rhashtable: Fix read-side crash during rehash

[mirror_ubuntu-artful-kernel.git] / lib / rhashtable.c

/*
 * Resizable, Scalable, Concurrent Hash Table
 *
 * Copyright (c) 2014-2015 Thomas Graf <tgraf@suug.ch>
 * Copyright (c) 2008-2014 Patrick McHardy <kaber@trash.net>
 *
 * Based on the following paper:
 * https://www.usenix.org/legacy/event/atc11/tech/final_files/Triplett.pdf
 *
 * Code partially derived from nft_hash
 *
 * 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/kernel.h>
#include <linux/init.h>
#include <linux/log2.h>
#include <linux/sched.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>

#define HASH_DEFAULT_SIZE       64UL
#define HASH_MIN_SIZE           4UL
#define BUCKET_LOCKS_PER_CPU   128UL

/* Base bits plus 1 bit for nulls marker */
#define HASH_RESERVED_SPACE     (RHT_BASE_BITS + 1)

enum {
        RHT_LOCK_NORMAL,
        RHT_LOCK_NESTED,
};

/* The bucket lock is selected based on the hash and protects mutations
 * on a group of hash buckets.
 *
 * A maximum of tbl->size/2 bucket locks is allocated. This ensures that
 * a single lock always covers both buckets which may both contains
 * entries which link to the same bucket of the old table during resizing.
 * This allows to simplify the locking as locking the bucket in both
 * tables during resize always guarantee protection.
 *
 * IMPORTANT: When holding the bucket lock of both the old and new table
 * during expansions and shrinking, the old bucket lock must always be
 * acquired first.
 */
static spinlock_t *bucket_lock(const struct bucket_table *tbl, u32 hash)
{
        return &tbl->locks[hash & tbl->locks_mask];
}

static void *rht_obj(const struct rhashtable *ht, const struct rhash_head *he)
{
        return (void *) he - ht->p.head_offset;
}

static u32 rht_bucket_index(const struct bucket_table *tbl, u32 hash)
{
        return (hash >> HASH_RESERVED_SPACE) & (tbl->size - 1);
}

static u32 key_hashfn(struct rhashtable *ht, const struct bucket_table *tbl,
                      const void *key)
{
        return rht_bucket_index(tbl, ht->p.hashfn(key, ht->p.key_len,
                                                  tbl->hash_rnd));
}

static u32 head_hashfn(struct rhashtable *ht,
                       const struct bucket_table *tbl,
                       const struct rhash_head *he)
{
        const char *ptr = rht_obj(ht, he);

        return likely(ht->p.key_len) ?
               key_hashfn(ht, tbl, ptr + ht->p.key_offset) :
               rht_bucket_index(tbl, ht->p.obj_hashfn(ptr, tbl->hash_rnd));
}

#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 = 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 int alloc_bucket_locks(struct rhashtable *ht, struct bucket_table *tbl)
{
        unsigned int i, size;
#if defined(CONFIG_PROVE_LOCKING)
        unsigned int nr_pcpus = 2;
#else
        unsigned int nr_pcpus = num_possible_cpus();
#endif

        nr_pcpus = min_t(unsigned int, nr_pcpus, 32UL);
        size = roundup_pow_of_two(nr_pcpus * ht->p.locks_mul);

        /* Never allocate more than 0.5 locks per bucket */
        size = min_t(unsigned int, size, tbl->size >> 1);

        if (sizeof(spinlock_t) != 0) {
#ifdef CONFIG_NUMA
                if (size * sizeof(spinlock_t) > PAGE_SIZE)
                        tbl->locks = vmalloc(size * sizeof(spinlock_t));
                else
#endif
                tbl->locks = kmalloc_array(size, sizeof(spinlock_t),
                                           GFP_KERNEL);
                if (!tbl->locks)
                        return -ENOMEM;
                for (i = 0; i < size; i++)
                        spin_lock_init(&tbl->locks[i]);
        }
        tbl->locks_mask = size - 1;

        return 0;
}

static void bucket_table_free(const struct bucket_table *tbl)
{
        if (tbl)
                kvfree(tbl->locks);

        kvfree(tbl);
}

static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
                                               size_t nbuckets, u32 hash_rnd)
{
        struct bucket_table *tbl = NULL;
        size_t size;
        int i;

        size = sizeof(*tbl) + nbuckets * sizeof(tbl->buckets[0]);
        if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
                tbl = kzalloc(size, GFP_KERNEL | __GFP_NOWARN | __GFP_NORETRY);
        if (tbl == NULL)
                tbl = vzalloc(size);
        if (tbl == NULL)
                return NULL;

        tbl->size = nbuckets;
        tbl->shift = ilog2(nbuckets);
        tbl->hash_rnd = hash_rnd;

        if (alloc_bucket_locks(ht, tbl) < 0) {
                bucket_table_free(tbl);
                return NULL;
        }

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

        return tbl;
}

/**
 * rht_grow_above_75 - returns true if nelems > 0.75 * table-size
 * @ht:         hash table
 * @tbl:        current table
 */
static bool rht_grow_above_75(const struct rhashtable *ht,
                              const struct bucket_table *tbl)
{
        /* Expand table when exceeding 75% load */
        return atomic_read(&ht->nelems) > (tbl->size / 4 * 3) &&
               (!ht->p.max_shift || tbl->shift < ht->p.max_shift);
}

/**
 * rht_shrink_below_30 - returns true if nelems < 0.3 * table-size
 * @ht:         hash table
 * @tbl:        current table
 */
static bool rht_shrink_below_30(const struct rhashtable *ht,
                                const struct bucket_table *tbl)
{
        /* Shrink table beneath 30% load */
        return atomic_read(&ht->nelems) < (tbl->size * 3 / 10) &&
               tbl->shift > ht->p.min_shift;
}

static int rhashtable_rehash_one(struct rhashtable *ht, unsigned old_hash)
{
        struct bucket_table *new_tbl = rht_dereference(ht->future_tbl, ht);
        struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
        struct rhash_head __rcu **pprev = &old_tbl->buckets[old_hash];
        int err = -ENOENT;
        struct rhash_head *head, *next, *entry;
        spinlock_t *new_bucket_lock;
        unsigned new_hash;

        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 = bucket_lock(new_tbl, new_hash);

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

        if (rht_is_a_nulls(head))
                INIT_RHT_NULLS_HEAD(entry->next, ht, new_hash);
        else
                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 void rhashtable_rehash_chain(struct rhashtable *ht, unsigned old_hash)
{
        struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
        spinlock_t *old_bucket_lock;

        old_bucket_lock = bucket_lock(old_tbl, old_hash);

        spin_lock_bh(old_bucket_lock);
        while (!rhashtable_rehash_one(ht, old_hash))
                ;
        spin_unlock_bh(old_bucket_lock);
}

static void rhashtable_rehash(struct rhashtable *ht,
                              struct bucket_table *new_tbl)
{
        struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
        unsigned old_hash;

        get_random_bytes(&new_tbl->hash_rnd, sizeof(new_tbl->hash_rnd));

        /* Make insertions go into the new, empty table right away. Deletions
         * and lookups will be attempted in both tables until we synchronize.
         * The synchronize_rcu() guarantees for the new table to be picked up
         * so no new additions go into the old table while we relink.
         */
        rcu_assign_pointer(ht->future_tbl, new_tbl);

        /* Ensure the new table is visible to readers. */
        smp_wmb();

        for (old_hash = 0; old_hash < old_tbl->size; old_hash++)
                rhashtable_rehash_chain(ht, old_hash);

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

        /* Wait for readers. All new readers will see the new
         * table, and thus no references to the old table will
         * remain.
         */
        synchronize_rcu();

        bucket_table_free(old_tbl);
}

/**
 * rhashtable_expand - Expand hash table while allowing concurrent lookups
 * @ht:         the hash table to expand
 *
 * A secondary bucket array is allocated and the hash entries are migrated.
 *
 * This function may only be called in a context where it is safe to call
 * synchronize_rcu(), e.g. not within a rcu_read_lock() section.
 *
 * The caller must ensure that no concurrent resizing occurs by holding
 * ht->mutex.
 *
 * It is valid to have concurrent insertions and deletions protected by per
 * bucket locks or concurrent RCU protected lookups and traversals.
 */
int rhashtable_expand(struct rhashtable *ht)
{
        struct bucket_table *new_tbl, *old_tbl = rht_dereference(ht->tbl, ht);

        ASSERT_RHT_MUTEX(ht);

        new_tbl = bucket_table_alloc(ht, old_tbl->size * 2, old_tbl->hash_rnd);
        if (new_tbl == NULL)
                return -ENOMEM;

        rhashtable_rehash(ht, new_tbl);
        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_expand);

/**
 * rhashtable_shrink - Shrink hash table while allowing concurrent lookups
 * @ht:         the hash table to shrink
 *
 * This function may only be called in a context where it is safe to call
 * synchronize_rcu(), e.g. not within a rcu_read_lock() section.
 *
 * 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.
 */
int rhashtable_shrink(struct rhashtable *ht)
{
        struct bucket_table *new_tbl, *old_tbl = rht_dereference(ht->tbl, ht);

        ASSERT_RHT_MUTEX(ht);

        new_tbl = bucket_table_alloc(ht, old_tbl->size / 2, old_tbl->hash_rnd);
        if (new_tbl == NULL)
                return -ENOMEM;

        rhashtable_rehash(ht, new_tbl);
        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_shrink);

static void rht_deferred_worker(struct work_struct *work)
{
        struct rhashtable *ht;
        struct bucket_table *tbl;
        struct rhashtable_walker *walker;

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

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

        list_for_each_entry(walker, &ht->walkers, list)
                walker->resize = true;

        if (rht_grow_above_75(ht, tbl))
                rhashtable_expand(ht);
        else if (rht_shrink_below_30(ht, tbl))
                rhashtable_shrink(ht);
unlock:
        mutex_unlock(&ht->mutex);
}

static bool __rhashtable_insert(struct rhashtable *ht, struct rhash_head *obj,
                                bool (*compare)(void *, void *), void *arg)
{
        struct bucket_table *tbl, *old_tbl;
        struct rhash_head *head;
        bool no_resize_running;
        unsigned hash;
        bool success = true;

        rcu_read_lock();

        old_tbl = rht_dereference_rcu(ht->tbl, ht);
        hash = head_hashfn(ht, old_tbl, obj);

        spin_lock_bh(bucket_lock(old_tbl, hash));

        /* Because we have already taken the bucket lock in old_tbl,
         * if we find that future_tbl is not yet visible then that
         * guarantees all other insertions of the same entry will
         * also grab the bucket lock in old_tbl because until the
         * rehash completes ht->tbl won't be changed.
         */
        tbl = rht_dereference_rcu(ht->future_tbl, ht);
        if (tbl != old_tbl) {
                hash = head_hashfn(ht, tbl, obj);
                spin_lock_nested(bucket_lock(tbl, hash), RHT_LOCK_NESTED);
        }

        if (compare &&
            rhashtable_lookup_compare(ht, rht_obj(ht, obj) + ht->p.key_offset,
                                      compare, arg)) {
                success = false;
                goto exit;
        }

        no_resize_running = tbl == old_tbl;

        head = rht_dereference_bucket(tbl->buckets[hash], tbl, hash);

        if (rht_is_a_nulls(head))
                INIT_RHT_NULLS_HEAD(obj->next, ht, hash);
        else
                RCU_INIT_POINTER(obj->next, head);

        rcu_assign_pointer(tbl->buckets[hash], obj);

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

exit:
        if (tbl != old_tbl) {
                hash = head_hashfn(ht, tbl, obj);
                spin_unlock(bucket_lock(tbl, hash));
        }

        hash = head_hashfn(ht, old_tbl, obj);
        spin_unlock_bh(bucket_lock(old_tbl, hash));

        rcu_read_unlock();

        return success;
}

/**
 * rhashtable_insert - insert object into hash table
 * @ht:         hash table
 * @obj:        pointer to hash head inside object
 *
 * Will take a per bucket spinlock to protect against mutual mutations
 * on the same bucket. Multiple insertions may occur in parallel unless
 * they map to the same bucket lock.
 *
 * It is safe to call this function from atomic context.
 *
 * Will trigger an automatic deferred table resizing if the size grows
 * beyond the watermark indicated by grow_decision() which can be passed
 * to rhashtable_init().
 */
void rhashtable_insert(struct rhashtable *ht, struct rhash_head *obj)
{
        __rhashtable_insert(ht, obj, NULL, NULL);
}
EXPORT_SYMBOL_GPL(rhashtable_insert);

static bool __rhashtable_remove(struct rhashtable *ht,
                                struct bucket_table *tbl,
                                struct rhash_head *obj)
{
        struct rhash_head __rcu **pprev;
        struct rhash_head *he;
        spinlock_t * lock;
        unsigned hash;
        bool ret = false;

        hash = head_hashfn(ht, tbl, obj);
        lock = bucket_lock(tbl, hash);

        spin_lock_bh(lock);

        pprev = &tbl->buckets[hash];
        rht_for_each(he, tbl, hash) {
                if (he != obj) {
                        pprev = &he->next;
                        continue;
                }

                rcu_assign_pointer(*pprev, obj->next);
                ret = true;
                break;
        }

        spin_unlock_bh(lock);

        return ret;
}

/**
 * rhashtable_remove - remove object from hash table
 * @ht:         hash table
 * @obj:        pointer to hash head inside object
 *
 * Since the hash chain is single linked, the removal operation needs to
 * walk the bucket chain upon removal. The removal operation is thus
 * considerable slow if the hash table is not correctly sized.
 *
 * Will automatically shrink the table via rhashtable_expand() if the
 * shrink_decision function specified at rhashtable_init() returns true.
 *
 * The caller must ensure that no concurrent table mutations occur. It is
 * however valid to have concurrent lookups if they are RCU protected.
 */
bool rhashtable_remove(struct rhashtable *ht, struct rhash_head *obj)
{
        struct bucket_table *tbl, *old_tbl;
        bool ret;

        rcu_read_lock();

        old_tbl = rht_dereference_rcu(ht->tbl, ht);
        ret = __rhashtable_remove(ht, old_tbl, obj);

        /* Because we have already taken (and released) the bucket
         * lock in old_tbl, if we find that future_tbl is not yet
         * visible then that guarantees the entry to still be in
         * old_tbl if it exists.
         */
        tbl = rht_dereference_rcu(ht->future_tbl, ht);
        if (!ret && old_tbl != tbl)
                ret = __rhashtable_remove(ht, tbl, obj);

        if (ret) {
                bool no_resize_running = tbl == old_tbl;

                atomic_dec(&ht->nelems);
                if (no_resize_running && rht_shrink_below_30(ht, tbl))
                        schedule_work(&ht->run_work);
        }

        rcu_read_unlock();

        return ret;
}
EXPORT_SYMBOL_GPL(rhashtable_remove);

struct rhashtable_compare_arg {
        struct rhashtable *ht;
        const void *key;
};

static bool rhashtable_compare(void *ptr, void *arg)
{
        struct rhashtable_compare_arg *x = arg;
        struct rhashtable *ht = x->ht;

        return !memcmp(ptr + ht->p.key_offset, x->key, ht->p.key_len);
}

/**
 * rhashtable_lookup - lookup key in hash table
 * @ht:         hash table
 * @key:        pointer to key
 *
 * Computes the hash value for the key and traverses the bucket chain looking
 * for a entry with an identical key. The first matching entry is returned.
 *
 * This lookup function may only be used for fixed key hash table (key_len
 * parameter set). It will BUG() if used inappropriately.
 *
 * Lookups may occur in parallel with hashtable mutations and resizing.
 */
void *rhashtable_lookup(struct rhashtable *ht, const void *key)
{
        struct rhashtable_compare_arg arg = {
                .ht = ht,
                .key = key,
        };

        BUG_ON(!ht->p.key_len);

        return rhashtable_lookup_compare(ht, key, &rhashtable_compare, &arg);
}
EXPORT_SYMBOL_GPL(rhashtable_lookup);

/**
 * rhashtable_lookup_compare - search hash table with compare function
 * @ht:         hash table
 * @key:        the pointer to the key
 * @compare:    compare function, must return true on match
 * @arg:        argument passed on to compare function
 *
 * Traverses the bucket chain behind the provided hash value and calls the
 * specified compare function for each entry.
 *
 * Lookups may occur in parallel with hashtable mutations and resizing.
 *
 * Returns the first entry on which the compare function returned true.
 */
void *rhashtable_lookup_compare(struct rhashtable *ht, const void *key,
                                bool (*compare)(void *, void *), void *arg)
{
        const struct bucket_table *tbl, *old_tbl;
        struct rhash_head *he;
        u32 hash;

        rcu_read_lock();

        tbl = rht_dereference_rcu(ht->tbl, ht);
restart:
        hash = key_hashfn(ht, tbl, key);
        rht_for_each_rcu(he, tbl, hash) {
                if (!compare(rht_obj(ht, he), arg))
                        continue;
                rcu_read_unlock();
                return rht_obj(ht, he);
        }

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

        old_tbl = tbl;
        tbl = rht_dereference_rcu(ht->future_tbl, ht);
        if (unlikely(tbl != old_tbl))
                goto restart;
        rcu_read_unlock();

        return NULL;
}
EXPORT_SYMBOL_GPL(rhashtable_lookup_compare);

/**
 * rhashtable_lookup_insert - lookup and insert object into hash table
 * @ht:         hash table
 * @obj:        pointer to hash head inside object
 *
 * Locks down the bucket chain in both the old and new table if a resize
 * is in progress to ensure that writers can't remove from the old table
 * and can't insert to the new table during the atomic operation of search
 * and insertion. Searches for duplicates in both the old and new table if
 * a resize is in progress.
 *
 * This lookup function may only be used for fixed key hash table (key_len
 * parameter set). It will BUG() if used inappropriately.
 *
 * It is safe to call this function from atomic context.
 *
 * Will trigger an automatic deferred table resizing if the size grows
 * beyond the watermark indicated by grow_decision() which can be passed
 * to rhashtable_init().
 */
bool rhashtable_lookup_insert(struct rhashtable *ht, struct rhash_head *obj)
{
        struct rhashtable_compare_arg arg = {
                .ht = ht,
                .key = rht_obj(ht, obj) + ht->p.key_offset,
        };

        BUG_ON(!ht->p.key_len);

        return rhashtable_lookup_compare_insert(ht, obj, &rhashtable_compare,
                                                &arg);
}
EXPORT_SYMBOL_GPL(rhashtable_lookup_insert);

/**
 * rhashtable_lookup_compare_insert - search and insert object to hash table
 *                                    with compare function
 * @ht:         hash table
 * @obj:        pointer to hash head inside object
 * @compare:    compare function, must return true on match
 * @arg:        argument passed on to compare function
 *
 * Locks down the bucket chain in both the old and new table if a resize
 * is in progress to ensure that writers can't remove from the old table
 * and can't insert to the new table during the atomic operation of search
 * and insertion. Searches for duplicates in both the old and new table if
 * a resize is in progress.
 *
 * Lookups may occur in parallel with hashtable mutations and resizing.
 *
 * Will trigger an automatic deferred table resizing if the size grows
 * beyond the watermark indicated by grow_decision() which can be passed
 * to rhashtable_init().
 */
bool rhashtable_lookup_compare_insert(struct rhashtable *ht,
                                      struct rhash_head *obj,
                                      bool (*compare)(void *, void *),
                                      void *arg)
{
        BUG_ON(!ht->p.key_len);

        return __rhashtable_insert(ht, obj, compare, arg);
}
EXPORT_SYMBOL_GPL(rhashtable_lookup_compare_insert);

/**
 * rhashtable_walk_init - 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 if this function returns
 * successfully.
 */
int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter)
{
        iter->ht = ht;
        iter->p = NULL;
        iter->slot = 0;
        iter->skip = 0;

        iter->walker = kmalloc(sizeof(*iter->walker), GFP_KERNEL);
        if (!iter->walker)
                return -ENOMEM;

        INIT_LIST_HEAD(&iter->walker->list);
        iter->walker->resize = false;

        mutex_lock(&ht->mutex);
        list_add(&iter->walker->list, &ht->walkers);
        mutex_unlock(&ht->mutex);

        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_init);

/**
 * 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)
{
        mutex_lock(&iter->ht->mutex);
        list_del(&iter->walker->list);
        mutex_unlock(&iter->ht->mutex);
        kfree(iter->walker);
}
EXPORT_SYMBOL_GPL(rhashtable_walk_exit);

/**
 * rhashtable_walk_start - Start a hash table walk
 * @iter:       Hash table iterator
 *
 * Start a hash table walk.  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.
 */
int rhashtable_walk_start(struct rhashtable_iter *iter)
{
        rcu_read_lock();

        if (iter->walker->resize) {
                iter->slot = 0;
                iter->skip = 0;
                iter->walker->resize = false;
                return -EAGAIN;
        }

        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_start);

/**
 * 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 occured.  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)
{
        const struct bucket_table *tbl;
        struct rhashtable *ht = iter->ht;
        struct rhash_head *p = iter->p;
        void *obj = NULL;

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

        if (p) {
                p = rht_dereference_bucket_rcu(p->next, tbl, iter->slot);
                goto next;
        }

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

                rht_for_each_rcu(p, tbl, iter->slot) {
                        if (!skip)
                                break;
                        skip--;
                }

next:
                if (!rht_is_a_nulls(p)) {
                        iter->skip++;
                        iter->p = p;
                        obj = rht_obj(ht, p);
                        goto out;
                }

                iter->skip = 0;
        }

        iter->p = NULL;

out:
        if (iter->walker->resize) {
                iter->p = NULL;
                iter->slot = 0;
                iter->skip = 0;
                iter->walker->resize = false;
                return ERR_PTR(-EAGAIN);
        }

        return obj;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_next);

/**
 * rhashtable_walk_stop - Finish a hash table walk
 * @iter:       Hash table iterator
 *
 * Finish a hash table walk.
 */
void rhashtable_walk_stop(struct rhashtable_iter *iter)
{
        rcu_read_unlock();
        iter->p = NULL;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_stop);

static size_t rounded_hashtable_size(struct rhashtable_params *params)
{
        return max(roundup_pow_of_two(params->nelem_hint * 4 / 3),
                   1UL << params->min_shift);
}

/**
 * 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 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, struct rhashtable_params *params)
{
        struct bucket_table *tbl;
        size_t size;
        u32 hash_rnd;

        size = HASH_DEFAULT_SIZE;

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

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

        params->min_shift = max_t(size_t, params->min_shift,
                                  ilog2(HASH_MIN_SIZE));

        if (params->nelem_hint)
                size = rounded_hashtable_size(params);

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

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

        get_random_bytes(&hash_rnd, sizeof(hash_rnd));

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

        atomic_set(&ht->nelems, 0);

        RCU_INIT_POINTER(ht->tbl, tbl);
        RCU_INIT_POINTER(ht->future_tbl, tbl);

        INIT_WORK(&ht->run_work, rht_deferred_worker);

        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_init);

/**
 * rhashtable_destroy - destroy hash table
 * @ht:         the hash table to destroy
 *
 * Frees the bucket array. This function is not rcu safe, therefore the caller
 * has to make sure that no resizing may happen by unpublishing the hashtable
 * and waiting for the quiescent cycle before releasing the bucket array.
 */
void rhashtable_destroy(struct rhashtable *ht)
{
        ht->being_destroyed = true;

        cancel_work_sync(&ht->run_work);

        mutex_lock(&ht->mutex);
        bucket_table_free(rht_dereference(ht->tbl, ht));
        mutex_unlock(&ht->mutex);
}
EXPORT_SYMBOL_GPL(rhashtable_destroy);