EVM: prevent array underflow in evm_write_xattrs()

[linux-2.6/btrfs-unstable.git] / lib / rhashtable.c

/*
 * Resizable, Scalable, Concurrent Hash Table
 *
 * Copyright (c) 2015 Herbert Xu <herbert@gondor.apana.org.au>
 * Copyright (c) 2014-2015 Thomas Graf <tgraf@suug.ch>
 * Copyright (c) 2008-2014 Patrick McHardy <kaber@trash.net>
 *
 * Code partially derived from nft_hash
 * Rewritten with rehash code from br_multicast plus single list
 * pointer as suggested by Josh Triplett
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */

#include <linux/atomic.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/log2.h>
#include <linux/sched.h>
#include <linux/rculist.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/mm.h>
#include <linux/jhash.h>
#include <linux/random.h>
#include <linux/rhashtable.h>
#include <linux/err.h>
#include <linux/export.h>

#define HASH_DEFAULT_SIZE       64UL
#define HASH_MIN_SIZE           4U
#define BUCKET_LOCKS_PER_CPU    32UL

union nested_table {
        union nested_table __rcu *table;
        struct rhash_head __rcu *bucket;
};

static u32 head_hashfn(struct rhashtable *ht,
                       const struct bucket_table *tbl,
                       const struct rhash_head *he)
{
        return rht_head_hashfn(ht, tbl, he, ht->p);
}

#ifdef CONFIG_PROVE_LOCKING
#define ASSERT_RHT_MUTEX(HT) BUG_ON(!lockdep_rht_mutex_is_held(HT))

int lockdep_rht_mutex_is_held(struct rhashtable *ht)
{
        return (debug_locks) ? lockdep_is_held(&ht->mutex) : 1;
}
EXPORT_SYMBOL_GPL(lockdep_rht_mutex_is_held);

int lockdep_rht_bucket_is_held(const struct bucket_table *tbl, u32 hash)
{
        spinlock_t *lock = rht_bucket_lock(tbl, hash);

        return (debug_locks) ? lockdep_is_held(lock) : 1;
}
EXPORT_SYMBOL_GPL(lockdep_rht_bucket_is_held);
#else
#define ASSERT_RHT_MUTEX(HT)
#endif

static void nested_table_free(union nested_table *ntbl, unsigned int size)
{
        const unsigned int shift = PAGE_SHIFT - ilog2(sizeof(void *));
        const unsigned int len = 1 << shift;
        unsigned int i;

        ntbl = rcu_dereference_raw(ntbl->table);
        if (!ntbl)
                return;

        if (size > len) {
                size >>= shift;
                for (i = 0; i < len; i++)
                        nested_table_free(ntbl + i, size);
        }

        kfree(ntbl);
}

static void nested_bucket_table_free(const struct bucket_table *tbl)
{
        unsigned int size = tbl->size >> tbl->nest;
        unsigned int len = 1 << tbl->nest;
        union nested_table *ntbl;
        unsigned int i;

        ntbl = (union nested_table *)rcu_dereference_raw(tbl->buckets[0]);

        for (i = 0; i < len; i++)
                nested_table_free(ntbl + i, size);

        kfree(ntbl);
}

static void bucket_table_free(const struct bucket_table *tbl)
{
        if (tbl->nest)
                nested_bucket_table_free(tbl);

        free_bucket_spinlocks(tbl->locks);
        kvfree(tbl);
}

static void bucket_table_free_rcu(struct rcu_head *head)
{
        bucket_table_free(container_of(head, struct bucket_table, rcu));
}

static union nested_table *nested_table_alloc(struct rhashtable *ht,
                                              union nested_table __rcu **prev,
                                              unsigned int shifted,
                                              unsigned int nhash)
{
        union nested_table *ntbl;
        int i;

        ntbl = rcu_dereference(*prev);
        if (ntbl)
                return ntbl;

        ntbl = kzalloc(PAGE_SIZE, GFP_ATOMIC);

        if (ntbl && shifted) {
                for (i = 0; i < PAGE_SIZE / sizeof(ntbl[0].bucket); i++)
                        INIT_RHT_NULLS_HEAD(ntbl[i].bucket, ht,
                                            (i << shifted) | nhash);
        }

        rcu_assign_pointer(*prev, ntbl);

        return ntbl;
}

static struct bucket_table *nested_bucket_table_alloc(struct rhashtable *ht,
                                                      size_t nbuckets,
                                                      gfp_t gfp)
{
        const unsigned int shift = PAGE_SHIFT - ilog2(sizeof(void *));
        struct bucket_table *tbl;
        size_t size;

        if (nbuckets < (1 << (shift + 1)))
                return NULL;

        size = sizeof(*tbl) + sizeof(tbl->buckets[0]);

        tbl = kzalloc(size, gfp);
        if (!tbl)
                return NULL;

        if (!nested_table_alloc(ht, (union nested_table __rcu **)tbl->buckets,
                                0, 0)) {
                kfree(tbl);
                return NULL;
        }

        tbl->nest = (ilog2(nbuckets) - 1) % shift + 1;

        return tbl;
}

static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
                                               size_t nbuckets,
                                               gfp_t gfp)
{
        struct bucket_table *tbl = NULL;
        size_t size, max_locks;
        int i;

        size = sizeof(*tbl) + nbuckets * sizeof(tbl->buckets[0]);
        if (gfp != GFP_KERNEL)
                tbl = kzalloc(size, gfp | __GFP_NOWARN | __GFP_NORETRY);
        else
                tbl = kvzalloc(size, gfp);

        size = nbuckets;

        if (tbl == NULL && gfp != GFP_KERNEL) {
                tbl = nested_bucket_table_alloc(ht, nbuckets, gfp);
                nbuckets = 0;
        }
        if (tbl == NULL)
                return NULL;

        tbl->size = size;

        max_locks = size >> 1;
        if (tbl->nest)
                max_locks = min_t(size_t, max_locks, 1U << tbl->nest);

        if (alloc_bucket_spinlocks(&tbl->locks, &tbl->locks_mask, max_locks,
                                   ht->p.locks_mul, gfp) < 0) {
                bucket_table_free(tbl);
                return NULL;
        }

        INIT_LIST_HEAD(&tbl->walkers);

        tbl->hash_rnd = get_random_u32();

        for (i = 0; i < nbuckets; i++)
                INIT_RHT_NULLS_HEAD(tbl->buckets[i], ht, i);

        return tbl;
}

static struct bucket_table *rhashtable_last_table(struct rhashtable *ht,
                                                  struct bucket_table *tbl)
{
        struct bucket_table *new_tbl;

        do {
                new_tbl = tbl;
                tbl = rht_dereference_rcu(tbl->future_tbl, ht);
        } while (tbl);

        return new_tbl;
}

static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
{
        struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
        struct bucket_table *new_tbl = rhashtable_last_table(ht,
                rht_dereference_rcu(old_tbl->future_tbl, ht));
        struct rhash_head __rcu **pprev = rht_bucket_var(old_tbl, old_hash);
        int err = -EAGAIN;
        struct rhash_head *head, *next, *entry;
        spinlock_t *new_bucket_lock;
        unsigned int new_hash;

        if (new_tbl->nest)
                goto out;

        err = -ENOENT;

        rht_for_each(entry, old_tbl, old_hash) {
                err = 0;
                next = rht_dereference_bucket(entry->next, old_tbl, old_hash);

                if (rht_is_a_nulls(next))
                        break;

                pprev = &entry->next;
        }

        if (err)
                goto out;

        new_hash = head_hashfn(ht, new_tbl, entry);

        new_bucket_lock = rht_bucket_lock(new_tbl, new_hash);

        spin_lock_nested(new_bucket_lock, SINGLE_DEPTH_NESTING);
        head = rht_dereference_bucket(new_tbl->buckets[new_hash],
                                      new_tbl, new_hash);

        RCU_INIT_POINTER(entry->next, head);

        rcu_assign_pointer(new_tbl->buckets[new_hash], entry);
        spin_unlock(new_bucket_lock);

        rcu_assign_pointer(*pprev, next);

out:
        return err;
}

static int rhashtable_rehash_chain(struct rhashtable *ht,
                                    unsigned int old_hash)
{
        struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
        spinlock_t *old_bucket_lock;
        int err;

        old_bucket_lock = rht_bucket_lock(old_tbl, old_hash);

        spin_lock_bh(old_bucket_lock);
        while (!(err = rhashtable_rehash_one(ht, old_hash)))
                ;

        if (err == -ENOENT) {
                old_tbl->rehash++;
                err = 0;
        }
        spin_unlock_bh(old_bucket_lock);

        return err;
}

static int rhashtable_rehash_attach(struct rhashtable *ht,
                                    struct bucket_table *old_tbl,
                                    struct bucket_table *new_tbl)
{
        /* Protect future_tbl using the first bucket lock. */
        spin_lock_bh(old_tbl->locks);

        /* Did somebody beat us to it? */
        if (rcu_access_pointer(old_tbl->future_tbl)) {
                spin_unlock_bh(old_tbl->locks);
                return -EEXIST;
        }

        /* Make insertions go into the new, empty table right away. Deletions
         * and lookups will be attempted in both tables until we synchronize.
         */
        rcu_assign_pointer(old_tbl->future_tbl, new_tbl);

        spin_unlock_bh(old_tbl->locks);

        return 0;
}

static int rhashtable_rehash_table(struct rhashtable *ht)
{
        struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
        struct bucket_table *new_tbl;
        struct rhashtable_walker *walker;
        unsigned int old_hash;
        int err;

        new_tbl = rht_dereference(old_tbl->future_tbl, ht);
        if (!new_tbl)
                return 0;

        for (old_hash = 0; old_hash < old_tbl->size; old_hash++) {
                err = rhashtable_rehash_chain(ht, old_hash);
                if (err)
                        return err;
                cond_resched();
        }

        /* Publish the new table pointer. */
        rcu_assign_pointer(ht->tbl, new_tbl);

        spin_lock(&ht->lock);
        list_for_each_entry(walker, &old_tbl->walkers, list)
                walker->tbl = NULL;
        spin_unlock(&ht->lock);

        /* Wait for readers. All new readers will see the new
         * table, and thus no references to the old table will
         * remain.
         */
        call_rcu(&old_tbl->rcu, bucket_table_free_rcu);

        return rht_dereference(new_tbl->future_tbl, ht) ? -EAGAIN : 0;
}

static int rhashtable_rehash_alloc(struct rhashtable *ht,
                                   struct bucket_table *old_tbl,
                                   unsigned int size)
{
        struct bucket_table *new_tbl;
        int err;

        ASSERT_RHT_MUTEX(ht);

        new_tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
        if (new_tbl == NULL)
                return -ENOMEM;

        err = rhashtable_rehash_attach(ht, old_tbl, new_tbl);
        if (err)
                bucket_table_free(new_tbl);

        return err;
}

/**
 * rhashtable_shrink - Shrink hash table while allowing concurrent lookups
 * @ht:         the hash table to shrink
 *
 * This function shrinks the hash table to fit, i.e., the smallest
 * size would not cause it to expand right away automatically.
 *
 * The caller must ensure that no concurrent resizing occurs by holding
 * ht->mutex.
 *
 * The caller must ensure that no concurrent table mutations take place.
 * It is however valid to have concurrent lookups if they are RCU protected.
 *
 * It is valid to have concurrent insertions and deletions protected by per
 * bucket locks or concurrent RCU protected lookups and traversals.
 */
static int rhashtable_shrink(struct rhashtable *ht)
{
        struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
        unsigned int nelems = atomic_read(&ht->nelems);
        unsigned int size = 0;

        if (nelems)
                size = roundup_pow_of_two(nelems * 3 / 2);
        if (size < ht->p.min_size)
                size = ht->p.min_size;

        if (old_tbl->size <= size)
                return 0;

        if (rht_dereference(old_tbl->future_tbl, ht))
                return -EEXIST;

        return rhashtable_rehash_alloc(ht, old_tbl, size);
}

static void rht_deferred_worker(struct work_struct *work)
{
        struct rhashtable *ht;
        struct bucket_table *tbl;
        int err = 0;

        ht = container_of(work, struct rhashtable, run_work);
        mutex_lock(&ht->mutex);

        tbl = rht_dereference(ht->tbl, ht);
        tbl = rhashtable_last_table(ht, tbl);

        if (rht_grow_above_75(ht, tbl))
                err = rhashtable_rehash_alloc(ht, tbl, tbl->size * 2);
        else if (ht->p.automatic_shrinking && rht_shrink_below_30(ht, tbl))
                err = rhashtable_shrink(ht);
        else if (tbl->nest)
                err = rhashtable_rehash_alloc(ht, tbl, tbl->size);

        if (!err)
                err = rhashtable_rehash_table(ht);

        mutex_unlock(&ht->mutex);

        if (err)
                schedule_work(&ht->run_work);
}

static int rhashtable_insert_rehash(struct rhashtable *ht,
                                    struct bucket_table *tbl)
{
        struct bucket_table *old_tbl;
        struct bucket_table *new_tbl;
        unsigned int size;
        int err;

        old_tbl = rht_dereference_rcu(ht->tbl, ht);

        size = tbl->size;

        err = -EBUSY;

        if (rht_grow_above_75(ht, tbl))
                size *= 2;
        /* Do not schedule more than one rehash */
        else if (old_tbl != tbl)
                goto fail;

        err = -ENOMEM;

        new_tbl = bucket_table_alloc(ht, size, GFP_ATOMIC);
        if (new_tbl == NULL)
                goto fail;

        err = rhashtable_rehash_attach(ht, tbl, new_tbl);
        if (err) {
                bucket_table_free(new_tbl);
                if (err == -EEXIST)
                        err = 0;
        } else
                schedule_work(&ht->run_work);

        return err;

fail:
        /* Do not fail the insert if someone else did a rehash. */
        if (likely(rcu_dereference_raw(tbl->future_tbl)))
                return 0;

        /* Schedule async rehash to retry allocation in process context. */
        if (err == -ENOMEM)
                schedule_work(&ht->run_work);

        return err;
}

static void *rhashtable_lookup_one(struct rhashtable *ht,
                                   struct bucket_table *tbl, unsigned int hash,
                                   const void *key, struct rhash_head *obj)
{
        struct rhashtable_compare_arg arg = {
                .ht = ht,
                .key = key,
        };
        struct rhash_head __rcu **pprev;
        struct rhash_head *head;
        int elasticity;

        elasticity = RHT_ELASTICITY;
        pprev = rht_bucket_var(tbl, hash);
        rht_for_each_continue(head, *pprev, tbl, hash) {
                struct rhlist_head *list;
                struct rhlist_head *plist;

                elasticity--;
                if (!key ||
                    (ht->p.obj_cmpfn ?
                     ht->p.obj_cmpfn(&arg, rht_obj(ht, head)) :
                     rhashtable_compare(&arg, rht_obj(ht, head)))) {
                        pprev = &head->next;
                        continue;
                }

                if (!ht->rhlist)
                        return rht_obj(ht, head);

                list = container_of(obj, struct rhlist_head, rhead);
                plist = container_of(head, struct rhlist_head, rhead);

                RCU_INIT_POINTER(list->next, plist);
                head = rht_dereference_bucket(head->next, tbl, hash);
                RCU_INIT_POINTER(list->rhead.next, head);
                rcu_assign_pointer(*pprev, obj);

                return NULL;
        }

        if (elasticity <= 0)
                return ERR_PTR(-EAGAIN);

        return ERR_PTR(-ENOENT);
}

static struct bucket_table *rhashtable_insert_one(struct rhashtable *ht,
                                                  struct bucket_table *tbl,
                                                  unsigned int hash,
                                                  struct rhash_head *obj,
                                                  void *data)
{
        struct rhash_head __rcu **pprev;
        struct bucket_table *new_tbl;
        struct rhash_head *head;

        if (!IS_ERR_OR_NULL(data))
                return ERR_PTR(-EEXIST);

        if (PTR_ERR(data) != -EAGAIN && PTR_ERR(data) != -ENOENT)
                return ERR_CAST(data);

        new_tbl = rcu_dereference(tbl->future_tbl);
        if (new_tbl)
                return new_tbl;

        if (PTR_ERR(data) != -ENOENT)
                return ERR_CAST(data);

        if (unlikely(rht_grow_above_max(ht, tbl)))
                return ERR_PTR(-E2BIG);

        if (unlikely(rht_grow_above_100(ht, tbl)))
                return ERR_PTR(-EAGAIN);

        pprev = rht_bucket_insert(ht, tbl, hash);
        if (!pprev)
                return ERR_PTR(-ENOMEM);

        head = rht_dereference_bucket(*pprev, tbl, hash);

        RCU_INIT_POINTER(obj->next, head);
        if (ht->rhlist) {
                struct rhlist_head *list;

                list = container_of(obj, struct rhlist_head, rhead);
                RCU_INIT_POINTER(list->next, NULL);
        }

        rcu_assign_pointer(*pprev, obj);

        atomic_inc(&ht->nelems);
        if (rht_grow_above_75(ht, tbl))
                schedule_work(&ht->run_work);

        return NULL;
}

static void *rhashtable_try_insert(struct rhashtable *ht, const void *key,
                                   struct rhash_head *obj)
{
        struct bucket_table *new_tbl;
        struct bucket_table *tbl;
        unsigned int hash;
        spinlock_t *lock;
        void *data;

        tbl = rcu_dereference(ht->tbl);

        /* All insertions must grab the oldest table containing
         * the hashed bucket that is yet to be rehashed.
         */
        for (;;) {
                hash = rht_head_hashfn(ht, tbl, obj, ht->p);
                lock = rht_bucket_lock(tbl, hash);
                spin_lock_bh(lock);

                if (tbl->rehash <= hash)
                        break;

                spin_unlock_bh(lock);
                tbl = rcu_dereference(tbl->future_tbl);
        }

        data = rhashtable_lookup_one(ht, tbl, hash, key, obj);
        new_tbl = rhashtable_insert_one(ht, tbl, hash, obj, data);
        if (PTR_ERR(new_tbl) != -EEXIST)
                data = ERR_CAST(new_tbl);

        while (!IS_ERR_OR_NULL(new_tbl)) {
                tbl = new_tbl;
                hash = rht_head_hashfn(ht, tbl, obj, ht->p);
                spin_lock_nested(rht_bucket_lock(tbl, hash),
                                 SINGLE_DEPTH_NESTING);

                data = rhashtable_lookup_one(ht, tbl, hash, key, obj);
                new_tbl = rhashtable_insert_one(ht, tbl, hash, obj, data);
                if (PTR_ERR(new_tbl) != -EEXIST)
                        data = ERR_CAST(new_tbl);

                spin_unlock(rht_bucket_lock(tbl, hash));
        }

        spin_unlock_bh(lock);

        if (PTR_ERR(data) == -EAGAIN)
                data = ERR_PTR(rhashtable_insert_rehash(ht, tbl) ?:
                               -EAGAIN);

        return data;
}

void *rhashtable_insert_slow(struct rhashtable *ht, const void *key,
                             struct rhash_head *obj)
{
        void *data;

        do {
                rcu_read_lock();
                data = rhashtable_try_insert(ht, key, obj);
                rcu_read_unlock();
        } while (PTR_ERR(data) == -EAGAIN);

        return data;
}
EXPORT_SYMBOL_GPL(rhashtable_insert_slow);

/**
 * rhashtable_walk_enter - Initialise an iterator
 * @ht:         Table to walk over
 * @iter:       Hash table Iterator
 *
 * This function prepares a hash table walk.
 *
 * Note that if you restart a walk after rhashtable_walk_stop you
 * may see the same object twice.  Also, you may miss objects if
 * there are removals in between rhashtable_walk_stop and the next
 * call to rhashtable_walk_start.
 *
 * For a completely stable walk you should construct your own data
 * structure outside the hash table.
 *
 * This function may sleep so you must not call it from interrupt
 * context or with spin locks held.
 *
 * You must call rhashtable_walk_exit after this function returns.
 */
void rhashtable_walk_enter(struct rhashtable *ht, struct rhashtable_iter *iter)
{
        iter->ht = ht;
        iter->p = NULL;
        iter->slot = 0;
        iter->skip = 0;
        iter->end_of_table = 0;

        spin_lock(&ht->lock);
        iter->walker.tbl =
                rcu_dereference_protected(ht->tbl, lockdep_is_held(&ht->lock));
        list_add(&iter->walker.list, &iter->walker.tbl->walkers);
        spin_unlock(&ht->lock);
}
EXPORT_SYMBOL_GPL(rhashtable_walk_enter);

/**
 * rhashtable_walk_exit - Free an iterator
 * @iter:       Hash table Iterator
 *
 * This function frees resources allocated by rhashtable_walk_init.
 */
void rhashtable_walk_exit(struct rhashtable_iter *iter)
{
        spin_lock(&iter->ht->lock);
        if (iter->walker.tbl)
                list_del(&iter->walker.list);
        spin_unlock(&iter->ht->lock);
}
EXPORT_SYMBOL_GPL(rhashtable_walk_exit);

/**
 * rhashtable_walk_start_check - Start a hash table walk
 * @iter:       Hash table iterator
 *
 * Start a hash table walk at the current iterator position.  Note that we take
 * the RCU lock in all cases including when we return an error.  So you must
 * always call rhashtable_walk_stop to clean up.
 *
 * Returns zero if successful.
 *
 * Returns -EAGAIN if resize event occured.  Note that the iterator
 * will rewind back to the beginning and you may use it immediately
 * by calling rhashtable_walk_next.
 *
 * rhashtable_walk_start is defined as an inline variant that returns
 * void. This is preferred in cases where the caller would ignore
 * resize events and always continue.
 */
int rhashtable_walk_start_check(struct rhashtable_iter *iter)
        __acquires(RCU)
{
        struct rhashtable *ht = iter->ht;

        rcu_read_lock();

        spin_lock(&ht->lock);
        if (iter->walker.tbl)
                list_del(&iter->walker.list);
        spin_unlock(&ht->lock);

        if (!iter->walker.tbl && !iter->end_of_table) {
                iter->walker.tbl = rht_dereference_rcu(ht->tbl, ht);
                return -EAGAIN;
        }

        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_start_check);

/**
 * __rhashtable_walk_find_next - Find the next element in a table (or the first
 * one in case of a new walk).
 *
 * @iter:       Hash table iterator
 *
 * Returns the found object or NULL when the end of the table is reached.
 *
 * Returns -EAGAIN if resize event occurred.
 */
static void *__rhashtable_walk_find_next(struct rhashtable_iter *iter)
{
        struct bucket_table *tbl = iter->walker.tbl;
        struct rhlist_head *list = iter->list;
        struct rhashtable *ht = iter->ht;
        struct rhash_head *p = iter->p;
        bool rhlist = ht->rhlist;

        if (!tbl)
                return NULL;

        for (; iter->slot < tbl->size; iter->slot++) {
                int skip = iter->skip;

                rht_for_each_rcu(p, tbl, iter->slot) {
                        if (rhlist) {
                                list = container_of(p, struct rhlist_head,
                                                    rhead);
                                do {
                                        if (!skip)
                                                goto next;
                                        skip--;
                                        list = rcu_dereference(list->next);
                                } while (list);

                                continue;
                        }
                        if (!skip)
                                break;
                        skip--;
                }

next:
                if (!rht_is_a_nulls(p)) {
                        iter->skip++;
                        iter->p = p;
                        iter->list = list;
                        return rht_obj(ht, rhlist ? &list->rhead : p);
                }

                iter->skip = 0;
        }

        iter->p = NULL;

        /* Ensure we see any new tables. */
        smp_rmb();

        iter->walker.tbl = rht_dereference_rcu(tbl->future_tbl, ht);
        if (iter->walker.tbl) {
                iter->slot = 0;
                iter->skip = 0;
                return ERR_PTR(-EAGAIN);
        } else {
                iter->end_of_table = true;
        }

        return NULL;
}

/**
 * rhashtable_walk_next - Return the next object and advance the iterator
 * @iter:       Hash table iterator
 *
 * Note that you must call rhashtable_walk_stop when you are finished
 * with the walk.
 *
 * Returns the next object or NULL when the end of the table is reached.
 *
 * Returns -EAGAIN if resize event occurred.  Note that the iterator
 * will rewind back to the beginning and you may continue to use it.
 */
void *rhashtable_walk_next(struct rhashtable_iter *iter)
{
        struct rhlist_head *list = iter->list;
        struct rhashtable *ht = iter->ht;
        struct rhash_head *p = iter->p;
        bool rhlist = ht->rhlist;

        if (p) {
                if (!rhlist || !(list = rcu_dereference(list->next))) {
                        p = rcu_dereference(p->next);
                        list = container_of(p, struct rhlist_head, rhead);
                }
                if (!rht_is_a_nulls(p)) {
                        iter->skip++;
                        iter->p = p;
                        iter->list = list;
                        return rht_obj(ht, rhlist ? &list->rhead : p);
                }

                /* At the end of this slot, switch to next one and then find
                 * next entry from that point.
                 */
                iter->skip = 0;
                iter->slot++;
        }

        return __rhashtable_walk_find_next(iter);
}
EXPORT_SYMBOL_GPL(rhashtable_walk_next);

/**
 * rhashtable_walk_peek - Return the next object but don't advance the iterator
 * @iter:       Hash table iterator
 *
 * Returns the next object or NULL when the end of the table is reached.
 *
 * Returns -EAGAIN if resize event occurred.  Note that the iterator
 * will rewind back to the beginning and you may continue to use it.
 */
void *rhashtable_walk_peek(struct rhashtable_iter *iter)
{
        struct rhlist_head *list = iter->list;
        struct rhashtable *ht = iter->ht;
        struct rhash_head *p = iter->p;

        if (p)
                return rht_obj(ht, ht->rhlist ? &list->rhead : p);

        /* No object found in current iter, find next one in the table. */

        if (iter->skip) {
                /* A nonzero skip value points to the next entry in the table
                 * beyond that last one that was found. Decrement skip so
                 * we find the current value. __rhashtable_walk_find_next
                 * will restore the original value of skip assuming that
                 * the table hasn't changed.
                 */
                iter->skip--;
        }

        return __rhashtable_walk_find_next(iter);
}
EXPORT_SYMBOL_GPL(rhashtable_walk_peek);

/**
 * rhashtable_walk_stop - Finish a hash table walk
 * @iter:       Hash table iterator
 *
 * Finish a hash table walk.  Does not reset the iterator to the start of the
 * hash table.
 */
void rhashtable_walk_stop(struct rhashtable_iter *iter)
        __releases(RCU)
{
        struct rhashtable *ht;
        struct bucket_table *tbl = iter->walker.tbl;

        if (!tbl)
                goto out;

        ht = iter->ht;

        spin_lock(&ht->lock);
        if (tbl->rehash < tbl->size)
                list_add(&iter->walker.list, &tbl->walkers);
        else
                iter->walker.tbl = NULL;
        spin_unlock(&ht->lock);

        iter->p = NULL;

out:
        rcu_read_unlock();
}
EXPORT_SYMBOL_GPL(rhashtable_walk_stop);

static size_t rounded_hashtable_size(const struct rhashtable_params *params)
{
        return max(roundup_pow_of_two(params->nelem_hint * 4 / 3),
                   (unsigned long)params->min_size);
}

static u32 rhashtable_jhash2(const void *key, u32 length, u32 seed)
{
        return jhash2(key, length, seed);
}

/**
 * rhashtable_init - initialize a new hash table
 * @ht:         hash table to be initialized
 * @params:     configuration parameters
 *
 * Initializes a new hash table based on the provided configuration
 * parameters. A table can be configured either with a variable or
 * fixed length key:
 *
 * Configuration Example 1: Fixed length keys
 * struct test_obj {
 *      int                     key;
 *      void *                  my_member;
 *      struct rhash_head       node;
 * };
 *
 * struct rhashtable_params params = {
 *      .head_offset = offsetof(struct test_obj, node),
 *      .key_offset = offsetof(struct test_obj, key),
 *      .key_len = sizeof(int),
 *      .hashfn = jhash,
 *      .nulls_base = (1U << RHT_BASE_SHIFT),
 * };
 *
 * Configuration Example 2: Variable length keys
 * struct test_obj {
 *      [...]
 *      struct rhash_head       node;
 * };
 *
 * u32 my_hash_fn(const void *data, u32 len, u32 seed)
 * {
 *      struct test_obj *obj = data;
 *
 *      return [... hash ...];
 * }
 *
 * struct rhashtable_params params = {
 *      .head_offset = offsetof(struct test_obj, node),
 *      .hashfn = jhash,
 *      .obj_hashfn = my_hash_fn,
 * };
 */
int rhashtable_init(struct rhashtable *ht,
                    const struct rhashtable_params *params)
{
        struct bucket_table *tbl;
        size_t size;

        size = HASH_DEFAULT_SIZE;

        if ((!params->key_len && !params->obj_hashfn) ||
            (params->obj_hashfn && !params->obj_cmpfn))
                return -EINVAL;

        if (params->nulls_base && params->nulls_base < (1U << RHT_BASE_SHIFT))
                return -EINVAL;

        memset(ht, 0, sizeof(*ht));
        mutex_init(&ht->mutex);
        spin_lock_init(&ht->lock);
        memcpy(&ht->p, params, sizeof(*params));

        if (params->min_size)
                ht->p.min_size = roundup_pow_of_two(params->min_size);

        /* Cap total entries at 2^31 to avoid nelems overflow. */
        ht->max_elems = 1u << 31;

        if (params->max_size) {
                ht->p.max_size = rounddown_pow_of_two(params->max_size);
                if (ht->p.max_size < ht->max_elems / 2)
                        ht->max_elems = ht->p.max_size * 2;
        }

        ht->p.min_size = max_t(u16, ht->p.min_size, HASH_MIN_SIZE);

        if (params->nelem_hint)
                size = rounded_hashtable_size(&ht->p);

        if (params->locks_mul)
                ht->p.locks_mul = roundup_pow_of_two(params->locks_mul);
        else
                ht->p.locks_mul = BUCKET_LOCKS_PER_CPU;

        ht->key_len = ht->p.key_len;
        if (!params->hashfn) {
                ht->p.hashfn = jhash;

                if (!(ht->key_len & (sizeof(u32) - 1))) {
                        ht->key_len /= sizeof(u32);
                        ht->p.hashfn = rhashtable_jhash2;
                }
        }

        tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
        if (tbl == NULL)
                return -ENOMEM;

        atomic_set(&ht->nelems, 0);

        RCU_INIT_POINTER(ht->tbl, tbl);

        INIT_WORK(&ht->run_work, rht_deferred_worker);

        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_init);

/**
 * rhltable_init - initialize a new hash list table
 * @hlt:        hash list table to be initialized
 * @params:     configuration parameters
 *
 * Initializes a new hash list table.
 *
 * See documentation for rhashtable_init.
 */
int rhltable_init(struct rhltable *hlt, const struct rhashtable_params *params)
{
        int err;

        /* No rhlist NULLs marking for now. */
        if (params->nulls_base)
                return -EINVAL;

        err = rhashtable_init(&hlt->ht, params);
        hlt->ht.rhlist = true;
        return err;
}
EXPORT_SYMBOL_GPL(rhltable_init);

static void rhashtable_free_one(struct rhashtable *ht, struct rhash_head *obj,
                                void (*free_fn)(void *ptr, void *arg),
                                void *arg)
{
        struct rhlist_head *list;

        if (!ht->rhlist) {
                free_fn(rht_obj(ht, obj), arg);
                return;
        }

        list = container_of(obj, struct rhlist_head, rhead);
        do {
                obj = &list->rhead;
                list = rht_dereference(list->next, ht);
                free_fn(rht_obj(ht, obj), arg);
        } while (list);
}

/**
 * rhashtable_free_and_destroy - free elements and destroy hash table
 * @ht:         the hash table to destroy
 * @free_fn:    callback to release resources of element
 * @arg:        pointer passed to free_fn
 *
 * Stops an eventual async resize. If defined, invokes free_fn for each
 * element to releasal resources. Please note that RCU protected
 * readers may still be accessing the elements. Releasing of resources
 * must occur in a compatible manner. Then frees the bucket array.
 *
 * This function will eventually sleep to wait for an async resize
 * to complete. The caller is responsible that no further write operations
 * occurs in parallel.
 */
void rhashtable_free_and_destroy(struct rhashtable *ht,
                                 void (*free_fn)(void *ptr, void *arg),
                                 void *arg)
{
        struct bucket_table *tbl;
        unsigned int i;

        cancel_work_sync(&ht->run_work);

        mutex_lock(&ht->mutex);
        tbl = rht_dereference(ht->tbl, ht);
        if (free_fn) {
                for (i = 0; i < tbl->size; i++) {
                        struct rhash_head *pos, *next;

                        cond_resched();
                        for (pos = rht_dereference(*rht_bucket(tbl, i), ht),
                             next = !rht_is_a_nulls(pos) ?
                                        rht_dereference(pos->next, ht) : NULL;
                             !rht_is_a_nulls(pos);
                             pos = next,
                             next = !rht_is_a_nulls(pos) ?
                                        rht_dereference(pos->next, ht) : NULL)
                                rhashtable_free_one(ht, pos, free_fn, arg);
                }
        }

        bucket_table_free(tbl);
        mutex_unlock(&ht->mutex);
}
EXPORT_SYMBOL_GPL(rhashtable_free_and_destroy);

void rhashtable_destroy(struct rhashtable *ht)
{
        return rhashtable_free_and_destroy(ht, NULL, NULL);
}
EXPORT_SYMBOL_GPL(rhashtable_destroy);

struct rhash_head __rcu **rht_bucket_nested(const struct bucket_table *tbl,
                                            unsigned int hash)
{
        const unsigned int shift = PAGE_SHIFT - ilog2(sizeof(void *));
        static struct rhash_head __rcu *rhnull =
                (struct rhash_head __rcu *)NULLS_MARKER(0);
        unsigned int index = hash & ((1 << tbl->nest) - 1);
        unsigned int size = tbl->size >> tbl->nest;
        unsigned int subhash = hash;
        union nested_table *ntbl;

        ntbl = (union nested_table *)rcu_dereference_raw(tbl->buckets[0]);
        ntbl = rht_dereference_bucket_rcu(ntbl[index].table, tbl, hash);
        subhash >>= tbl->nest;

        while (ntbl && size > (1 << shift)) {
                index = subhash & ((1 << shift) - 1);
                ntbl = rht_dereference_bucket_rcu(ntbl[index].table,
                                                  tbl, hash);
                size >>= shift;
                subhash >>= shift;
        }

        if (!ntbl)
                return &rhnull;

        return &ntbl[subhash].bucket;

}
EXPORT_SYMBOL_GPL(rht_bucket_nested);

struct rhash_head __rcu **rht_bucket_nested_insert(struct rhashtable *ht,
                                                   struct bucket_table *tbl,
                                                   unsigned int hash)
{
        const unsigned int shift = PAGE_SHIFT - ilog2(sizeof(void *));
        unsigned int index = hash & ((1 << tbl->nest) - 1);
        unsigned int size = tbl->size >> tbl->nest;
        union nested_table *ntbl;
        unsigned int shifted;
        unsigned int nhash;

        ntbl = (union nested_table *)rcu_dereference_raw(tbl->buckets[0]);
        hash >>= tbl->nest;
        nhash = index;
        shifted = tbl->nest;
        ntbl = nested_table_alloc(ht, &ntbl[index].table,
                                  size <= (1 << shift) ? shifted : 0, nhash);

        while (ntbl && size > (1 << shift)) {
                index = hash & ((1 << shift) - 1);
                size >>= shift;
                hash >>= shift;
                nhash |= index << shifted;
                shifted += shift;
                ntbl = nested_table_alloc(ht, &ntbl[index].table,
                                          size <= (1 << shift) ? shifted : 0,
                                          nhash);
        }

        if (!ntbl)
                return NULL;

        return &ntbl[hash].bucket;

}
EXPORT_SYMBOL_GPL(rht_bucket_nested_insert);