kernel/fs/zfs/zfs_rlock.c

   1 /*
   2  * CDDL HEADER START
   3  *
   4  * The contents of this file are subject to the terms of the
   5  * Common Development and Distribution License (the "License").
   6  * You may not use this file except in compliance with the License.
   7  *
   8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
   9  * or http://www.opensolaris.org/os/licensing.
  10  * See the License for the specific language governing permissions
  11  * and limitations under the License.
  12  *
  13  * When distributing Covered Code, include this CDDL HEADER in each
  14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
  15  * If applicable, add the following below this CDDL HEADER, with the
  16  * fields enclosed by brackets "[]" replaced with your own identifying
  17  * information: Portions Copyright [yyyy] [name of copyright owner]
  18  *
  19  * CDDL HEADER END
  20  */
  21 /*
  22  * Copyright 2010 Sun Microsystems, Inc.  All rights reserved.
  23  * Use is subject to license terms.
  24  */
  25 /*
  26  * Copyright (c) 2012 by Delphix. All rights reserved.
  27  */
  28
  29 /*
  30  * This file contains the code to implement file range locking in
  31  * ZFS, although there isn't much specific to ZFS (all that comes to mind is
  32  * support for growing the blocksize).
  33  *
  34  * Interface
  35  * ---------
  36  * Defined in zfs_rlock.h but essentially:
  37  *      rl = zfs_range_lock(zp, off, len, lock_type);
  38  *      zfs_range_unlock(rl);
  39  *      zfs_range_reduce(rl, off, len);
  40  *
  41  * AVL tree
  42  * --------
  43  * An AVL tree is used to maintain the state of the existing ranges
  44  * that are locked for exclusive (writer) or shared (reader) use.
  45  * The starting range offset is used for searching and sorting the tree.
  46  *
  47  * Common case
  48  * -----------
  49  * The (hopefully) usual case is of no overlaps or contention for
  50  * locks. On entry to zfs_lock_range() a rl_t is allocated; the tree
  51  * searched that finds no overlap, and *this* rl_t is placed in the tree.
  52  *
  53  * Overlaps/Reference counting/Proxy locks
  54  * ---------------------------------------
  55  * The avl code only allows one node at a particular offset. Also it's very
  56  * inefficient to search through all previous entries looking for overlaps
  57  * (because the very 1st in the ordered list might be at offset 0 but
  58  * cover the whole file).
  59  * So this implementation uses reference counts and proxy range locks.
  60  * Firstly, only reader locks use reference counts and proxy locks,
  61  * because writer locks are exclusive.
  62  * When a reader lock overlaps with another then a proxy lock is created
  63  * for that range and replaces the original lock. If the overlap
  64  * is exact then the reference count of the proxy is simply incremented.
  65  * Otherwise, the proxy lock is split into smaller lock ranges and
  66  * new proxy locks created for non overlapping ranges.
  67  * The reference counts are adjusted accordingly.
  68  * Meanwhile, the orginal lock is kept around (this is the callers handle)
  69  * and its offset and length are used when releasing the lock.
  70  *
  71  * Thread coordination
  72  * -------------------
  73  * In order to make wakeups efficient and to ensure multiple continuous
  74  * readers on a range don't starve a writer for the same range lock,
  75  * two condition variables are allocated in each rl_t.
  76  * If a writer (or reader) can't get a range it initialises the writer
  77  * (or reader) cv; sets a flag saying there's a writer (or reader) waiting;
  78  * and waits on that cv. When a thread unlocks that range it wakes up all
  79  * writers then all readers before destroying the lock.
  80  *
  81  * Append mode writes
  82  * ------------------
  83  * Append mode writes need to lock a range at the end of a file.
  84  * The offset of the end of the file is determined under the
  85  * range locking mutex, and the lock type converted from RL_APPEND to
  86  * RL_WRITER and the range locked.
  87  *
  88  * Grow block handling
  89  * -------------------
  90  * ZFS supports multiple block sizes currently upto 128K. The smallest
  91  * block size is used for the file which is grown as needed. During this
  92  * growth all other writers and readers must be excluded.
  93  * So if the block size needs to be grown then the whole file is
  94  * exclusively locked, then later the caller will reduce the lock
  95  * range to just the range to be written using zfs_reduce_range.
  96  */
  97
  98 #include <sys/zfs_rlock.h>
  99
 100 /*
 101  * Check if a write lock can be grabbed, or wait and recheck until available.
 102  */
 103 static void
 104 zfs_range_lock_writer(znode_t *zp, rl_t *new)
 105 {
 106         avl_tree_t *tree = &zp->z_range_avl;
 107         rl_t *rl;
 108         avl_index_t where;
 109         uint64_t end_size;
 110         uint64_t off = new->r_off;
 111         uint64_t len = new->r_len;
 112
 113         for (;;) {
 114                 /*
 115                  * Range locking is also used by zvol and uses a
 116                  * dummied up znode. However, for zvol, we don't need to
 117                  * append or grow blocksize, and besides we don't have
 118                  * a "sa" data or z_zfsvfs - so skip that processing.
 119                  *
 120                  * Yes, this is ugly, and would be solved by not handling
 121                  * grow or append in range lock code. If that was done then
 122                  * we could make the range locking code generically available
 123                  * to other non-zfs consumers.
 124                  */
 125                 if (zp->z_vnode) { /* caller is ZPL */
 126                         /*
 127                          * If in append mode pick up the current end of file.
 128                          * This is done under z_range_lock to avoid races.
 129                          */
 130                         if (new->r_type == RL_APPEND)
 131                                 new->r_off = zp->z_size;
 132
 133                         /*
 134                          * If we need to grow the block size then grab the whole
 135                          * file range. This is also done under z_range_lock to
 136                          * avoid races.
 137                          */
 138                         end_size = MAX(zp->z_size, new->r_off + len);
 139                         if (end_size > zp->z_blksz && (!ISP2(zp->z_blksz) ||
 140                             zp->z_blksz < zp->z_zfsvfs->z_max_blksz)) {
 141                                 new->r_off = 0;
 142                                 new->r_len = UINT64_MAX;
 143                         }
 144                 }
 145
 146                 /*
 147                  * First check for the usual case of no locks
 148                  */
 149                 if (avl_numnodes(tree) == 0) {
 150                         new->r_type = RL_WRITER; /* convert to writer */
 151                         avl_add(tree, new);
 152                         return;
 153                 }
 154
 155                 /*
 156                  * Look for any locks in the range.
 157                  */
 158                 rl = avl_find(tree, new, &where);
 159                 if (rl)
 160                         goto wait; /* already locked at same offset */
 161
 162                 rl = (rl_t *)avl_nearest(tree, where, AVL_AFTER);
 163                 if (rl && (rl->r_off < new->r_off + new->r_len))
 164                         goto wait;
 165
 166                 rl = (rl_t *)avl_nearest(tree, where, AVL_BEFORE);
 167                 if (rl && rl->r_off + rl->r_len > new->r_off)
 168                         goto wait;
 169
 170                 new->r_type = RL_WRITER; /* convert possible RL_APPEND */
 171                 avl_insert(tree, new, where);
 172                 return;
 173 wait:
 174                 if (!rl->r_write_wanted) {
 175                         cv_init(&rl->r_wr_cv, NULL, CV_DEFAULT, NULL);
 176                         rl->r_write_wanted = B_TRUE;
 177                 }
 178                 cv_wait(&rl->r_wr_cv, &zp->z_range_lock);
 179
 180                 /* reset to original */
 181                 new->r_off = off;
 182                 new->r_len = len;
 183         }
 184 }
 185
 186 /*
 187  * If this is an original (non-proxy) lock then replace it by
 188  * a proxy and return the proxy.
 189  */
 190 static rl_t *
 191 zfs_range_proxify(avl_tree_t *tree, rl_t *rl)
 192 {
 193         rl_t *proxy;
 194
 195         if (rl->r_proxy)
 196                 return (rl); /* already a proxy */
 197
 198         ASSERT3U(rl->r_cnt, ==, 1);
 199         ASSERT(rl->r_write_wanted == B_FALSE);
 200         ASSERT(rl->r_read_wanted == B_FALSE);
 201         avl_remove(tree, rl);
 202         rl->r_cnt = 0;
 203
 204         /* create a proxy range lock */
 205         proxy = kmem_alloc(sizeof (rl_t), KM_SLEEP);
 206         proxy->r_off = rl->r_off;
 207         proxy->r_len = rl->r_len;
 208         proxy->r_cnt = 1;
 209         proxy->r_type = RL_READER;
 210         proxy->r_proxy = B_TRUE;
 211         proxy->r_write_wanted = B_FALSE;
 212         proxy->r_read_wanted = B_FALSE;
 213         avl_add(tree, proxy);
 214
 215         return (proxy);
 216 }
 217
 218 /*
 219  * Split the range lock at the supplied offset
 220  * returning the *front* proxy.
 221  */
 222 static rl_t *
 223 zfs_range_split(avl_tree_t *tree, rl_t *rl, uint64_t off)
 224 {
 225         rl_t *front, *rear;
 226
 227         ASSERT3U(rl->r_len, >, 1);
 228         ASSERT3U(off, >, rl->r_off);
 229         ASSERT3U(off, <, rl->r_off + rl->r_len);
 230         ASSERT(rl->r_write_wanted == B_FALSE);
 231         ASSERT(rl->r_read_wanted == B_FALSE);
 232
 233         /* create the rear proxy range lock */
 234         rear = kmem_alloc(sizeof (rl_t), KM_SLEEP);
 235         rear->r_off = off;
 236         rear->r_len = rl->r_off + rl->r_len - off;
 237         rear->r_cnt = rl->r_cnt;
 238         rear->r_type = RL_READER;
 239         rear->r_proxy = B_TRUE;
 240         rear->r_write_wanted = B_FALSE;
 241         rear->r_read_wanted = B_FALSE;
 242
 243         front = zfs_range_proxify(tree, rl);
 244         front->r_len = off - rl->r_off;
 245
 246         avl_insert_here(tree, rear, front, AVL_AFTER);
 247         return (front);
 248 }
 249
 250 /*
 251  * Create and add a new proxy range lock for the supplied range.
 252  */
 253 static void
 254 zfs_range_new_proxy(avl_tree_t *tree, uint64_t off, uint64_t len)
 255 {
 256         rl_t *rl;
 257
 258         ASSERT(len);
 259         rl = kmem_alloc(sizeof (rl_t), KM_SLEEP);
 260         rl->r_off = off;
 261         rl->r_len = len;
 262         rl->r_cnt = 1;
 263         rl->r_type = RL_READER;
 264         rl->r_proxy = B_TRUE;
 265         rl->r_write_wanted = B_FALSE;
 266         rl->r_read_wanted = B_FALSE;
 267         avl_add(tree, rl);
 268 }
 269
 270 static void
 271 zfs_range_add_reader(avl_tree_t *tree, rl_t *new, rl_t *prev, avl_index_t where)
 272 {
 273         rl_t *next;
 274         uint64_t off = new->r_off;
 275         uint64_t len = new->r_len;
 276
 277         /*
 278          * prev arrives either:
 279          * - pointing to an entry at the same offset
 280          * - pointing to the entry with the closest previous offset whose
 281          *   range may overlap with the new range
 282          * - null, if there were no ranges starting before the new one
 283          */
 284         if (prev) {
 285                 if (prev->r_off + prev->r_len <= off) {
 286                         prev = NULL;
 287                 } else if (prev->r_off != off) {
 288                         /*
 289                          * convert to proxy if needed then
 290                          * split this entry and bump ref count
 291                          */
 292                         prev = zfs_range_split(tree, prev, off);
 293                         prev = AVL_NEXT(tree, prev); /* move to rear range */
 294                 }
 295         }
 296         ASSERT((prev == NULL) || (prev->r_off == off));
 297
 298         if (prev)
 299                 next = prev;
 300         else
 301                 next = (rl_t *)avl_nearest(tree, where, AVL_AFTER);
 302
 303         if (next == NULL || off + len <= next->r_off) {
 304                 /* no overlaps, use the original new rl_t in the tree */
 305                 avl_insert(tree, new, where);
 306                 return;
 307         }
 308
 309         if (off < next->r_off) {
 310                 /* Add a proxy for initial range before the overlap */
 311                 zfs_range_new_proxy(tree, off, next->r_off - off);
 312         }
 313
 314         new->r_cnt = 0; /* will use proxies in tree */
 315         /*
 316          * We now search forward through the ranges, until we go past the end
 317          * of the new range. For each entry we make it a proxy if it
 318          * isn't already, then bump its reference count. If there's any
 319          * gaps between the ranges then we create a new proxy range.
 320          */
 321         for (prev = NULL; next; prev = next, next = AVL_NEXT(tree, next)) {
 322                 if (off + len <= next->r_off)
 323                         break;
 324                 if (prev && prev->r_off + prev->r_len < next->r_off) {
 325                         /* there's a gap */
 326                         ASSERT3U(next->r_off, >, prev->r_off + prev->r_len);
 327                         zfs_range_new_proxy(tree, prev->r_off + prev->r_len,
 328                             next->r_off - (prev->r_off + prev->r_len));
 329                 }
 330                 if (off + len == next->r_off + next->r_len) {
 331                         /* exact overlap with end */
 332                         next = zfs_range_proxify(tree, next);
 333                         next->r_cnt++;
 334                         return;
 335                 }
 336                 if (off + len < next->r_off + next->r_len) {
 337                         /* new range ends in the middle of this block */
 338                         next = zfs_range_split(tree, next, off + len);
 339                         next->r_cnt++;
 340                         return;
 341                 }
 342                 ASSERT3U(off + len, >, next->r_off + next->r_len);
 343                 next = zfs_range_proxify(tree, next);
 344                 next->r_cnt++;
 345         }
 346
 347         /* Add the remaining end range. */
 348         zfs_range_new_proxy(tree, prev->r_off + prev->r_len,
 349             (off + len) - (prev->r_off + prev->r_len));
 350 }
 351
 352 /*
 353  * Check if a reader lock can be grabbed, or wait and recheck until available.
 354  */
 355 static void
 356 zfs_range_lock_reader(znode_t *zp, rl_t *new)
 357 {
 358         avl_tree_t *tree = &zp->z_range_avl;
 359         rl_t *prev, *next;
 360         avl_index_t where;
 361         uint64_t off = new->r_off;
 362         uint64_t len = new->r_len;
 363
 364         /*
 365          * Look for any writer locks in the range.
 366          */
 367 retry:
 368         prev = avl_find(tree, new, &where);
 369         if (prev == NULL)
 370                 prev = (rl_t *)avl_nearest(tree, where, AVL_BEFORE);
 371
 372         /*
 373          * Check the previous range for a writer lock overlap.
 374          */
 375         if (prev && (off < prev->r_off + prev->r_len)) {
 376                 if ((prev->r_type == RL_WRITER) || (prev->r_write_wanted)) {
 377                         if (!prev->r_read_wanted) {
 378                                 cv_init(&prev->r_rd_cv, NULL, CV_DEFAULT, NULL);
 379                                 prev->r_read_wanted = B_TRUE;
 380                         }
 381                         cv_wait(&prev->r_rd_cv, &zp->z_range_lock);
 382                         goto retry;
 383                 }
 384                 if (off + len < prev->r_off + prev->r_len)
 385                         goto got_lock;
 386         }
 387
 388         /*
 389          * Search through the following ranges to see if there's
 390          * write lock any overlap.
 391          */
 392         if (prev)
 393                 next = AVL_NEXT(tree, prev);
 394         else
 395                 next = (rl_t *)avl_nearest(tree, where, AVL_AFTER);
 396         for (; next; next = AVL_NEXT(tree, next)) {
 397                 if (off + len <= next->r_off)
 398                         goto got_lock;
 399                 if ((next->r_type == RL_WRITER) || (next->r_write_wanted)) {
 400                         if (!next->r_read_wanted) {
 401                                 cv_init(&next->r_rd_cv, NULL, CV_DEFAULT, NULL);
 402                                 next->r_read_wanted = B_TRUE;
 403                         }
 404                         cv_wait(&next->r_rd_cv, &zp->z_range_lock);
 405                         goto retry;
 406                 }
 407                 if (off + len <= next->r_off + next->r_len)
 408                         goto got_lock;
 409         }
 410
 411 got_lock:
 412         /*
 413          * Add the read lock, which may involve splitting existing
 414          * locks and bumping ref counts (r_cnt).
 415          */
 416         zfs_range_add_reader(tree, new, prev, where);
 417 }
 418
 419 /*
 420  * Lock a range (offset, length) as either shared (RL_READER)
 421  * or exclusive (RL_WRITER). Returns the range lock structure
 422  * for later unlocking or reduce range (if entire file
 423  * previously locked as RL_WRITER).
 424  */
 425 rl_t *
 426 zfs_range_lock(znode_t *zp, uint64_t off, uint64_t len, rl_type_t type)
 427 {
 428         rl_t *new;
 429
 430         ASSERT(type == RL_READER || type == RL_WRITER || type == RL_APPEND);
 431
 432         new = kmem_alloc(sizeof (rl_t), KM_SLEEP);
 433         new->r_zp = zp;
 434         new->r_off = off;
 435         if (len + off < off)    /* overflow */
 436                 len = UINT64_MAX - off;
 437         new->r_len = len;
 438         new->r_cnt = 1; /* assume it's going to be in the tree */
 439         new->r_type = type;
 440         new->r_proxy = B_FALSE;
 441         new->r_write_wanted = B_FALSE;
 442         new->r_read_wanted = B_FALSE;
 443
 444         mutex_enter(&zp->z_range_lock);
 445         if (type == RL_READER) {
 446                 /*
 447                  * First check for the usual case of no locks
 448                  */
 449                 if (avl_numnodes(&zp->z_range_avl) == 0)
 450                         avl_add(&zp->z_range_avl, new);
 451                 else
 452                         zfs_range_lock_reader(zp, new);
 453         } else
 454                 zfs_range_lock_writer(zp, new); /* RL_WRITER or RL_APPEND */
 455         mutex_exit(&zp->z_range_lock);
 456         return (new);
 457 }
 458
 459 /*
 460  * Unlock a reader lock
 461  */
 462 static void
 463 zfs_range_unlock_reader(znode_t *zp, rl_t *remove)
 464 {
 465         avl_tree_t *tree = &zp->z_range_avl;
 466         rl_t *rl, *next = NULL;
 467         uint64_t len;
 468
 469         /*
 470          * The common case is when the remove entry is in the tree
 471          * (cnt == 1) meaning there's been no other reader locks overlapping
 472          * with this one. Otherwise the remove entry will have been
 473          * removed from the tree and replaced by proxies (one or
 474          * more ranges mapping to the entire range).
 475          */
 476         if (remove->r_cnt == 1) {
 477                 avl_remove(tree, remove);
 478                 if (remove->r_write_wanted) {
 479                         cv_broadcast(&remove->r_wr_cv);
 480                         cv_destroy(&remove->r_wr_cv);
 481                 }
 482                 if (remove->r_read_wanted) {
 483                         cv_broadcast(&remove->r_rd_cv);
 484                         cv_destroy(&remove->r_rd_cv);
 485                 }
 486         } else {
 487                 ASSERT0(remove->r_cnt);
 488                 ASSERT0(remove->r_write_wanted);
 489                 ASSERT0(remove->r_read_wanted);
 490                 /*
 491                  * Find start proxy representing this reader lock,
 492                  * then decrement ref count on all proxies
 493                  * that make up this range, freeing them as needed.
 494                  */
 495                 rl = avl_find(tree, remove, NULL);
 496                 ASSERT(rl);
 497                 ASSERT(rl->r_cnt);
 498                 ASSERT(rl->r_type == RL_READER);
 499                 for (len = remove->r_len; len != 0; rl = next) {
 500                         len -= rl->r_len;
 501                         if (len) {
 502                                 next = AVL_NEXT(tree, rl);
 503                                 ASSERT(next);
 504                                 ASSERT(rl->r_off + rl->r_len == next->r_off);
 505                                 ASSERT(next->r_cnt);
 506                                 ASSERT(next->r_type == RL_READER);
 507                         }
 508                         rl->r_cnt--;
 509                         if (rl->r_cnt == 0) {
 510                                 avl_remove(tree, rl);
 511                                 if (rl->r_write_wanted) {
 512                                         cv_broadcast(&rl->r_wr_cv);
 513                                         cv_destroy(&rl->r_wr_cv);
 514                                 }
 515                                 if (rl->r_read_wanted) {
 516                                         cv_broadcast(&rl->r_rd_cv);
 517                                         cv_destroy(&rl->r_rd_cv);
 518                                 }
 519                                 kmem_free(rl, sizeof (rl_t));
 520                         }
 521                 }
 522         }
 523         kmem_free(remove, sizeof (rl_t));
 524 }
 525
 526 /*
 527  * Unlock range and destroy range lock structure.
 528  */
 529 void
 530 zfs_range_unlock(rl_t *rl)
 531 {
 532         znode_t *zp = rl->r_zp;
 533
 534         ASSERT(rl->r_type == RL_WRITER || rl->r_type == RL_READER);
 535         ASSERT(rl->r_cnt == 1 || rl->r_cnt == 0);
 536         ASSERT(!rl->r_proxy);
 537
 538         mutex_enter(&zp->z_range_lock);
 539         if (rl->r_type == RL_WRITER) {
 540                 /* writer locks can't be shared or split */
 541                 avl_remove(&zp->z_range_avl, rl);
 542                 mutex_exit(&zp->z_range_lock);
 543                 if (rl->r_write_wanted) {
 544                         cv_broadcast(&rl->r_wr_cv);
 545                         cv_destroy(&rl->r_wr_cv);
 546                 }
 547                 if (rl->r_read_wanted) {
 548                         cv_broadcast(&rl->r_rd_cv);
 549                         cv_destroy(&rl->r_rd_cv);
 550                 }
 551                 kmem_free(rl, sizeof (rl_t));
 552         } else {
 553                 /*
 554                  * lock may be shared, let zfs_range_unlock_reader()
 555                  * release the lock and free the rl_t
 556                  */
 557                 zfs_range_unlock_reader(zp, rl);
 558                 mutex_exit(&zp->z_range_lock);
 559         }
 560 }
 561
 562 /*
 563  * Reduce range locked as RL_WRITER from whole file to specified range.
 564  * Asserts the whole file is exclusivly locked and so there's only one
 565  * entry in the tree.
 566  */
 567 void
 568 zfs_range_reduce(rl_t *rl, uint64_t off, uint64_t len)
 569 {
 570         znode_t *zp = rl->r_zp;
 571
 572         /* Ensure there are no other locks */
 573         ASSERT(avl_numnodes(&zp->z_range_avl) == 1);
 574         ASSERT(rl->r_off == 0);
 575         ASSERT(rl->r_type == RL_WRITER);
 576         ASSERT(!rl->r_proxy);
 577         ASSERT3U(rl->r_len, ==, UINT64_MAX);
 578         ASSERT3U(rl->r_cnt, ==, 1);
 579
 580         mutex_enter(&zp->z_range_lock);
 581         rl->r_off = off;
 582         rl->r_len = len;
 583         mutex_exit(&zp->z_range_lock);
 584         if (rl->r_write_wanted)
 585                 cv_broadcast(&rl->r_wr_cv);
 586         if (rl->r_read_wanted)
 587                 cv_broadcast(&rl->r_rd_cv);
 588 }
 589
 590 /*
 591  * AVL comparison function used to order range locks
 592  * Locks are ordered on the start offset of the range.
 593  */
 594 int
 595 zfs_range_compare(const void *arg1, const void *arg2)
 596 {
 597         const rl_t *rl1 = arg1;
 598         const rl_t *rl2 = arg2;
 599
 600         if (rl1->r_off > rl2->r_off)
 601                 return (1);
 602         if (rl1->r_off < rl2->r_off)
 603                 return (-1);
 604         return (0);
 605 }