xref: /linux/lib/rhashtable.c (revision 8c515ff87534a700823fbef194a10fc354a16654)

// SPDX-License-Identifier: GPL-2.0-only
/*
 * 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
 */

#include <linux/atomic.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/irq_work.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>
#include <linux/workqueue.h>

#define HASH_DEFAULT_SIZE	64UL
#define HASH_MIN_SIZE		4U

union nested_table {
	union nested_table __rcu *table;
	struct rhash_lock_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)
{
	if (!debug_locks)
		return 1;
	if (unlikely(tbl->nest))
		return 1;
	return bit_spin_is_locked(0, (unsigned long *)&tbl->buckets[hash]);
}
EXPORT_SYMBOL_GPL(lockdep_rht_bucket_is_held);
#else
#define ASSERT_RHT_MUTEX(HT)
#endif

static inline union nested_table *nested_table_top(
	const struct bucket_table *tbl)
{
	/* The top-level bucket entry does not need RCU protection
	 * because it's set at the same time as tbl->nest.
	 */
	return (void *)rcu_dereference_protected(tbl->buckets[0], 1);
}

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_protected(ntbl->table, 1);
	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 = nested_table_top(tbl);

	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);

	kvfree(tbl);
}

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

	kvfree_atomic(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,
					      bool leaf)
{
	union nested_table *ntbl;
	int i;

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

	ntbl = alloc_hooks_tag(ht->alloc_tag,
			kmalloc_noprof(PAGE_SIZE, GFP_ATOMIC|__GFP_ZERO));

	if (ntbl && leaf) {
		for (i = 0; i < PAGE_SIZE / sizeof(ntbl[0]); i++)
			INIT_RHT_NULLS_HEAD(ntbl[i].bucket);
	}

	if (cmpxchg((union nested_table **)prev, NULL, ntbl) == NULL)
		return ntbl;
	/* Raced with another thread. */
	kfree(ntbl);
	return rcu_dereference(*prev);
}

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 = alloc_hooks_tag(ht->alloc_tag,
			kmalloc_noprof(size, gfp|__GFP_ZERO));
	if (!tbl)
		return NULL;

	if (!nested_table_alloc(ht, (union nested_table __rcu **)tbl->buckets,
				false)) {
		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;
	int i;

	tbl = alloc_hooks_tag(ht->alloc_tag,
			kvmalloc_node_align_noprof(struct_size(tbl, buckets, nbuckets),
					     1, gfp|__GFP_ZERO, NUMA_NO_NODE));

	size = nbuckets;

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

	if (tbl == NULL)
		return NULL;

#ifdef CONFIG_LOCKDEP
	lockdep_init_map(&tbl->dep_map, "rhashtable_bucket", ht->lockdep_key, 0);
#endif

	tbl->size = size;

	rcu_head_init(&tbl->rcu);
	INIT_LIST_HEAD(&tbl->walkers);

	tbl->hash_rnd = get_random_u32();

	for (i = 0; i < nbuckets; i++)
		INIT_RHT_NULLS_HEAD(tbl->buckets[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,
				 struct rhash_lock_head __rcu **bkt,
				 unsigned int old_hash)
{
	struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
	struct bucket_table *new_tbl = rhashtable_last_table(ht, old_tbl);
	int err = -EAGAIN;
	struct rhash_head *head, *next, *entry;
	struct rhash_head __rcu **pprev = NULL;
	unsigned int new_hash;
	unsigned long flags;

	if (new_tbl->nest)
		goto out;

	err = -ENOENT;

	rht_for_each_from(entry, rht_ptr(bkt, old_tbl, old_hash),
			  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);

	flags = rht_lock_nested(new_tbl, &new_tbl->buckets[new_hash],
				SINGLE_DEPTH_NESTING);

	head = rht_ptr(new_tbl->buckets + new_hash, new_tbl, new_hash);

	RCU_INIT_POINTER(entry->next, head);

	rht_assign_unlock(new_tbl, &new_tbl->buckets[new_hash], entry, flags);

	if (pprev)
		rcu_assign_pointer(*pprev, next);
	else
		/* Need to preserved the bit lock. */
		rht_assign_locked(bkt, 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);
	struct rhash_lock_head __rcu **bkt = rht_bucket_var(old_tbl, old_hash);
	unsigned long flags;
	int err;

	if (!bkt)
		return 0;
	flags = rht_lock(old_tbl, bkt);

	while (!(err = rhashtable_rehash_one(ht, bkt, old_hash)))
		;

	if (err == -ENOENT)
		err = 0;
	rht_unlock(old_tbl, bkt, flags);

	return err;
}

static int rhashtable_rehash_attach(struct rhashtable *ht,
				    struct bucket_table *old_tbl,
				    struct bucket_table *new_tbl)
{
	/* Make insertions go into the new, empty table right away. Deletions
	 * and lookups will be attempted in both tables until we synchronize.
	 * As cmpxchg() provides strong barriers, we do not need
	 * rcu_assign_pointer().
	 */

	if (cmpxchg((struct bucket_table **)&old_tbl->future_tbl, NULL,
		    new_tbl) != NULL)
		return -EEXIST;

	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;

	/* Wait for readers. All new readers will see the new
	 * table, and thus no references to the old table will
	 * remain.
	 * We do this inside the locked region so that
	 * rhashtable_walk_stop() can use rcu_head_after_call_rcu()
	 * to check if it should not re-link the table.
	 */
	call_rcu(&old_tbl->rcu, bucket_table_free_rcu);
	spin_unlock(&ht->lock);

	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)
	__must_hold(&ht->mutex)
{
	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)
	__must_hold(&ht->mutex)
{
	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 == -EEXIST) {
		int nerr;

		nerr = rhashtable_rehash_table(ht);
		err = err ?: nerr;
	}

	mutex_unlock(&ht->mutex);

	/*
	 * Re-arm via @run_work, not @run_irq_work.
	 * rhashtable_free_and_destroy() drains async work as irq_work_sync()
	 * followed by cancel_work_sync(). If this site queued irq_work while
	 * cancel_work_sync() was waiting for us, irq_work_sync() would already
	 * have returned and the stale irq_work could fire post-teardown.
	 * cancel_work_sync() natively handles self-requeue on @run_work.
	 */
	if (err)
		schedule_work(&ht->run_work);
}

/*
 * Insert-path callers can run under a raw spinlock (e.g. an insecure_elasticity
 * user). Calling schedule_work() under that lock records caller_lock ->
 * pool->lock -> pi_lock -> rq->__lock, closing a locking cycle if any of
 * these is acquired in the reverse direction elsewhere. Bounce through
 * irq_work so the schedule_work() runs with the caller's lock no longer held.
 */
static void rht_deferred_irq_work(struct irq_work *irq_work)
{
	struct rhashtable *ht = container_of(irq_work, struct rhashtable,
					     run_irq_work);

	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 | __GFP_NOWARN);
	if (new_tbl == NULL)
		goto fail;

	err = rhashtable_rehash_attach(ht, tbl, new_tbl);
	if (err) {
		bucket_table_free_atomic(new_tbl);
		if (err == -EEXIST)
			err = 0;
	} else
		irq_work_queue(&ht->run_irq_work);

	return err;

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

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

	return err;
}

static void *rhashtable_lookup_one(struct rhashtable *ht,
				   struct rhash_lock_head __rcu **bkt,
				   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 = NULL;
	struct rhash_head *head;
	int elasticity;

	elasticity = RHT_ELASTICITY;
	rht_for_each_from(head, rht_ptr(bkt, tbl, hash), 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), ht->p))) {
			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);
		if (pprev)
			rcu_assign_pointer(*pprev, obj);
		else
			/* Need to preserve the bit lock */
			rht_assign_locked(bkt, obj);

		return NULL;
	}

	if (elasticity <= 0 && !ht->p.insecure_elasticity)
		return ERR_PTR(-EAGAIN);

	return ERR_PTR(-ENOENT);
}

static struct bucket_table *rhashtable_insert_one(
	struct rhashtable *ht, struct rhash_lock_head __rcu **bkt,
	struct bucket_table *tbl, unsigned int hash, struct rhash_head *obj,
	void *data)
{
	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 = rht_dereference_rcu(tbl->future_tbl, ht);
	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)) &&
	    !ht->p.insecure_elasticity)
		return ERR_PTR(-EAGAIN);

	head = rht_ptr(bkt, 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);
	}

	/* bkt is always the head of the list, so it holds
	 * the lock, which we need to preserve
	 */
	rht_assign_locked(bkt, obj);

	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;
	struct rhash_lock_head __rcu **bkt;
	unsigned long flags;
	unsigned int hash;
	void *data;

	new_tbl = rcu_dereference(ht->tbl);

	do {
		tbl = new_tbl;
		hash = rht_head_hashfn(ht, tbl, obj, ht->p);
		if (rcu_access_pointer(tbl->future_tbl))
			/* Failure is OK */
			bkt = rht_bucket_var(tbl, hash);
		else
			bkt = rht_bucket_insert(ht, tbl, hash);
		if (bkt == NULL) {
			new_tbl = rht_dereference_rcu(tbl->future_tbl, ht);
			data = ERR_PTR(-EAGAIN);
		} else {
			bool inserted;

			flags = rht_lock(tbl, bkt);
			data = rhashtable_lookup_one(ht, bkt, tbl,
						     hash, key, obj);
			new_tbl = rhashtable_insert_one(ht, bkt, tbl,
							hash, obj, data);
			inserted = data && !new_tbl;
			if (inserted)
				atomic_inc(&ht->nelems);
			if (PTR_ERR(new_tbl) != -EEXIST)
				data = ERR_CAST(new_tbl);

			rht_unlock(tbl, bkt, flags);

			if (inserted && rht_grow_above_75(ht, tbl))
				irq_work_queue(&ht->run_irq_work);
		}
	} while (!IS_ERR_OR_NULL(new_tbl));

	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);

/* Scan one element forward from prev_key's position in @tbl.
 * Returns first rhash_head whose bucket > prev_key's bucket, or the
 * element immediately after prev_key inside prev_key's bucket.
 * Returns the first element if prev_key is NULL, NULL when @tbl is
 * exhausted, or ERR_PTR(-ENOENT) if prev_key is not found in @tbl.
 */
static struct rhash_head *__rhashtable_next_in_table(
	struct rhashtable *ht, struct bucket_table *tbl,
	const void *prev_key)
{
	struct rhashtable_compare_arg arg = { .ht = ht, .key = prev_key };
	const struct rhashtable_params params = ht->p;
	struct rhash_head *he;
	unsigned int b = 0;
	bool found = false;

	if (prev_key) {
		b = rht_key_hashfn(ht, tbl, prev_key, params);
		rht_for_each_rcu(he, tbl, b) {
			bool match = params.obj_cmpfn
				     ? !params.obj_cmpfn(&arg, rht_obj(ht, he))
				     : !rhashtable_compare(&arg, rht_obj(ht, he),
						   params);
			if (found) {
				if (match)
					continue;
				return he;
			}
			if (match)
				found = true;
		}
		if (!found)
			return ERR_PTR(-ENOENT);
		b++;
	}

	for (; b < tbl->size; b++)
		rht_for_each_rcu(he, tbl, b)
			return he;
	return NULL;
}

/**
 * rhashtable_next_key - return next element after a given key
 * @ht:		hash table
 * @prev_key:	pointer to previous key, or NULL for the first element
 *
 * WARNING: this walk is highly unstable. Unlike rhashtable_walk_*(),
 * it cannot detect a concurrent resize or rehash, so a full iteration
 * is NOT guaranteed to terminate under adversarial or sustained
 * rehashing. Callers MUST tolerate skipped and duplicated elements and
 * SHOULD bound their loop externally.
 *
 * Returns the next element in best-effort iteration order, walking the
 * @tbl chain (including any future_tbl in flight). Caller must hold RCU.
 *
 * Pass @prev_key == NULL to obtain the first element. To iterate, set
 * @prev_key to the key of the previously returned element on each call,
 * and stop when NULL is returned.
 *
 * Best-effort semantics:
 *   - Across the tbl->future_tbl chain, an element being migrated may
 *     transiently appear in both tables and be observed twice.
 *   - Concurrent inserts may or may not be observed.
 *   - Termination of a full iteration loop is NOT guaranteed under
 *     adversarial continuous rehash; callers MUST tolerate skips and
 *     repeats and SHOULD bound their loop externally.
 *   - Behavior on tables that contain duplicate keys is undefined:
 *     duplicates may be skipped, repeated, or trap the walk in a
 *     cycle. Callers requiring duplicate-key iteration must use
 *     rhashtable_walk_*() instead.
 *   - rhltable instances are not supported and return
 *     ERR_PTR(-EOPNOTSUPP).
 *   - If prev_key was concurrently deleted and is not present in any
 *     in-flight table, returns ERR_PTR(-ENOENT).
 *
 * Returns entry of the next element, or NULL when iteration is exhausted,
 * or ERR_PTR(-ENOENT) if prev_key is not found, or
 * ERR_PTR(-EOPNOTSUPP) if @ht is an rhltable.
 */
void *rhashtable_next_key(struct rhashtable *ht, const void *prev_key)
{
	struct bucket_table *tbl;
	struct rhash_head *he;

	if (unlikely(ht->rhlist))
		return ERR_PTR(-EOPNOTSUPP);

	tbl = rht_dereference_rcu(ht->tbl, ht);
	do {
		he = __rhashtable_next_in_table(ht, tbl, prev_key);
		if (!IS_ERR_OR_NULL(he))
			return rht_obj(ht, he);
		if (!he)
			prev_key = NULL;
		/* See any new future_tbl attached during a rehash. */
		smp_rmb();
		tbl = rht_dereference_rcu(tbl->future_tbl, ht);
	} while (tbl);
	return he; /* NULL or -ENOENT */
}
EXPORT_SYMBOL_GPL(rhashtable_next_key);

/**
 * 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 be called from any process context, including
 * non-preemptible context, but cannot be called from softirq or
 * hardirq context.
 *
 * 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_enter.
 */
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 occurred.  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_shared(RCU)
{
	struct rhashtable *ht = iter->ht;
	bool rhlist = ht->rhlist;

	rcu_read_lock();

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

	if (iter->end_of_table)
		return 0;
	if (!iter->walker.tbl) {
		iter->walker.tbl = rht_dereference_rcu(ht->tbl, ht);
		iter->slot = 0;
		iter->skip = 0;
		iter->p = NULL;
		return -EAGAIN;
	}

	if (iter->p && !rhlist) {
		/*
		 * We need to validate that 'p' is still in the table, and
		 * if so, update 'skip'
		 */
		struct rhash_head *p;
		int skip = 0;
		rht_for_each_rcu(p, iter->walker.tbl, iter->slot) {
			skip++;
			if (p == iter->p) {
				iter->skip = skip;
				goto found;
			}
		}
		iter->p = NULL;
	} else if (iter->p && rhlist) {
		/* Need to validate that 'list' is still in the table, and
		 * if so, update 'skip' and 'p'.
		 */
		struct rhash_head *p;
		struct rhlist_head *list;
		int skip = 0;
		rht_for_each_rcu(p, iter->walker.tbl, iter->slot) {
			for (list = container_of(p, struct rhlist_head, rhead);
			     list;
			     list = rcu_dereference(list->next)) {
				skip++;
				if (list == iter->list) {
					iter->p = p;
					iter->skip = skip;
					goto found;
				}
			}
		}
		iter->p = NULL;
	}
found:
	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)
{
	struct rhashtable *ht;
	struct bucket_table *tbl = iter->walker.tbl;

	if (!tbl)
		goto out;

	ht = iter->ht;

	spin_lock(&ht->lock);
	if (rcu_head_after_call_rcu(&tbl->rcu, bucket_table_free_rcu))
		/* This bucket table is being freed, don't re-link it. */
		iter->walker.tbl = NULL;
	else
		list_add(&iter->walker.list, &tbl->walkers);
	spin_unlock(&ht->lock);

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

static size_t rounded_hashtable_size(const struct rhashtable_params *params)
{
	size_t retsize;

	if (params->nelem_hint)
		retsize = max(roundup_pow_of_two(params->nelem_hint * 4 / 3),
			      (unsigned long)params->min_size);
	else
		retsize = max(HASH_DEFAULT_SIZE,
			      (unsigned long)params->min_size);

	return retsize;
}

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

int __rhashtable_init_noprof(struct rhashtable *ht,
			     const struct rhashtable_params *params,
			     struct rhashtable_lockdep_keys *keys)
{
	struct bucket_table *tbl;
	size_t size;

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

	memset(ht, 0, sizeof(*ht));
	mutex_init_with_key(&ht->mutex, &keys->mutex_key);
	spin_lock_init(&ht->lock);
	lockdep_set_class(&ht->lock, &keys->lock_key);
#ifdef CONFIG_LOCKDEP
	ht->lockdep_key = &keys->bucket_key;
#endif
	memcpy(&ht->p, params, sizeof(*params));

	alloc_tag_record(ht->alloc_tag);

	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);

	size = rounded_hashtable_size(&ht->p);

	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;
		}
	}

	/*
	 * This is api initialization and thus we need to guarantee the
	 * initial rhashtable allocation. Upon failure, retry with the
	 * smallest possible size with __GFP_NOFAIL semantics.
	 */
	tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
	if (unlikely(tbl == NULL)) {
		size = max_t(u16, ht->p.min_size, HASH_MIN_SIZE);
		tbl = bucket_table_alloc(ht, size, GFP_KERNEL | __GFP_NOFAIL);
	}

	atomic_set(&ht->nelems, 0);

	RCU_INIT_POINTER(ht->tbl, tbl);

	INIT_WORK(&ht->run_work, rht_deferred_worker);
	init_irq_work(&ht->run_irq_work, rht_deferred_irq_work);

	return 0;
}
EXPORT_SYMBOL_GPL(__rhashtable_init_noprof);

int __rhltable_init_noprof(struct rhltable *hlt,
			   const struct rhashtable_params *params,
			   struct rhashtable_lockdep_keys *keys)
{
	int err;

	err = __rhashtable_init_noprof(&hlt->ht, params, keys);
	hlt->ht.rhlist = true;
	return err;
}
EXPORT_SYMBOL_GPL(__rhltable_init_noprof);

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 = rcu_dereference_raw(list->next);
		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.
 *
 * After cancel_work_sync() has returned, the deferred rehash worker is
 * quiesced and, per the contract above, no other concurrent access to the
 * rhashtable is possible. The tables are therefore owned exclusively by
 * this function and can be walked without ht->mutex held.
 */
void rhashtable_free_and_destroy(struct rhashtable *ht,
				 void (*free_fn)(void *ptr, void *arg),
				 void *arg)
{
	struct bucket_table *tbl, *next_tbl;
	unsigned int i;

	irq_work_sync(&ht->run_irq_work);
	cancel_work_sync(&ht->run_work);

	/*
	 * Do NOT take ht->mutex here. The rehash worker establishes
	 * ht->mutex -> fs_reclaim via GFP_KERNEL bucket allocation under
	 * the mutex; callers on the reclaim path (e.g. simple_xattr_ht_free()
	 * from evict() under the dcache shrinker for shmem/kernfs/pidfs
	 * inodes) would otherwise close a circular dependency
	 * fs_reclaim -> ht->mutex.
	 */
	tbl = rcu_dereference_raw(ht->tbl);
restart:
	if (free_fn) {
		for (i = 0; i < tbl->size; i++) {
			struct rhash_head *pos, *next;

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

	next_tbl = rcu_dereference_raw(tbl->future_tbl);
	bucket_table_free(tbl);
	if (next_tbl) {
		tbl = next_tbl;
		goto restart;
	}
}
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_lock_head __rcu **__rht_bucket_nested(
	const 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;
	unsigned int subhash = hash;
	union nested_table *ntbl;

	ntbl = nested_table_top(tbl);
	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 NULL;

	return &ntbl[subhash].bucket;

}
EXPORT_SYMBOL_GPL(__rht_bucket_nested);

struct rhash_lock_head __rcu **rht_bucket_nested(
	const struct bucket_table *tbl, unsigned int hash)
{
	static struct rhash_lock_head __rcu *rhnull;

	if (!rhnull)
		INIT_RHT_NULLS_HEAD(rhnull);
	return __rht_bucket_nested(tbl, hash) ?: &rhnull;
}
EXPORT_SYMBOL_GPL(rht_bucket_nested);

struct rhash_lock_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;

	ntbl = nested_table_top(tbl);
	hash >>= tbl->nest;
	ntbl = nested_table_alloc(ht, &ntbl[index].table,
				  size <= (1 << shift));

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

	if (!ntbl)
		return NULL;

	return &ntbl[hash].bucket;

}
EXPORT_SYMBOL_GPL(rht_bucket_nested_insert);