lib/rhashtable.c

   1 /*
   2  * Resizable, Scalable, Concurrent Hash Table
   3  *
   4  * Copyright (c) 2015 Herbert Xu <herbert@gondor.apana.org.au>
   5  * Copyright (c) 2014-2015 Thomas Graf <tgraf@suug.ch>
   6  * Copyright (c) 2008-2014 Patrick McHardy <kaber@trash.net>
   7  *
   8  * Code partially derived from nft_hash
   9  * Rewritten with rehash code from br_multicast plus single list
  10  * pointer as suggested by Josh Triplett
  11  *
  12  * This program is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU General Public License version 2 as
  14  * published by the Free Software Foundation.
  15  */
  16
  17 #include <linux/atomic.h>
  18 #include <linux/kernel.h>
  19 #include <linux/init.h>
  20 #include <linux/log2.h>
  21 #include <linux/sched.h>
  22 #include <linux/slab.h>
  23 #include <linux/vmalloc.h>
  24 #include <linux/mm.h>
  25 #include <linux/jhash.h>
  26 #include <linux/random.h>
  27 #include <linux/rhashtable.h>
  28 #include <linux/err.h>
  29 #include <linux/export.h>
  30
  31 #define HASH_DEFAULT_SIZE       64UL
  32 #define HASH_MIN_SIZE           4U
  33 #define BUCKET_LOCKS_PER_CPU   128UL
  34
  35 static u32 head_hashfn(struct rhashtable *ht,
  36                        const struct bucket_table *tbl,
  37                        const struct rhash_head *he)
  38 {
  39         return rht_head_hashfn(ht, tbl, he, ht->p);
  40 }
  41
  42 #ifdef CONFIG_PROVE_LOCKING
  43 #define ASSERT_RHT_MUTEX(HT) BUG_ON(!lockdep_rht_mutex_is_held(HT))
  44
  45 int lockdep_rht_mutex_is_held(struct rhashtable *ht)
  46 {
  47         return (debug_locks) ? lockdep_is_held(&ht->mutex) : 1;
  48 }
  49 EXPORT_SYMBOL_GPL(lockdep_rht_mutex_is_held);
  50
  51 int lockdep_rht_bucket_is_held(const struct bucket_table *tbl, u32 hash)
  52 {
  53         spinlock_t *lock = rht_bucket_lock(tbl, hash);
  54
  55         return (debug_locks) ? lockdep_is_held(lock) : 1;
  56 }
  57 EXPORT_SYMBOL_GPL(lockdep_rht_bucket_is_held);
  58 #else
  59 #define ASSERT_RHT_MUTEX(HT)
  60 #endif
  61
  62
  63 static int alloc_bucket_locks(struct rhashtable *ht, struct bucket_table *tbl,
  64                               gfp_t gfp)
  65 {
  66         unsigned int i, size;
  67 #if defined(CONFIG_PROVE_LOCKING)
  68         unsigned int nr_pcpus = 2;
  69 #else
  70         unsigned int nr_pcpus = num_possible_cpus();
  71 #endif
  72
  73         nr_pcpus = min_t(unsigned int, nr_pcpus, 32UL);
  74         size = roundup_pow_of_two(nr_pcpus * ht->p.locks_mul);
  75
  76         /* Never allocate more than 0.5 locks per bucket */
  77         size = min_t(unsigned int, size, tbl->size >> 1);
  78
  79         if (sizeof(spinlock_t) != 0) {
  80 #ifdef CONFIG_NUMA
  81                 if (size * sizeof(spinlock_t) > PAGE_SIZE &&
  82                     gfp == GFP_KERNEL)
  83                         tbl->locks = vmalloc(size * sizeof(spinlock_t));
  84                 else
  85 #endif
  86                 tbl->locks = kmalloc_array(size, sizeof(spinlock_t),
  87                                            gfp);
  88                 if (!tbl->locks)
  89                         return -ENOMEM;
  90                 for (i = 0; i < size; i++)
  91                         spin_lock_init(&tbl->locks[i]);
  92         }
  93         tbl->locks_mask = size - 1;
  94
  95         return 0;
  96 }
  97
  98 static void bucket_table_free(const struct bucket_table *tbl)
  99 {
 100         if (tbl)
 101                 kvfree(tbl->locks);
 102
 103         kvfree(tbl);
 104 }
 105
 106 static void bucket_table_free_rcu(struct rcu_head *head)
 107 {
 108         bucket_table_free(container_of(head, struct bucket_table, rcu));
 109 }
 110
 111 static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
 112                                                size_t nbuckets,
 113                                                gfp_t gfp)
 114 {
 115         struct bucket_table *tbl = NULL;
 116         size_t size;
 117         int i;
 118
 119         size = sizeof(*tbl) + nbuckets * sizeof(tbl->buckets[0]);
 120         if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER) ||
 121             gfp != GFP_KERNEL)
 122                 tbl = kzalloc(size, gfp | __GFP_NOWARN | __GFP_NORETRY);
 123         if (tbl == NULL && gfp == GFP_KERNEL)
 124                 tbl = vzalloc(size);
 125         if (tbl == NULL)
 126                 return NULL;
 127
 128         tbl->size = nbuckets;
 129
 130         if (alloc_bucket_locks(ht, tbl, gfp) < 0) {
 131                 bucket_table_free(tbl);
 132                 return NULL;
 133         }
 134
 135         INIT_LIST_HEAD(&tbl->walkers);
 136
 137         get_random_bytes(&tbl->hash_rnd, sizeof(tbl->hash_rnd));
 138
 139         for (i = 0; i < nbuckets; i++)
 140                 INIT_RHT_NULLS_HEAD(tbl->buckets[i], ht, i);
 141
 142         return tbl;
 143 }
 144
 145 static struct bucket_table *rhashtable_last_table(struct rhashtable *ht,
 146                                                   struct bucket_table *tbl)
 147 {
 148         struct bucket_table *new_tbl;
 149
 150         do {
 151                 new_tbl = tbl;
 152                 tbl = rht_dereference_rcu(tbl->future_tbl, ht);
 153         } while (tbl);
 154
 155         return new_tbl;
 156 }
 157
 158 static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
 159 {
 160         struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
 161         struct bucket_table *new_tbl = rhashtable_last_table(ht,
 162                 rht_dereference_rcu(old_tbl->future_tbl, ht));
 163         struct rhash_head __rcu **pprev = &old_tbl->buckets[old_hash];
 164         int err = -ENOENT;
 165         struct rhash_head *head, *next, *entry;
 166         spinlock_t *new_bucket_lock;
 167         unsigned int new_hash;
 168
 169         rht_for_each(entry, old_tbl, old_hash) {
 170                 err = 0;
 171                 next = rht_dereference_bucket(entry->next, old_tbl, old_hash);
 172
 173                 if (rht_is_a_nulls(next))
 174                         break;
 175
 176                 pprev = &entry->next;
 177         }
 178
 179         if (err)
 180                 goto out;
 181
 182         new_hash = head_hashfn(ht, new_tbl, entry);
 183
 184         new_bucket_lock = rht_bucket_lock(new_tbl, new_hash);
 185
 186         spin_lock_nested(new_bucket_lock, SINGLE_DEPTH_NESTING);
 187         head = rht_dereference_bucket(new_tbl->buckets[new_hash],
 188                                       new_tbl, new_hash);
 189
 190         if (rht_is_a_nulls(head))
 191                 INIT_RHT_NULLS_HEAD(entry->next, ht, new_hash);
 192         else
 193                 RCU_INIT_POINTER(entry->next, head);
 194
 195         rcu_assign_pointer(new_tbl->buckets[new_hash], entry);
 196         spin_unlock(new_bucket_lock);
 197
 198         rcu_assign_pointer(*pprev, next);
 199
 200 out:
 201         return err;
 202 }
 203
 204 static void rhashtable_rehash_chain(struct rhashtable *ht,
 205                                     unsigned int old_hash)
 206 {
 207         struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
 208         spinlock_t *old_bucket_lock;
 209
 210         old_bucket_lock = rht_bucket_lock(old_tbl, old_hash);
 211
 212         spin_lock_bh(old_bucket_lock);
 213         while (!rhashtable_rehash_one(ht, old_hash))
 214                 ;
 215         old_tbl->rehash++;
 216         spin_unlock_bh(old_bucket_lock);
 217 }
 218
 219 static int rhashtable_rehash_attach(struct rhashtable *ht,
 220                                     struct bucket_table *old_tbl,
 221                                     struct bucket_table *new_tbl)
 222 {
 223         /* Protect future_tbl using the first bucket lock. */
 224         spin_lock_bh(old_tbl->locks);
 225
 226         /* Did somebody beat us to it? */
 227         if (rcu_access_pointer(old_tbl->future_tbl)) {
 228                 spin_unlock_bh(old_tbl->locks);
 229                 return -EEXIST;
 230         }
 231
 232         /* Make insertions go into the new, empty table right away. Deletions
 233          * and lookups will be attempted in both tables until we synchronize.
 234          */
 235         rcu_assign_pointer(old_tbl->future_tbl, new_tbl);
 236
 237         /* Ensure the new table is visible to readers. */
 238         smp_wmb();
 239
 240         spin_unlock_bh(old_tbl->locks);
 241
 242         return 0;
 243 }
 244
 245 static int rhashtable_rehash_table(struct rhashtable *ht)
 246 {
 247         struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
 248         struct bucket_table *new_tbl;
 249         struct rhashtable_walker *walker;
 250         unsigned int old_hash;
 251
 252         new_tbl = rht_dereference(old_tbl->future_tbl, ht);
 253         if (!new_tbl)
 254                 return 0;
 255
 256         for (old_hash = 0; old_hash < old_tbl->size; old_hash++)
 257                 rhashtable_rehash_chain(ht, old_hash);
 258
 259         /* Publish the new table pointer. */
 260         rcu_assign_pointer(ht->tbl, new_tbl);
 261
 262         spin_lock(&ht->lock);
 263         list_for_each_entry(walker, &old_tbl->walkers, list)
 264                 walker->tbl = NULL;
 265         spin_unlock(&ht->lock);
 266
 267         /* Wait for readers. All new readers will see the new
 268          * table, and thus no references to the old table will
 269          * remain.
 270          */
 271         call_rcu(&old_tbl->rcu, bucket_table_free_rcu);
 272
 273         return rht_dereference(new_tbl->future_tbl, ht) ? -EAGAIN : 0;
 274 }
 275
 276 /**
 277  * rhashtable_expand - Expand hash table while allowing concurrent lookups
 278  * @ht:         the hash table to expand
 279  *
 280  * A secondary bucket array is allocated and the hash entries are migrated.
 281  *
 282  * This function may only be called in a context where it is safe to call
 283  * synchronize_rcu(), e.g. not within a rcu_read_lock() section.
 284  *
 285  * The caller must ensure that no concurrent resizing occurs by holding
 286  * ht->mutex.
 287  *
 288  * It is valid to have concurrent insertions and deletions protected by per
 289  * bucket locks or concurrent RCU protected lookups and traversals.
 290  */
 291 static int rhashtable_expand(struct rhashtable *ht)
 292 {
 293         struct bucket_table *new_tbl, *old_tbl = rht_dereference(ht->tbl, ht);
 294         int err;
 295
 296         ASSERT_RHT_MUTEX(ht);
 297
 298         old_tbl = rhashtable_last_table(ht, old_tbl);
 299
 300         new_tbl = bucket_table_alloc(ht, old_tbl->size * 2, GFP_KERNEL);
 301         if (new_tbl == NULL)
 302                 return -ENOMEM;
 303
 304         err = rhashtable_rehash_attach(ht, old_tbl, new_tbl);
 305         if (err)
 306                 bucket_table_free(new_tbl);
 307
 308         return err;
 309 }
 310
 311 /**
 312  * rhashtable_shrink - Shrink hash table while allowing concurrent lookups
 313  * @ht:         the hash table to shrink
 314  *
 315  * This function shrinks the hash table to fit, i.e., the smallest
 316  * size would not cause it to expand right away automatically.
 317  *
 318  * The caller must ensure that no concurrent resizing occurs by holding
 319  * ht->mutex.
 320  *
 321  * The caller must ensure that no concurrent table mutations take place.
 322  * It is however valid to have concurrent lookups if they are RCU protected.
 323  *
 324  * It is valid to have concurrent insertions and deletions protected by per
 325  * bucket locks or concurrent RCU protected lookups and traversals.
 326  */
 327 static int rhashtable_shrink(struct rhashtable *ht)
 328 {
 329         struct bucket_table *new_tbl, *old_tbl = rht_dereference(ht->tbl, ht);
 330         unsigned int size;
 331         int err;
 332
 333         ASSERT_RHT_MUTEX(ht);
 334
 335         size = roundup_pow_of_two(atomic_read(&ht->nelems) * 3 / 2);
 336         if (size < ht->p.min_size)
 337                 size = ht->p.min_size;
 338
 339         if (old_tbl->size <= size)
 340                 return 0;
 341
 342         if (rht_dereference(old_tbl->future_tbl, ht))
 343                 return -EEXIST;
 344
 345         new_tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
 346         if (new_tbl == NULL)
 347                 return -ENOMEM;
 348
 349         err = rhashtable_rehash_attach(ht, old_tbl, new_tbl);
 350         if (err)
 351                 bucket_table_free(new_tbl);
 352
 353         return err;
 354 }
 355
 356 static void rht_deferred_worker(struct work_struct *work)
 357 {
 358         struct rhashtable *ht;
 359         struct bucket_table *tbl;
 360         int err = 0;
 361
 362         ht = container_of(work, struct rhashtable, run_work);
 363         mutex_lock(&ht->mutex);
 364
 365         tbl = rht_dereference(ht->tbl, ht);
 366         tbl = rhashtable_last_table(ht, tbl);
 367
 368         if (rht_grow_above_75(ht, tbl))
 369                 rhashtable_expand(ht);
 370         else if (ht->p.automatic_shrinking && rht_shrink_below_30(ht, tbl))
 371                 rhashtable_shrink(ht);
 372
 373         err = rhashtable_rehash_table(ht);
 374
 375         mutex_unlock(&ht->mutex);
 376
 377         if (err)
 378                 schedule_work(&ht->run_work);
 379 }
 380
 381 static bool rhashtable_check_elasticity(struct rhashtable *ht,
 382                                         struct bucket_table *tbl,
 383                                         unsigned int hash)
 384 {
 385         unsigned int elasticity = ht->elasticity;
 386         struct rhash_head *head;
 387
 388         rht_for_each(head, tbl, hash)
 389                 if (!--elasticity)
 390                         return true;
 391
 392         return false;
 393 }
 394
 395 int rhashtable_insert_rehash(struct rhashtable *ht)
 396 {
 397         struct bucket_table *old_tbl;
 398         struct bucket_table *new_tbl;
 399         struct bucket_table *tbl;
 400         unsigned int size;
 401         int err;
 402
 403         old_tbl = rht_dereference_rcu(ht->tbl, ht);
 404         tbl = rhashtable_last_table(ht, old_tbl);
 405
 406         size = tbl->size;
 407
 408         if (rht_grow_above_75(ht, tbl))
 409                 size *= 2;
 410         /* Do not schedule more than one rehash */
 411         else if (old_tbl != tbl)
 412                 return -EBUSY;
 413
 414         new_tbl = bucket_table_alloc(ht, size, GFP_ATOMIC);
 415         if (new_tbl == NULL) {
 416                 /* Schedule async resize/rehash to try allocation
 417                  * non-atomic context.
 418                  */
 419                 schedule_work(&ht->run_work);
 420                 return -ENOMEM;
 421         }
 422
 423         err = rhashtable_rehash_attach(ht, tbl, new_tbl);
 424         if (err) {
 425                 bucket_table_free(new_tbl);
 426                 if (err == -EEXIST)
 427                         err = 0;
 428         } else
 429                 schedule_work(&ht->run_work);
 430
 431         return err;
 432 }
 433 EXPORT_SYMBOL_GPL(rhashtable_insert_rehash);
 434
 435 int rhashtable_insert_slow(struct rhashtable *ht, const void *key,
 436                            struct rhash_head *obj,
 437                            struct bucket_table *tbl)
 438 {
 439         struct rhash_head *head;
 440         unsigned int hash;
 441         int err;
 442
 443         tbl = rhashtable_last_table(ht, tbl);
 444         hash = head_hashfn(ht, tbl, obj);
 445         spin_lock_nested(rht_bucket_lock(tbl, hash), SINGLE_DEPTH_NESTING);
 446
 447         err = -EEXIST;
 448         if (key && rhashtable_lookup_fast(ht, key, ht->p))
 449                 goto exit;
 450
 451         err = -E2BIG;
 452         if (unlikely(rht_grow_above_max(ht, tbl)))
 453                 goto exit;
 454
 455         err = -EAGAIN;
 456         if (rhashtable_check_elasticity(ht, tbl, hash) ||
 457             rht_grow_above_100(ht, tbl))
 458                 goto exit;
 459
 460         err = 0;
 461
 462         head = rht_dereference_bucket(tbl->buckets[hash], tbl, hash);
 463
 464         RCU_INIT_POINTER(obj->next, head);
 465
 466         rcu_assign_pointer(tbl->buckets[hash], obj);
 467
 468         atomic_inc(&ht->nelems);
 469
 470 exit:
 471         spin_unlock(rht_bucket_lock(tbl, hash));
 472
 473         return err;
 474 }
 475 EXPORT_SYMBOL_GPL(rhashtable_insert_slow);
 476
 477 /**
 478  * rhashtable_walk_init - Initialise an iterator
 479  * @ht:         Table to walk over
 480  * @iter:       Hash table Iterator
 481  *
 482  * This function prepares a hash table walk.
 483  *
 484  * Note that if you restart a walk after rhashtable_walk_stop you
 485  * may see the same object twice.  Also, you may miss objects if
 486  * there are removals in between rhashtable_walk_stop and the next
 487  * call to rhashtable_walk_start.
 488  *
 489  * For a completely stable walk you should construct your own data
 490  * structure outside the hash table.
 491  *
 492  * This function may sleep so you must not call it from interrupt
 493  * context or with spin locks held.
 494  *
 495  * You must call rhashtable_walk_exit if this function returns
 496  * successfully.
 497  */
 498 int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter)
 499 {
 500         iter->ht = ht;
 501         iter->p = NULL;
 502         iter->slot = 0;
 503         iter->skip = 0;
 504
 505         iter->walker = kmalloc(sizeof(*iter->walker), GFP_KERNEL);
 506         if (!iter->walker)
 507                 return -ENOMEM;
 508
 509         mutex_lock(&ht->mutex);
 510         iter->walker->tbl = rht_dereference(ht->tbl, ht);
 511         list_add(&iter->walker->list, &iter->walker->tbl->walkers);
 512         mutex_unlock(&ht->mutex);
 513
 514         return 0;
 515 }
 516 EXPORT_SYMBOL_GPL(rhashtable_walk_init);
 517
 518 /**
 519  * rhashtable_walk_exit - Free an iterator
 520  * @iter:       Hash table Iterator
 521  *
 522  * This function frees resources allocated by rhashtable_walk_init.
 523  */
 524 void rhashtable_walk_exit(struct rhashtable_iter *iter)
 525 {
 526         mutex_lock(&iter->ht->mutex);
 527         if (iter->walker->tbl)
 528                 list_del(&iter->walker->list);
 529         mutex_unlock(&iter->ht->mutex);
 530         kfree(iter->walker);
 531 }
 532 EXPORT_SYMBOL_GPL(rhashtable_walk_exit);
 533
 534 /**
 535  * rhashtable_walk_start - Start a hash table walk
 536  * @iter:       Hash table iterator
 537  *
 538  * Start a hash table walk.  Note that we take the RCU lock in all
 539  * cases including when we return an error.  So you must always call
 540  * rhashtable_walk_stop to clean up.
 541  *
 542  * Returns zero if successful.
 543  *
 544  * Returns -EAGAIN if resize event occured.  Note that the iterator
 545  * will rewind back to the beginning and you may use it immediately
 546  * by calling rhashtable_walk_next.
 547  */
 548 int rhashtable_walk_start(struct rhashtable_iter *iter)
 549         __acquires(RCU)
 550 {
 551         struct rhashtable *ht = iter->ht;
 552
 553         mutex_lock(&ht->mutex);
 554
 555         if (iter->walker->tbl)
 556                 list_del(&iter->walker->list);
 557
 558         rcu_read_lock();
 559
 560         mutex_unlock(&ht->mutex);
 561
 562         if (!iter->walker->tbl) {
 563                 iter->walker->tbl = rht_dereference_rcu(ht->tbl, ht);
 564                 return -EAGAIN;
 565         }
 566
 567         return 0;
 568 }
 569 EXPORT_SYMBOL_GPL(rhashtable_walk_start);
 570
 571 /**
 572  * rhashtable_walk_next - Return the next object and advance the iterator
 573  * @iter:       Hash table iterator
 574  *
 575  * Note that you must call rhashtable_walk_stop when you are finished
 576  * with the walk.
 577  *
 578  * Returns the next object or NULL when the end of the table is reached.
 579  *
 580  * Returns -EAGAIN if resize event occured.  Note that the iterator
 581  * will rewind back to the beginning and you may continue to use it.
 582  */
 583 void *rhashtable_walk_next(struct rhashtable_iter *iter)
 584 {
 585         struct bucket_table *tbl = iter->walker->tbl;
 586         struct rhashtable *ht = iter->ht;
 587         struct rhash_head *p = iter->p;
 588
 589         if (p) {
 590                 p = rht_dereference_bucket_rcu(p->next, tbl, iter->slot);
 591                 goto next;
 592         }
 593
 594         for (; iter->slot < tbl->size; iter->slot++) {
 595                 int skip = iter->skip;
 596
 597                 rht_for_each_rcu(p, tbl, iter->slot) {
 598                         if (!skip)
 599                                 break;
 600                         skip--;
 601                 }
 602
 603 next:
 604                 if (!rht_is_a_nulls(p)) {
 605                         iter->skip++;
 606                         iter->p = p;
 607                         return rht_obj(ht, p);
 608                 }
 609
 610                 iter->skip = 0;
 611         }
 612
 613         iter->p = NULL;
 614
 615         /* Ensure we see any new tables. */
 616         smp_rmb();
 617
 618         iter->walker->tbl = rht_dereference_rcu(tbl->future_tbl, ht);
 619         if (iter->walker->tbl) {
 620                 iter->slot = 0;
 621                 iter->skip = 0;
 622                 return ERR_PTR(-EAGAIN);
 623         }
 624
 625         return NULL;
 626 }
 627 EXPORT_SYMBOL_GPL(rhashtable_walk_next);
 628
 629 /**
 630  * rhashtable_walk_stop - Finish a hash table walk
 631  * @iter:       Hash table iterator
 632  *
 633  * Finish a hash table walk.
 634  */
 635 void rhashtable_walk_stop(struct rhashtable_iter *iter)
 636         __releases(RCU)
 637 {
 638         struct rhashtable *ht;
 639         struct bucket_table *tbl = iter->walker->tbl;
 640
 641         if (!tbl)
 642                 goto out;
 643
 644         ht = iter->ht;
 645
 646         spin_lock(&ht->lock);
 647         if (tbl->rehash < tbl->size)
 648                 list_add(&iter->walker->list, &tbl->walkers);
 649         else
 650                 iter->walker->tbl = NULL;
 651         spin_unlock(&ht->lock);
 652
 653         iter->p = NULL;
 654
 655 out:
 656         rcu_read_unlock();
 657 }
 658 EXPORT_SYMBOL_GPL(rhashtable_walk_stop);
 659
 660 static size_t rounded_hashtable_size(const struct rhashtable_params *params)
 661 {
 662         return max(roundup_pow_of_two(params->nelem_hint * 4 / 3),
 663                    (unsigned long)params->min_size);
 664 }
 665
 666 static u32 rhashtable_jhash2(const void *key, u32 length, u32 seed)
 667 {
 668         return jhash2(key, length, seed);
 669 }
 670
 671 /**
 672  * rhashtable_init - initialize a new hash table
 673  * @ht:         hash table to be initialized
 674  * @params:     configuration parameters
 675  *
 676  * Initializes a new hash table based on the provided configuration
 677  * parameters. A table can be configured either with a variable or
 678  * fixed length key:
 679  *
 680  * Configuration Example 1: Fixed length keys
 681  * struct test_obj {
 682  *      int                     key;
 683  *      void *                  my_member;
 684  *      struct rhash_head       node;
 685  * };
 686  *
 687  * struct rhashtable_params params = {
 688  *      .head_offset = offsetof(struct test_obj, node),
 689  *      .key_offset = offsetof(struct test_obj, key),
 690  *      .key_len = sizeof(int),
 691  *      .hashfn = jhash,
 692  *      .nulls_base = (1U << RHT_BASE_SHIFT),
 693  * };
 694  *
 695  * Configuration Example 2: Variable length keys
 696  * struct test_obj {
 697  *      [...]
 698  *      struct rhash_head       node;
 699  * };
 700  *
 701  * u32 my_hash_fn(const void *data, u32 len, u32 seed)
 702  * {
 703  *      struct test_obj *obj = data;
 704  *
 705  *      return [... hash ...];
 706  * }
 707  *
 708  * struct rhashtable_params params = {
 709  *      .head_offset = offsetof(struct test_obj, node),
 710  *      .hashfn = jhash,
 711  *      .obj_hashfn = my_hash_fn,
 712  * };
 713  */
 714 int rhashtable_init(struct rhashtable *ht,
 715                     const struct rhashtable_params *params)
 716 {
 717         struct bucket_table *tbl;
 718         size_t size;
 719
 720         size = HASH_DEFAULT_SIZE;
 721
 722         if ((!params->key_len && !params->obj_hashfn) ||
 723             (params->obj_hashfn && !params->obj_cmpfn))
 724                 return -EINVAL;
 725
 726         if (params->nulls_base && params->nulls_base < (1U << RHT_BASE_SHIFT))
 727                 return -EINVAL;
 728
 729         if (params->nelem_hint)
 730                 size = rounded_hashtable_size(params);
 731
 732         memset(ht, 0, sizeof(*ht));
 733         mutex_init(&ht->mutex);
 734         spin_lock_init(&ht->lock);
 735         memcpy(&ht->p, params, sizeof(*params));
 736
 737         if (params->min_size)
 738                 ht->p.min_size = roundup_pow_of_two(params->min_size);
 739
 740         if (params->max_size)
 741                 ht->p.max_size = rounddown_pow_of_two(params->max_size);
 742
 743         if (params->insecure_max_entries)
 744                 ht->p.insecure_max_entries =
 745                         rounddown_pow_of_two(params->insecure_max_entries);
 746         else
 747                 ht->p.insecure_max_entries = ht->p.max_size * 2;
 748
 749         ht->p.min_size = max(ht->p.min_size, HASH_MIN_SIZE);
 750
 751         /* The maximum (not average) chain length grows with the
 752          * size of the hash table, at a rate of (log N)/(log log N).
 753          * The value of 16 is selected so that even if the hash
 754          * table grew to 2^32 you would not expect the maximum
 755          * chain length to exceed it unless we are under attack
 756          * (or extremely unlucky).
 757          *
 758          * As this limit is only to detect attacks, we don't need
 759          * to set it to a lower value as you'd need the chain
 760          * length to vastly exceed 16 to have any real effect
 761          * on the system.
 762          */
 763         if (!params->insecure_elasticity)
 764                 ht->elasticity = 16;
 765
 766         if (params->locks_mul)
 767                 ht->p.locks_mul = roundup_pow_of_two(params->locks_mul);
 768         else
 769                 ht->p.locks_mul = BUCKET_LOCKS_PER_CPU;
 770
 771         ht->key_len = ht->p.key_len;
 772         if (!params->hashfn) {
 773                 ht->p.hashfn = jhash;
 774
 775                 if (!(ht->key_len & (sizeof(u32) - 1))) {
 776                         ht->key_len /= sizeof(u32);
 777                         ht->p.hashfn = rhashtable_jhash2;
 778                 }
 779         }
 780
 781         tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
 782         if (tbl == NULL)
 783                 return -ENOMEM;
 784
 785         atomic_set(&ht->nelems, 0);
 786
 787         RCU_INIT_POINTER(ht->tbl, tbl);
 788
 789         INIT_WORK(&ht->run_work, rht_deferred_worker);
 790
 791         return 0;
 792 }
 793 EXPORT_SYMBOL_GPL(rhashtable_init);
 794
 795 /**
 796  * rhashtable_free_and_destroy - free elements and destroy hash table
 797  * @ht:         the hash table to destroy
 798  * @free_fn:    callback to release resources of element
 799  * @arg:        pointer passed to free_fn
 800  *
 801  * Stops an eventual async resize. If defined, invokes free_fn for each
 802  * element to releasal resources. Please note that RCU protected
 803  * readers may still be accessing the elements. Releasing of resources
 804  * must occur in a compatible manner. Then frees the bucket array.
 805  *
 806  * This function will eventually sleep to wait for an async resize
 807  * to complete. The caller is responsible that no further write operations
 808  * occurs in parallel.
 809  */
 810 void rhashtable_free_and_destroy(struct rhashtable *ht,
 811                                  void (*free_fn)(void *ptr, void *arg),
 812                                  void *arg)
 813 {
 814         const struct bucket_table *tbl;
 815         unsigned int i;
 816
 817         cancel_work_sync(&ht->run_work);
 818
 819         mutex_lock(&ht->mutex);
 820         tbl = rht_dereference(ht->tbl, ht);
 821         if (free_fn) {
 822                 for (i = 0; i < tbl->size; i++) {
 823                         struct rhash_head *pos, *next;
 824
 825                         for (pos = rht_dereference(tbl->buckets[i], ht),
 826                              next = !rht_is_a_nulls(pos) ?
 827                                         rht_dereference(pos->next, ht) : NULL;
 828                              !rht_is_a_nulls(pos);
 829                              pos = next,
 830                              next = !rht_is_a_nulls(pos) ?
 831                                         rht_dereference(pos->next, ht) : NULL)
 832                                 free_fn(rht_obj(ht, pos), arg);
 833                 }
 834         }
 835
 836         bucket_table_free(tbl);
 837         mutex_unlock(&ht->mutex);
 838 }
 839 EXPORT_SYMBOL_GPL(rhashtable_free_and_destroy);
 840
 841 void rhashtable_destroy(struct rhashtable *ht)
 842 {
 843         return rhashtable_free_and_destroy(ht, NULL, NULL);
 844 }
 845 EXPORT_SYMBOL_GPL(rhashtable_destroy);