linux-next/include/linux/rhashtable.h

/*
 * Resizable, Scalable, Concurrent Hash Table
 *
 * Copyright (c) 2015 Herbert Xu <herbert@gondor.apana.org.au>
 * Copyright (c) 2014 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.
 */

#ifndef _LINUX_RHASHTABLE_H
#define _LINUX_RHASHTABLE_H

#include <linux/compiler.h>
#include <linux/errno.h>
#include <linux/list_nulls.h>
#include <linux/workqueue.h>
#include <linux/mutex.h>
#include <linux/rcupdate.h>

/*
 * The end of the chain is marked with a special nulls marks which has
 * the following format:
 *
 * +-------+-----------------------------------------------------+-+
 * | Base  |                      Hash                           |1|
 * +-------+-----------------------------------------------------+-+
 *
 * Base (4 bits) : Reserved to distinguish between multiple tables.
 *                 Specified via &struct rhashtable_params.nulls_base.
 * Hash (27 bits): Full hash (unmasked) of first element added to bucket
 * 1 (1 bit)     : Nulls marker (always set)
 *
 * The remaining bits of the next pointer remain unused for now.
 */
#define RHT_BASE_BITS		4
#define RHT_HASH_BITS		27
#define RHT_BASE_SHIFT		RHT_HASH_BITS

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

struct rhash_head {
	struct rhash_head __rcu		*next;
};

/**
 * struct bucket_table - Table of hash buckets
 * @size: Number of hash buckets
 * @rehash: Current bucket being rehashed
 * @hash_rnd: Random seed to fold into hash
 * @locks_mask: Mask to apply before accessing locks[]
 * @locks: Array of spinlocks protecting individual buckets
 * @walkers: List of active walkers
 * @rcu: RCU structure for freeing the table
 * @future_tbl: Table under construction during rehashing
 * @buckets: size * hash buckets
 */
struct bucket_table {
	unsigned int		size;
	unsigned int		rehash;
	u32			hash_rnd;
	unsigned int		locks_mask;
	spinlock_t		*locks;
	struct list_head	walkers;
	struct rcu_head		rcu;

	struct bucket_table __rcu *future_tbl;

	struct rhash_head __rcu	*buckets[] ____cacheline_aligned_in_smp;
};

/**
 * struct rhashtable_compare_arg - Key for the function rhashtable_compare
 * @ht: Hash table
 * @key: Key to compare against
 */
struct rhashtable_compare_arg {
	struct rhashtable *ht;
	const void *key;
};

typedef u32 (*rht_hashfn_t)(const void *data, u32 len, u32 seed);
typedef u32 (*rht_obj_hashfn_t)(const void *data, u32 seed);
typedef int (*rht_obj_cmpfn_t)(struct rhashtable_compare_arg *arg,
			       const void *obj);

struct rhashtable;

/**
 * struct rhashtable_params - Hash table construction parameters
 * @nelem_hint: Hint on number of elements, should be 75% of desired size
 * @key_len: Length of key
 * @key_offset: Offset of key in struct to be hashed
 * @head_offset: Offset of rhash_head in struct to be hashed
 * @max_size: Maximum size while expanding
 * @min_size: Minimum size while shrinking
 * @nulls_base: Base value to generate nulls marker
 * @locks_mul: Number of bucket locks to allocate per cpu (default: 128)
 * @hashfn: Function to hash key
 * @obj_hashfn: Function to hash object
 * @obj_cmpfn: Function to compare key with object
 */
struct rhashtable_params {
	size_t			nelem_hint;
	size_t			key_len;
	size_t			key_offset;
	size_t			head_offset;
	unsigned int		max_size;
	unsigned int		min_size;
	u32			nulls_base;
	size_t			locks_mul;
	rht_hashfn_t		hashfn;
	rht_obj_hashfn_t	obj_hashfn;
	rht_obj_cmpfn_t		obj_cmpfn;
};

/**
 * struct rhashtable - Hash table handle
 * @tbl: Bucket table
 * @nelems: Number of elements in table
 * @p: Configuration parameters
 * @run_work: Deferred worker to expand/shrink asynchronously
 * @mutex: Mutex to protect current/future table swapping
 * @being_destroyed: True if table is set up for destruction
 */
struct rhashtable {
	struct bucket_table __rcu	*tbl;
	atomic_t			nelems;
	bool                            being_destroyed;
	struct rhashtable_params	p;
	struct work_struct		run_work;
	struct mutex                    mutex;
};

/**
 * struct rhashtable_walker - Hash table walker
 * @list: List entry on list of walkers
 * @tbl: The table that we were walking over
 */
struct rhashtable_walker {
	struct list_head list;
	struct bucket_table *tbl;
};

/**
 * struct rhashtable_iter - Hash table iterator, fits into netlink cb
 * @ht: Table to iterate through
 * @p: Current pointer
 * @walker: Associated rhashtable walker
 * @slot: Current slot
 * @skip: Number of entries to skip in slot
 */
struct rhashtable_iter {
	struct rhashtable *ht;
	struct rhash_head *p;
	struct rhashtable_walker *walker;
	unsigned int slot;
	unsigned int skip;
};

static inline unsigned long rht_marker(const struct rhashtable *ht, u32 hash)
{
	return NULLS_MARKER(ht->p.nulls_base + hash);
}

#define INIT_RHT_NULLS_HEAD(ptr, ht, hash) \
	((ptr) = (typeof(ptr)) rht_marker(ht, hash))

static inline bool rht_is_a_nulls(const struct rhash_head *ptr)
{
	return ((unsigned long) ptr & 1);
}

static inline unsigned long rht_get_nulls_value(const struct rhash_head *ptr)
{
	return ((unsigned long) ptr) >> 1;
}

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

static inline unsigned int rht_bucket_index(const struct bucket_table *tbl,
					    unsigned int hash)
{
	return (hash >> RHT_HASH_RESERVED_SPACE) & (tbl->size - 1);
}

static inline unsigned int rht_key_hashfn(
	struct rhashtable *ht, const struct bucket_table *tbl,
	const void *key, const struct rhashtable_params params)
{
	/* params must be equal to ht->p if it isn't constant. */
	unsigned key_len = __builtin_constant_p(params.key_len) ?
			   (params.key_len ?: ht->p.key_len) :
			   params.key_len;

	return rht_bucket_index(tbl, params.hashfn(key, key_len,
						   tbl->hash_rnd));
}

static inline unsigned int rht_head_hashfn(
	struct rhashtable *ht, const struct bucket_table *tbl,
	const struct rhash_head *he, const struct rhashtable_params params)
{
	const char *ptr = rht_obj(ht, he);

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

/**
 * rht_grow_above_75 - returns true if nelems > 0.75 * table-size
 * @ht:		hash table
 * @tbl:	current table
 */
static inline 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_size || tbl->size < ht->p.max_size);
}

/**
 * rht_shrink_below_30 - returns true if nelems < 0.3 * table-size
 * @ht:		hash table
 * @tbl:	current table
 */
static inline 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->size > ht->p.min_size;
}

/* 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 inline spinlock_t *rht_bucket_lock(const struct bucket_table *tbl,
					  unsigned int hash)
{
	return &tbl->locks[hash & tbl->locks_mask];
}

#ifdef CONFIG_PROVE_LOCKING
int lockdep_rht_mutex_is_held(struct rhashtable *ht);
int lockdep_rht_bucket_is_held(const struct bucket_table *tbl, u32 hash);
#else
static inline int lockdep_rht_mutex_is_held(struct rhashtable *ht)
{
	return 1;
}

static inline int lockdep_rht_bucket_is_held(const struct bucket_table *tbl,
					     u32 hash)
{
	return 1;
}
#endif /* CONFIG_PROVE_LOCKING */

int rhashtable_init(struct rhashtable *ht,
		    const struct rhashtable_params *params);

int rhashtable_insert_slow(struct rhashtable *ht, const void *key,
			   struct rhash_head *obj,
			   struct bucket_table *old_tbl);

int rhashtable_expand(struct rhashtable *ht);
int rhashtable_shrink(struct rhashtable *ht);

int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter);
void rhashtable_walk_exit(struct rhashtable_iter *iter);
int rhashtable_walk_start(struct rhashtable_iter *iter) __acquires(RCU);
void *rhashtable_walk_next(struct rhashtable_iter *iter);
void rhashtable_walk_stop(struct rhashtable_iter *iter) __releases(RCU);

void rhashtable_destroy(struct rhashtable *ht);

#define rht_dereference(p, ht) \
	rcu_dereference_protected(p, lockdep_rht_mutex_is_held(ht))

#define rht_dereference_rcu(p, ht) \
	rcu_dereference_check(p, lockdep_rht_mutex_is_held(ht))

#define rht_dereference_bucket(p, tbl, hash) \
	rcu_dereference_protected(p, lockdep_rht_bucket_is_held(tbl, hash))

#define rht_dereference_bucket_rcu(p, tbl, hash) \
	rcu_dereference_check(p, lockdep_rht_bucket_is_held(tbl, hash))

#define rht_entry(tpos, pos, member) \
	({ tpos = container_of(pos, typeof(*tpos), member); 1; })

/**
 * rht_for_each_continue - continue iterating over hash chain
 * @pos:	the &struct rhash_head to use as a loop cursor.
 * @head:	the previous &struct rhash_head to continue from
 * @tbl:	the &struct bucket_table
 * @hash:	the hash value / bucket index
 */
#define rht_for_each_continue(pos, head, tbl, hash) \
	for (pos = rht_dereference_bucket(head, tbl, hash); \
	     !rht_is_a_nulls(pos); \
	     pos = rht_dereference_bucket((pos)->next, tbl, hash))

/**
 * rht_for_each - iterate over hash chain
 * @pos:	the &struct rhash_head to use as a loop cursor.
 * @tbl:	the &struct bucket_table
 * @hash:	the hash value / bucket index
 */
#define rht_for_each(pos, tbl, hash) \
	rht_for_each_continue(pos, (tbl)->buckets[hash], tbl, hash)

/**
 * rht_for_each_entry_continue - continue iterating over hash chain
 * @tpos:	the type * to use as a loop cursor.
 * @pos:	the &struct rhash_head to use as a loop cursor.
 * @head:	the previous &struct rhash_head to continue from
 * @tbl:	the &struct bucket_table
 * @hash:	the hash value / bucket index
 * @member:	name of the &struct rhash_head within the hashable struct.
 */
#define rht_for_each_entry_continue(tpos, pos, head, tbl, hash, member)	\
	for (pos = rht_dereference_bucket(head, tbl, hash);		\
	     (!rht_is_a_nulls(pos)) && rht_entry(tpos, pos, member);	\
	     pos = rht_dereference_bucket((pos)->next, tbl, hash))

/**
 * rht_for_each_entry - iterate over hash chain of given type
 * @tpos:	the type * to use as a loop cursor.
 * @pos:	the &struct rhash_head to use as a loop cursor.
 * @tbl:	the &struct bucket_table
 * @hash:	the hash value / bucket index
 * @member:	name of the &struct rhash_head within the hashable struct.
 */
#define rht_for_each_entry(tpos, pos, tbl, hash, member)		\
	rht_for_each_entry_continue(tpos, pos, (tbl)->buckets[hash],	\
				    tbl, hash, member)

/**
 * rht_for_each_entry_safe - safely iterate over hash chain of given type
 * @tpos:	the type * to use as a loop cursor.
 * @pos:	the &struct rhash_head to use as a loop cursor.
 * @next:	the &struct rhash_head to use as next in loop cursor.
 * @tbl:	the &struct bucket_table
 * @hash:	the hash value / bucket index
 * @member:	name of the &struct rhash_head within the hashable struct.
 *
 * This hash chain list-traversal primitive allows for the looped code to
 * remove the loop cursor from the list.
 */
#define rht_for_each_entry_safe(tpos, pos, next, tbl, hash, member)	    \
	for (pos = rht_dereference_bucket((tbl)->buckets[hash], tbl, hash), \
	     next = !rht_is_a_nulls(pos) ?				    \
		       rht_dereference_bucket(pos->next, tbl, hash) : NULL; \
	     (!rht_is_a_nulls(pos)) && rht_entry(tpos, pos, member);	    \
	     pos = next,						    \
	     next = !rht_is_a_nulls(pos) ?				    \
		       rht_dereference_bucket(pos->next, tbl, hash) : NULL)

/**
 * rht_for_each_rcu_continue - continue iterating over rcu hash chain
 * @pos:	the &struct rhash_head to use as a loop cursor.
 * @head:	the previous &struct rhash_head to continue from
 * @tbl:	the &struct bucket_table
 * @hash:	the hash value / bucket index
 *
 * This hash chain list-traversal primitive may safely run concurrently with
 * the _rcu mutation primitives such as rhashtable_insert() as long as the
 * traversal is guarded by rcu_read_lock().
 */
#define rht_for_each_rcu_continue(pos, head, tbl, hash)			\
	for (({barrier(); }),						\
	     pos = rht_dereference_bucket_rcu(head, tbl, hash);		\
	     !rht_is_a_nulls(pos);					\
	     pos = rcu_dereference_raw(pos->next))

/**
 * rht_for_each_rcu - iterate over rcu hash chain
 * @pos:	the &struct rhash_head to use as a loop cursor.
 * @tbl:	the &struct bucket_table
 * @hash:	the hash value / bucket index
 *
 * This hash chain list-traversal primitive may safely run concurrently with
 * the _rcu mutation primitives such as rhashtable_insert() as long as the
 * traversal is guarded by rcu_read_lock().
 */
#define rht_for_each_rcu(pos, tbl, hash)				\
	rht_for_each_rcu_continue(pos, (tbl)->buckets[hash], tbl, hash)

/**
 * rht_for_each_entry_rcu_continue - continue iterating over rcu hash chain
 * @tpos:	the type * to use as a loop cursor.
 * @pos:	the &struct rhash_head to use as a loop cursor.
 * @head:	the previous &struct rhash_head to continue from
 * @tbl:	the &struct bucket_table
 * @hash:	the hash value / bucket index
 * @member:	name of the &struct rhash_head within the hashable struct.
 *
 * This hash chain list-traversal primitive may safely run concurrently with
 * the _rcu mutation primitives such as rhashtable_insert() as long as the
 * traversal is guarded by rcu_read_lock().
 */
#define rht_for_each_entry_rcu_continue(tpos, pos, head, tbl, hash, member) \
	for (({barrier(); }),						    \
	     pos = rht_dereference_bucket_rcu(head, tbl, hash);		    \
	     (!rht_is_a_nulls(pos)) && rht_entry(tpos, pos, member);	    \
	     pos = rht_dereference_bucket_rcu(pos->next, tbl, hash))

/**
 * rht_for_each_entry_rcu - iterate over rcu hash chain of given type
 * @tpos:	the type * to use as a loop cursor.
 * @pos:	the &struct rhash_head to use as a loop cursor.
 * @tbl:	the &struct bucket_table
 * @hash:	the hash value / bucket index
 * @member:	name of the &struct rhash_head within the hashable struct.
 *
 * This hash chain list-traversal primitive may safely run concurrently with
 * the _rcu mutation primitives such as rhashtable_insert() as long as the
 * traversal is guarded by rcu_read_lock().
 */
#define rht_for_each_entry_rcu(tpos, pos, tbl, hash, member)		\
	rht_for_each_entry_rcu_continue(tpos, pos, (tbl)->buckets[hash],\
					tbl, hash, member)

static inline int rhashtable_compare(struct rhashtable_compare_arg *arg,
				     const void *obj)
{
	struct rhashtable *ht = arg->ht;
	const char *ptr = obj;

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

/**
 * rhashtable_lookup_fast - search hash table, inlined version
 * @ht:		hash table
 * @key:	the pointer to the key
 * @params:	hash table parameters
 *
 * 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.
 *
 * Returns the first entry on which the compare function returned true.
 */
static inline void *rhashtable_lookup_fast(
	struct rhashtable *ht, const void *key,
	const struct rhashtable_params params)
{
	struct rhashtable_compare_arg arg = {
		.ht = ht,
		.key = key,
	};
	const struct bucket_table *tbl;
	struct rhash_head *he;
	unsigned hash;

	rcu_read_lock();

	tbl = rht_dereference_rcu(ht->tbl, ht);
restart:
	hash = rht_key_hashfn(ht, tbl, key, params);
	rht_for_each_rcu(he, tbl, hash) {
		if (params.obj_cmpfn ?
		    params.obj_cmpfn(&arg, rht_obj(ht, he)) :
		    rhashtable_compare(&arg, rht_obj(ht, he)))
			continue;
		rcu_read_unlock();
		return rht_obj(ht, he);
	}

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

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

	return NULL;
}

static inline int __rhashtable_insert_fast(
	struct rhashtable *ht, const void *key, struct rhash_head *obj,
	const struct rhashtable_params params)
{
	struct rhashtable_compare_arg arg = {
		.ht = ht,
		.key = key,
	};
	int err = -EEXIST;
	struct bucket_table *tbl, *new_tbl;
	struct rhash_head *head;
	spinlock_t *lock;
	unsigned hash;

	rcu_read_lock();

	tbl = rht_dereference_rcu(ht->tbl, ht);
	hash = rht_head_hashfn(ht, tbl, obj, params);
	lock = rht_bucket_lock(tbl, hash);

	spin_lock_bh(lock);

	/* Because we have already taken the bucket lock in 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 tbl because until
	 * the rehash completes ht->tbl won't be changed.
	 */
	new_tbl = rht_dereference_rcu(tbl->future_tbl, ht);
	if (unlikely(new_tbl)) {
		err = rhashtable_insert_slow(ht, key, obj, new_tbl);
		goto out;
	}

	if (!key)
		goto skip_lookup;

	rht_for_each(head, tbl, hash) {
		if (unlikely(!(params.obj_cmpfn ?
			       params.obj_cmpfn(&arg, rht_obj(ht, head)) :
			       rhashtable_compare(&arg, rht_obj(ht, head)))))
			goto out;
	}

skip_lookup:
	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);
	if (rht_grow_above_75(ht, tbl))
		schedule_work(&ht->run_work);

out:
	spin_unlock_bh(lock);
	rcu_read_unlock();

	return err;
}

/**
 * rhashtable_insert_fast - insert object into hash table
 * @ht:		hash table
 * @obj:	pointer to hash head inside object
 * @params:	hash table parameters
 *
 * 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().
 */
static inline int rhashtable_insert_fast(
	struct rhashtable *ht, struct rhash_head *obj,
	const struct rhashtable_params params)
{
	return __rhashtable_insert_fast(ht, NULL, obj, params);
}

/**
 * rhashtable_lookup_insert_fast - lookup and insert object into hash table
 * @ht:		hash table
 * @obj:	pointer to hash head inside object
 * @params:	hash table parameters
 *
 * 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().
 */
static inline int rhashtable_lookup_insert_fast(
	struct rhashtable *ht, struct rhash_head *obj,
	const struct rhashtable_params params)
{
	const char *key = rht_obj(ht, obj);

	BUG_ON(ht->p.obj_hashfn);

	return __rhashtable_insert_fast(ht, key + ht->p.key_offset, obj,
					params);
}

/**
 * rhashtable_lookup_insert_key - search and insert object to hash table
 *				  with explicit key
 * @ht:		hash table
 * @key:	key
 * @obj:	pointer to hash head inside object
 * @params:	hash table parameters
 *
 * 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().
 *
 * Returns zero on success.
 */
static inline int rhashtable_lookup_insert_key(
	struct rhashtable *ht, const void *key, struct rhash_head *obj,
	const struct rhashtable_params params)
{
	BUG_ON(!ht->p.obj_hashfn || !key);

	return __rhashtable_insert_fast(ht, key, obj, params);
}

static inline int __rhashtable_remove_fast(
	struct rhashtable *ht, struct bucket_table *tbl,
	struct rhash_head *obj, const struct rhashtable_params params)
{
	struct rhash_head __rcu **pprev;
	struct rhash_head *he;
	spinlock_t * lock;
	unsigned hash;
	int err = -ENOENT;

	hash = rht_head_hashfn(ht, tbl, obj, params);
	lock = rht_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);
		err = 0;
		break;
	}

	spin_unlock_bh(lock);

	return err;
}

/**
 * rhashtable_remove_fast - remove object from hash table
 * @ht:		hash table
 * @obj:	pointer to hash head inside object
 * @params:	hash table parameters
 *
 * 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.
 *
 * Returns zero on success, -ENOENT if the entry could not be found.
 */
static inline int rhashtable_remove_fast(
	struct rhashtable *ht, struct rhash_head *obj,
	const struct rhashtable_params params)
{
	struct bucket_table *tbl;
	int err;

	rcu_read_lock();

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

	/* 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
	 * the old tbl if it exists.
	 */
	while ((err = __rhashtable_remove_fast(ht, tbl, obj, params)) &&
	       (tbl = rht_dereference_rcu(tbl->future_tbl, ht)))
		;

	if (err)
		goto out;

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

out:
	rcu_read_unlock();

	return err;
}

#endif /* _LINUX_RHASHTABLE_H */