_t size = seq_get_buf(m, &buf);
 458        int res = -ENAMETOOLONG;
 459
 460        if (size) {
 461                char *p;
 462
 463                p = __d_path(path, root, buf, size);
 464                if (!p)
 465                        return SEQ_SKIP;
 466                res = PTR_ERR(p);
 467                if (!IS_ERR(p)) {
 468                        char *end = mangle_path(buf, p, esc);
 469                        if (end)
 470                                res = end - buf;
 471                        else
 472                                res = -ENAMETOOLONG;
 473                }
 474        }
 475        seq_commit(m, res);
 476
 477        return res < 0 && res != -ENAMETOOLONG ? res : 0;
 478}
 479
 480/*
 481 * returns the path of the 'dentry' from the root of its filesystem.
 482 */
 483int seq_dentry(struct seq_file *m, struct dentry *dentry, char *esc)
 484{
 485        char *buf;
 486        size_t size = seq_get_buf(m, &buf);
 487        int res = -1;
 488
 489        if (size) {
 490                char *p = dentry_path(dentry, buf, size);
 491                if (!IS_ERR(p)) {
 492                        char *end = mangle_path(buf, p, esc);
 493                        if (end)
 494                                res = end - buf;
 495                }
 496        }
 497        seq_commit(m, res);
 498
 499        return res;
 500}
 501
 502int seq_bitmap(struct seq_file *m, const unsigned long *bits,
 503                                   unsigned int nr_bits)
 504{
 505        if (m->count < m->size) {
 506                int len = bitmap_scnprintf(m->buf + m->count,
 507                                m->size - m->count, bits, nr_bits);
 508                if (m->count + len < m->size) {
 509                        m->count += len;
 510                        return 0;
 511                }
 512        }
 513        m->count = m->size;
 514        return -1;
 515}
 516EXPORT_SYMBOL(seq_bitmap);
 517
 518int seq_bitmap_list(struct seq_file *m, const unsigned long *bits,
 519                unsigned int nr_bits)
 520{
 521        if (m->count < m->size) {
 522                int len = bitmap_scnlistprintf(m->buf + m->count,
 523                                m->size - m->count, bits, nr_bits);
 524                if (m->count + len < m->size) {
 525                        m->count += len;
 526                        return 0;
 527                }
 528        }
 529        m->count = m->size;
 530        return -1;
 531}
 532EXPORT_SYMBOL(seq_bitmap_list);
 533
 534static void *single_start(struct seq_file *p, loff_t *pos)
 535{
 536        return NULL + (*pos == 0);
 537}
 538
 539static void *single_next(struct seq_file *p, void *v, loff_t *pos)
 540{
 541        ++*pos;
 542        return NULL;
 543}
 544
 545static void single_stop(struct seq_file *p, void *v)
 546{
 547}
 548
 549int single_open(struct file *file, int (*show)(struct seq_file *, void *),
 550                void *data)
 551{
 552        struct seq_operations *op = kmalloc(sizeof(*op), GFP_KERNEL);
 553        int res = -ENOMEM;
 554
 555        if (op) {
 556                op->start = single_start;
 557                op->next = single_next;
 558                op->stop = single_stop;
 559                op->show = show;
 560                res = seq_open(file, op);
 561                if (!res)
 562                        ((struct seq_file *)file->private_data)->private = data;
 563                else
 564                        kfree(op);
 565        }
 566        return res;
 567}
 568EXPORT_SYMBOL(single_open);
 569
 570int single_release(struct inode *inode, struct file *file)
 571{
 572        const struct seq_operations *op = ((struct seq_file *)file->private_data)->op;
 573        int res = seq_release(inode, file);
 574        kfree(op);
 575        return res;
 576}
 577EXPORT_SYMBOL(single_release);
 578
 579int seq_release_private(struct inode *inode, struct file *file)
 580{
 581        struct seq_file *seq = file->private_data;
 582
 583        kfree(seq->private);
 584        seq->private = NULL;
 585        return seq_release(inode, file);
 586}
 587EXPORT_SYMBOL(seq_release_private);
 588
 589void *__seq_open_private(struct file *f, const struct seq_operations *ops,
 590                int psize)
 591{
 592        int rc;
 593        void *private;
 594        struct seq_file *seq;
 595
 596        private = kzalloc(psize, GFP_KERNEL);
 597        if (private == NULL)
 598                goto out;
 599
 600        rc = seq_open(f, ops);
 601        if (rc < 0)
 602                goto out_free;
 603
 604        seq = f->private_data;
 605        seq->private = private;
 606        return private;
 607
 608out_free:
 609        kfree(private);
 610out:
 611        return NULL;
 612}
 613EXPORT_SYMBOL(__seq_open_private);
 614
 615int seq_open_private(struct file *filp, const struct seq_operations *ops,
 616                int psize)
 617{
 618        return __seq_open_private(filp, ops, psize) ? 0 : -ENOMEM;
 619}
 620EXPORT_SYMBOL(seq_open_private);
 621
 622int seq_putc(struct seq_file *m, char c)
 623{
 624        if (m->count < m->size) {
 625                m->buf[m->count++] = c;
 626                return 0;
 627        }
 628        return -1;
 629}
 630EXPORT_SYMBOL(seq_putc);
 631
 632int seq_puts(struct seq_file *m, const char *s)
 633{
 634        int len = strlen(s);
 635        if (m->count + len < m->size) {
 636                memcpy(m->buf + m->count, s, len);
 637                m->count += len;
 638                return 0;
 639        }
 640        m->count = m->size;
 641        return -1;
 642}
 643EXPORT_SYMBOL(seq_puts);
 644
 645/**
 646 * seq_write - write arbitrary data to buffer
 647 * @seq: seq_file identifying the buffer to which data should be written
 648 * @data: data address
 649 * @len: number of bytes
 650 *
 651 * Return 0 on success, non-zero otherwise.
 652 */
 653int seq_write(struct seq_file *seq, const void *data, size_t len)
 654{
 655        if (seq->count + len < seq->size) {
 656                memcpy(seq->buf + seq->count, data, len);
 657                seq->count += len;
 658                return 0;
 659        }
 660        seq->count = seq->size;
 661        return -1;
 662}
 663EXPORT_SYMBOL(seq_write);
 664
 665struct list_head *seq_list_start(struct list_head *head, loff_t pos)
 666{
 667        struct list_head *lh;
 668
 669        list_for_each(lh, head)
 670                if (pos-- == 0)
 671                        return lh;
 672
 673        return NULL;
 674}
 675EXPORT_SYMBOL(seq_list_start);
 676
 677struct list_head *seq_list_start_head(struct list_head *head, loff_t pos)
 678{
 679        if (!pos)
 680                return head;
 681
 682        return seq_list_start(head, pos - 1);
 683}
 684EXPORT_SYMBOL(seq_list_start_head);
 685
 686struct list_head *seq_list_next(void *v, struct list_head *head, loff_t *ppos)
 687{
 688        struct list_head *lh;
 689
 690        lh = ((struct list_head *)v)->next;
 691        ++*ppos;
 692        return lh == head ? NULL : lh;
 693}
 694EXPORT_SYMBOL(seq_list_next);
 695
 696/**
 697 * seq_hlist_start - start an iteration of a hlist
 698 * @head: the head of the hlist
 699 * @pos:  the start position of the sequence
 700 *
 701 * Called at seq_file->op->start().
 702 */
 703struct hlist_node *seq_hlist_start(struct hlist_head *head, loff_t pos)
 704{
 705        struct hlist_node *node;
 706
 707        hlist_for_each(node, head)
 708                if (pos-- == 0)
 709                        return node;
 710        return NULL;
 711}
 712EXPORT_SYMBOL(seq_hlist_start);
 713
 714/**
 715 * seq_hlist_start_head - start an iteration of a hlist
 716 * @head: the head of the hlist
 717 * @pos:  the start position of the sequence
 718 *
 719 * Called at seq_file->op->start(). Call this function if you want to
 720 * print a header at the top of the output.
 721 */
 722struct hlist_node *seq_hlist_start_head(struct hlist_head *head, loff_t pos)
 723{
 724        if (!pos)
 725                return SEQ_START_TOKEN;
 726
 727        return seq_hlist_start(head, pos - 1);
 728}
 729EXPORT_SYMBOL(seq_hlist_start_head);
 730
 731/**
 732 * seq_hlist_next - move to the next position of the hlist
 733 * @v:    the current iterator
 734 * @head: the head of the hlist
 735 * @ppos: the current position
 736 *
 737 * Called at seq_file->op->next().
 738 */
 739struct hlist_node *seq_hlist_next(void *v, struct hlist_head *head,
 740                                  loff_t *ppos)
 741{
 742        struct hlist_node *node = v;
 743
 744        ++*ppos;
 745        if (v == SEQ_START_TOKEN)
 746                return head->first;
 747        else
 748                return node->next;
 749}
 750EXPORT_SYMBOL(seq_hlist_next);
 751
 752/**
 753 * seq_hlist_start_rcu - start an iteration of a hlist protected by RCU
 754 * @head: the head of the hlist
 755 * @pos:  the start position of the sequence
 756 *
 757 * Called at seq_file->op->start().
 758 *
 759 * This list-traversal primitive may safely run concurrently with
 760 * the _rcu list-mutation primitives such as hlist_add_head_rcu()
 761 * as long as the traversal is guarded by rcu_read_lock().
 762 */
 763struct hlist_node *seq_hlist_start_rcu(struct hlist_head *head,
 764                                       loff_t pos)
 765{
 766        struct hlist_node *node;
 767
 768        __hlist_for_each_rcu(node, head)
 769                if (pos-- == 0)
 770                        return node;
 771        return NULL;
 772}
 773EXPORT_SYMBOL(seq_hlist_start_rcu);
 774
 775/**
 776 * seq_hlist_start_head_rcu - start an iteration of a hlist protected by RCU
 777 * @head: the head of the hlist
 778 * @pos:  the start position of the sequence
 779 *
 780 * Called at seq_file->op->start(). Call this function if you want to
 781 * print a header at the top of the output.
 782 *
 783 * This list-traversal primitive may safely run concurrently with
 784 * the _rcu list-mutation primitives such as hlist_add_head_rcu()
 785 * as long as the traversal is guarded by rcu_read_lock().
 786 */
 787struct hlist_node *seq_hlist_start_head_rcu(struct hlist_head *head,
 788                                            loff_t pos)
 789{
 790        if (!pos)
 791                return SEQ_START_TOKEN;
 792
 793        return seq_hlist_start_rcu(head, pos - 1);
 794}
 795EXPORT_SYMBOL(seq_hlist_start_head_rcu);
 796
 797/**
 798 * seq_hlist_next_rcu - move to the next position of the hlist protected by RCU
 799 * @v:    the current iterator
 800 * @head: the head of the hlist
 801 * @ppos: the current position
 802 *
 803 * Called at seq_file->op->next().
 804 *
 805 * This list-traversal primitive may safely run concurrently with
 806 * the _rcu list-mutation primitives such as hlist_add_head_rcu()
 807 * as long as the traversal is guarded by rcu_read_lock().
 808 */
 809struct hlist_node *seq_hlist_next_rcu(void *v,
 810                                      struct hlist_head *head,
 811                                      loff_t *ppos)
 812{
 813        struct hlist_node *node = v;
 814
 815        ++*ppos;
 816        if (v == SEQ_START_TOKEN)
 817                return rcu_dereference(head->first);
 818        else
 819                return rcu_dereference(node->next);
 820}
 821EXPORT_SYMBOL(seq_hlist_next_rcu);
 822
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