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32 results

rhashtable.c

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  • rhashtable.c 20.64 KiB
    /*
     * 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);
    
    	/* Ensure the new table is visible to readers. */
    	smp_wmb();
    
    	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
     *
     * 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;
    
    	mutex_lock(&ht->mutex);
    	iter->walker->tbl = rht_dereference(ht->tbl, ht);
    	list_add(&iter->walker->list, &iter->walker->tbl->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);
    	if (iter->walker->tbl)
    		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)
    	__acquires(RCU)
    {
    	struct rhashtable *ht = iter->ht;
    
    	mutex_lock(&ht->mutex);
    
    	if (iter->walker->tbl)
    		list_del(&iter->walker->list);
    
    	rcu_read_lock();
    
    	mutex_unlock(&ht->mutex);
    
    	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;
    
    	if (params->nelem_hint)
    		size = rounded_hashtable_size(params);
    
    	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);
    
    	/* 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);