ext4: fix deadlock during page writeback

[linux/fpc-iii.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/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   128UL

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 int alloc_bucket_locks(struct rhashtable *ht, struct bucket_table *tbl,
                              gfp_t gfp)
{
        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 &&
                    gfp == GFP_KERNEL)
                        tbl->locks = vmalloc(size * sizeof(spinlock_t));
                else
#endif
                tbl->locks = kmalloc_array(size, sizeof(spinlock_t),
                                           gfp);
                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 void bucket_table_free_rcu(struct rcu_head *head)
{
        bucket_table_free(container_of(head, struct bucket_table, rcu));
}

static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
                                               size_t nbuckets,
                                               gfp_t gfp)
{
        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) ||
            gfp != GFP_KERNEL)
                tbl = kzalloc(size, gfp | __GFP_NOWARN | __GFP_NORETRY);
        if (tbl == NULL && gfp == GFP_KERNEL)
                tbl = vzalloc(size);
        if (tbl == NULL)
                return NULL;

        tbl->size = nbuckets;

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

        INIT_LIST_HEAD(&tbl->walkers);

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

        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 = &old_tbl->buckets[old_hash];
        int err = -ENOENT;
        struct rhash_head *head, *next, *entry;
        spinlock_t *new_bucket_lock;
        unsigned int 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 = 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 void 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;

        old_bucket_lock = rht_bucket_lock(old_tbl, old_hash);

        spin_lock_bh(old_bucket_lock);
        while (!rhashtable_rehash_one(ht, old_hash))
                ;
        old_tbl->rehash++;
        spin_unlock_bh(old_bucket_lock);
}

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;

        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++)
                rhashtable_rehash_chain(ht, old_hash);

        /* 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;
}

/**
 * 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.
 */
static int rhashtable_expand(struct rhashtable *ht)
{
        struct bucket_table *new_tbl, *old_tbl = rht_dereference(ht->tbl, ht);
        int err;

        ASSERT_RHT_MUTEX(ht);

        old_tbl = rhashtable_last_table(ht, old_tbl);

        new_tbl = bucket_table_alloc(ht, old_tbl->size * 2, 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 *new_tbl, *old_tbl = rht_dereference(ht->tbl, ht);
        unsigned int size;
        int err;

        ASSERT_RHT_MUTEX(ht);

        size = roundup_pow_of_two(atomic_read(&ht->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;

        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;
}

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))
                rhashtable_expand(ht);
        else if (ht->p.automatic_shrinking && rht_shrink_below_30(ht, tbl))
                rhashtable_shrink(ht);

        err = rhashtable_rehash_table(ht);

        mutex_unlock(&ht->mutex);

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

static bool rhashtable_check_elasticity(struct rhashtable *ht,
                                        struct bucket_table *tbl,
                                        unsigned int hash)
{
        unsigned int elasticity = ht->elasticity;
        struct rhash_head *head;

        rht_for_each(head, tbl, hash)
                if (!--elasticity)
                        return true;

        return false;
}

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;
}
EXPORT_SYMBOL_GPL(rhashtable_insert_rehash);

struct bucket_table *rhashtable_insert_slow(struct rhashtable *ht,
                                            const void *key,
                                            struct rhash_head *obj,
                                            struct bucket_table *tbl)
{
        struct rhash_head *head;
        unsigned int hash;
        int err;

        tbl = rhashtable_last_table(ht, tbl);
        hash = head_hashfn(ht, tbl, obj);
        spin_lock_nested(rht_bucket_lock(tbl, hash), SINGLE_DEPTH_NESTING);

        err = -EEXIST;
        if (key && rhashtable_lookup_fast(ht, key, ht->p))
                goto exit;

        err = -E2BIG;
        if (unlikely(rht_grow_above_max(ht, tbl)))
                goto exit;

        err = -EAGAIN;
        if (rhashtable_check_elasticity(ht, tbl, hash) ||
            rht_grow_above_100(ht, tbl))
                goto exit;

        err = 0;

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

        RCU_INIT_POINTER(obj->next, head);

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

        atomic_inc(&ht->nelems);

exit:
        spin_unlock(rht_bucket_lock(tbl, hash));

        if (err == 0)
                return NULL;
        else if (err == -EAGAIN)
                return tbl;
        else
                return ERR_PTR(err);
}
EXPORT_SYMBOL_GPL(rhashtable_insert_slow);

/**
 * rhashtable_walk_init - Initialise an iterator
 * @ht:         Table to walk over
 * @iter:       Hash table Iterator
 * @gfp:        GFP flags for allocations
 *
 * 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,
                         gfp_t gfp)
{
        iter->ht = ht;
        iter->p = NULL;
        iter->slot = 0;
        iter->skip = 0;

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

        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);

        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)
{
        spin_lock(&iter->ht->lock);
        if (iter->walker->tbl)
                list_del(&iter->walker->list);
        spin_unlock(&iter->ht->lock);
        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)
        __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->walker->tbl = rht_dereference_rcu(ht->tbl, ht);
                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)
{
        struct bucket_table *tbl = iter->walker->tbl;
        struct rhashtable *ht = iter->ht;
        struct rhash_head *p = iter->p;

        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;
                        return rht_obj(ht, 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);
        }

        return NULL;
}
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)
        __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);

        if (params->max_size)
                ht->p.max_size = rounddown_pow_of_two(params->max_size);

        if (params->insecure_max_entries)
                ht->p.insecure_max_entries =
                        rounddown_pow_of_two(params->insecure_max_entries);
        else
                ht->p.insecure_max_entries = ht->p.max_size * 2;

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

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

        /* The maximum (not average) chain length grows with the
         * size of the hash table, at a rate of (log N)/(log log N).
         * The value of 16 is selected so that even if the hash
         * table grew to 2^32 you would not expect the maximum
         * chain length to exceed it unless we are under attack
         * (or extremely unlucky).
         *
         * As this limit is only to detect attacks, we don't need
         * to set it to a lower value as you'd need the chain
         * length to vastly exceed 16 to have any real effect
         * on the system.
         */
        if (!params->insecure_elasticity)
                ht->elasticity = 16;

        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);

/**
 * 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)
{
        const 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;

                        for (pos = rht_dereference(tbl->buckets[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)
                                free_fn(rht_obj(ht, pos), 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);