rhashtable: better high order allocation attempts

[deliverable/linux.git] / lib / rhashtable.c

/*
 * Resizable, Scalable, Concurrent Hash Table
 *
 * Copyright (c) 2014-2015 Thomas Graf <tgraf@suug.ch>
 * Copyright (c) 2008-2014 Patrick McHardy <kaber@trash.net>
 *
 * Based on the following paper:
 * https://www.usenix.org/legacy/event/atc11/tech/final_files/Triplett.pdf
 *
 * Code partially derived from nft_hash
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/log2.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/mm.h>
#include <linux/jhash.h>
#include <linux/random.h>
#include <linux/rhashtable.h>
#include <linux/err.h>

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

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

enum {
        RHT_LOCK_NORMAL,
        RHT_LOCK_NESTED,
};

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

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

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

static u32 obj_raw_hashfn(const struct rhashtable *ht, const void *ptr)
{
        u32 hash;

        if (unlikely(!ht->p.key_len))
                hash = ht->p.obj_hashfn(ptr, ht->p.hash_rnd);
        else
                hash = ht->p.hashfn(ptr + ht->p.key_offset, ht->p.key_len,
                                    ht->p.hash_rnd);

        return hash >> HASH_RESERVED_SPACE;
}

static u32 key_hashfn(struct rhashtable *ht, const void *key, u32 len)
{
        return ht->p.hashfn(key, len, ht->p.hash_rnd) >> HASH_RESERVED_SPACE;
}

static u32 head_hashfn(const struct rhashtable *ht,
                       const struct bucket_table *tbl,
                       const struct rhash_head *he)
{
        return rht_bucket_index(tbl, obj_raw_hashfn(ht, rht_obj(ht, he)));
}

#ifdef CONFIG_PROVE_LOCKING
static void debug_dump_buckets(const struct rhashtable *ht,
                               const struct bucket_table *tbl)
{
        struct rhash_head *he;
        unsigned int i, hash;

        for (i = 0; i < tbl->size; i++) {
                pr_warn(" [Bucket %d] ", i);
                rht_for_each_rcu(he, tbl, i) {
                        hash = head_hashfn(ht, tbl, he);
                        pr_cont("[hash = %#x, lock = %p] ",
                                hash, bucket_lock(tbl, hash));
                }
                pr_cont("\n");
        }

}

static void debug_dump_table(struct rhashtable *ht,
                             const struct bucket_table *tbl,
                             unsigned int hash)
{
        struct bucket_table *old_tbl, *future_tbl;

        pr_emerg("BUG: lock for hash %#x in table %p not held\n",
                 hash, tbl);

        rcu_read_lock();
        future_tbl = rht_dereference_rcu(ht->future_tbl, ht);
        old_tbl = rht_dereference_rcu(ht->tbl, ht);
        if (future_tbl != old_tbl) {
                pr_warn("Future table %p (size: %zd)\n",
                        future_tbl, future_tbl->size);
                debug_dump_buckets(ht, future_tbl);
        }

        pr_warn("Table %p (size: %zd)\n", old_tbl, old_tbl->size);
        debug_dump_buckets(ht, old_tbl);

        rcu_read_unlock();
}

#define ASSERT_RHT_MUTEX(HT) BUG_ON(!lockdep_rht_mutex_is_held(HT))
#define ASSERT_BUCKET_LOCK(HT, TBL, HASH)                               \
        do {                                                            \
                if (unlikely(!lockdep_rht_bucket_is_held(TBL, HASH))) { \
                        debug_dump_table(HT, TBL, HASH);                \
                        BUG();                                          \
                }                                                       \
        } while (0)

int lockdep_rht_mutex_is_held(struct rhashtable *ht)
{
        return (debug_locks) ? lockdep_is_held(&ht->mutex) : 1;
}
EXPORT_SYMBOL_GPL(lockdep_rht_mutex_is_held);

int lockdep_rht_bucket_is_held(const struct bucket_table *tbl, u32 hash)
{
        spinlock_t *lock = bucket_lock(tbl, hash);

        return (debug_locks) ? lockdep_is_held(lock) : 1;
}
EXPORT_SYMBOL_GPL(lockdep_rht_bucket_is_held);
#else
#define ASSERT_RHT_MUTEX(HT)
#define ASSERT_BUCKET_LOCK(HT, TBL, HASH)
#endif


static struct rhash_head __rcu **bucket_tail(struct bucket_table *tbl, u32 n)
{
        struct rhash_head __rcu **pprev;

        for (pprev = &tbl->buckets[n];
             !rht_is_a_nulls(rht_dereference_bucket(*pprev, tbl, n));
             pprev = &rht_dereference_bucket(*pprev, tbl, n)->next)
                ;

        return pprev;
}

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

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

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

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

        return 0;
}

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

        kvfree(tbl);
}

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

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

        tbl->size = nbuckets;

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

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

        return tbl;
}

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

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

static void lock_buckets(struct bucket_table *new_tbl,
                         struct bucket_table *old_tbl, unsigned int hash)
        __acquires(old_bucket_lock)
{
        spin_lock_bh(bucket_lock(old_tbl, hash));
        if (new_tbl != old_tbl)
                spin_lock_bh_nested(bucket_lock(new_tbl, hash),
                                    RHT_LOCK_NESTED);
}

static void unlock_buckets(struct bucket_table *new_tbl,
                           struct bucket_table *old_tbl, unsigned int hash)
        __releases(old_bucket_lock)
{
        if (new_tbl != old_tbl)
                spin_unlock_bh(bucket_lock(new_tbl, hash));
        spin_unlock_bh(bucket_lock(old_tbl, hash));
}

/**
 * Unlink entries on bucket which hash to different bucket.
 *
 * Returns true if no more work needs to be performed on the bucket.
 */
static bool hashtable_chain_unzip(struct rhashtable *ht,
                                  const struct bucket_table *new_tbl,
                                  struct bucket_table *old_tbl,
                                  size_t old_hash)
{
        struct rhash_head *he, *p, *next;
        unsigned int new_hash, new_hash2;

        ASSERT_BUCKET_LOCK(ht, old_tbl, old_hash);

        /* Old bucket empty, no work needed. */
        p = rht_dereference_bucket(old_tbl->buckets[old_hash], old_tbl,
                                   old_hash);
        if (rht_is_a_nulls(p))
                return false;

        new_hash = head_hashfn(ht, new_tbl, p);
        ASSERT_BUCKET_LOCK(ht, new_tbl, new_hash);

        /* Advance the old bucket pointer one or more times until it
         * reaches a node that doesn't hash to the same bucket as the
         * previous node p. Call the previous node p;
         */
        rht_for_each_continue(he, p->next, old_tbl, old_hash) {
                new_hash2 = head_hashfn(ht, new_tbl, he);
                ASSERT_BUCKET_LOCK(ht, new_tbl, new_hash2);

                if (new_hash != new_hash2)
                        break;
                p = he;
        }
        rcu_assign_pointer(old_tbl->buckets[old_hash], p->next);

        /* Find the subsequent node which does hash to the same
         * bucket as node P, or NULL if no such node exists.
         */
        INIT_RHT_NULLS_HEAD(next, ht, old_hash);
        if (!rht_is_a_nulls(he)) {
                rht_for_each_continue(he, he->next, old_tbl, old_hash) {
                        if (head_hashfn(ht, new_tbl, he) == new_hash) {
                                next = he;
                                break;
                        }
                }
        }

        /* Set p's next pointer to that subsequent node pointer,
         * bypassing the nodes which do not hash to p's bucket
         */
        rcu_assign_pointer(p->next, next);

        p = rht_dereference_bucket(old_tbl->buckets[old_hash], old_tbl,
                                   old_hash);

        return !rht_is_a_nulls(p);
}

static void link_old_to_new(struct rhashtable *ht, struct bucket_table *new_tbl,
                            unsigned int new_hash, struct rhash_head *entry)
{
        ASSERT_BUCKET_LOCK(ht, new_tbl, new_hash);

        rcu_assign_pointer(*bucket_tail(new_tbl, new_hash), entry);
}

/**
 * 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
 * while keeping them on both lists until the end of the RCU grace period.
 *
 * This function may only be called in a context where it is safe to call
 * synchronize_rcu(), e.g. not within a rcu_read_lock() section.
 *
 * The caller must ensure that no concurrent resizing occurs by holding
 * ht->mutex.
 *
 * It is valid to have concurrent insertions and deletions protected by per
 * bucket locks or concurrent RCU protected lookups and traversals.
 */
int rhashtable_expand(struct rhashtable *ht)
{
        struct bucket_table *new_tbl, *old_tbl = rht_dereference(ht->tbl, ht);
        struct rhash_head *he;
        unsigned int new_hash, old_hash;
        bool complete = false;

        ASSERT_RHT_MUTEX(ht);

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

        atomic_inc(&ht->shift);

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

        /* For each new bucket, search the corresponding old bucket for the
         * first entry that hashes to the new bucket, and link the end of
         * newly formed bucket chain (containing entries added to future
         * table) to that entry. Since all the entries which will end up in
         * the new bucket appear in the same old bucket, this constructs an
         * entirely valid new hash table, but with multiple buckets
         * "zipped" together into a single imprecise chain.
         */
        for (new_hash = 0; new_hash < new_tbl->size; new_hash++) {
                old_hash = rht_bucket_index(old_tbl, new_hash);
                lock_buckets(new_tbl, old_tbl, new_hash);
                rht_for_each(he, old_tbl, old_hash) {
                        if (head_hashfn(ht, new_tbl, he) == new_hash) {
                                link_old_to_new(ht, new_tbl, new_hash, he);
                                break;
                        }
                }
                unlock_buckets(new_tbl, old_tbl, new_hash);
        }

        /* Unzip interleaved hash chains */
        while (!complete && !ht->being_destroyed) {
                /* Wait for readers. All new readers will see the new
                 * table, and thus no references to the old table will
                 * remain.
                 */
                synchronize_rcu();

                /* For each bucket in the old table (each of which
                 * contains items from multiple buckets of the new
                 * table): ...
                 */
                complete = true;
                for (old_hash = 0; old_hash < old_tbl->size; old_hash++) {
                        lock_buckets(new_tbl, old_tbl, old_hash);

                        if (hashtable_chain_unzip(ht, new_tbl, old_tbl,
                                                  old_hash))
                                complete = false;

                        unlock_buckets(new_tbl, old_tbl, old_hash);
                }
        }

        rcu_assign_pointer(ht->tbl, new_tbl);
        synchronize_rcu();

        bucket_table_free(old_tbl);
        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_expand);

/**
 * rhashtable_shrink - Shrink hash table while allowing concurrent lookups
 * @ht:         the hash table to shrink
 *
 * This function may only be called in a context where it is safe to call
 * synchronize_rcu(), e.g. not within a rcu_read_lock() section.
 *
 * The caller must ensure that no concurrent resizing occurs by holding
 * ht->mutex.
 *
 * The caller must ensure that no concurrent table mutations take place.
 * It is however valid to have concurrent lookups if they are RCU protected.
 *
 * It is valid to have concurrent insertions and deletions protected by per
 * bucket locks or concurrent RCU protected lookups and traversals.
 */
int rhashtable_shrink(struct rhashtable *ht)
{
        struct bucket_table *new_tbl, *tbl = rht_dereference(ht->tbl, ht);
        unsigned int new_hash;

        ASSERT_RHT_MUTEX(ht);

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

        rcu_assign_pointer(ht->future_tbl, new_tbl);
        synchronize_rcu();

        /* Link the first entry in the old bucket to the end of the
         * bucket in the new table. As entries are concurrently being
         * added to the new table, lock down the new bucket. As we
         * always divide the size in half when shrinking, each bucket
         * in the new table maps to exactly two buckets in the old
         * table.
         */
        for (new_hash = 0; new_hash < new_tbl->size; new_hash++) {
                lock_buckets(new_tbl, tbl, new_hash);

                rcu_assign_pointer(*bucket_tail(new_tbl, new_hash),
                                   tbl->buckets[new_hash]);
                ASSERT_BUCKET_LOCK(ht, tbl, new_hash + new_tbl->size);
                rcu_assign_pointer(*bucket_tail(new_tbl, new_hash),
                                   tbl->buckets[new_hash + new_tbl->size]);

                unlock_buckets(new_tbl, tbl, new_hash);
        }

        /* Publish the new, valid hash table */
        rcu_assign_pointer(ht->tbl, new_tbl);
        atomic_dec(&ht->shift);

        /* Wait for readers. No new readers will have references to the
         * old hash table.
         */
        synchronize_rcu();

        bucket_table_free(tbl);

        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_shrink);

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

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

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

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

        if (ht->p.grow_decision && ht->p.grow_decision(ht, tbl->size))
                rhashtable_expand(ht);
        else if (ht->p.shrink_decision && ht->p.shrink_decision(ht, tbl->size))
                rhashtable_shrink(ht);

unlock:
        mutex_unlock(&ht->mutex);
}

static void rhashtable_probe_expand(struct rhashtable *ht)
{
        const struct bucket_table *new_tbl = rht_dereference_rcu(ht->future_tbl, ht);
        const struct bucket_table *tbl = rht_dereference_rcu(ht->tbl, ht);

        /* Only adjust the table if no resizing is currently in progress. */
        if (tbl == new_tbl && ht->p.grow_decision &&
            ht->p.grow_decision(ht, tbl->size))
                schedule_work(&ht->run_work);
}

static void rhashtable_probe_shrink(struct rhashtable *ht)
{
        const struct bucket_table *new_tbl = rht_dereference_rcu(ht->future_tbl, ht);
        const struct bucket_table *tbl = rht_dereference_rcu(ht->tbl, ht);

        /* Only adjust the table if no resizing is currently in progress. */
        if (tbl == new_tbl && ht->p.shrink_decision &&
            ht->p.shrink_decision(ht, tbl->size))
                schedule_work(&ht->run_work);
}

static void __rhashtable_insert(struct rhashtable *ht, struct rhash_head *obj,
                                struct bucket_table *tbl, u32 hash)
{
        struct rhash_head *head;

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

        ASSERT_BUCKET_LOCK(ht, tbl, hash);

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

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

        atomic_inc(&ht->nelems);

        rhashtable_probe_expand(ht);
}

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

        rcu_read_lock();

        tbl = rht_dereference_rcu(ht->future_tbl, ht);
        old_tbl = rht_dereference_rcu(ht->tbl, ht);
        hash = obj_raw_hashfn(ht, rht_obj(ht, obj));

        lock_buckets(tbl, old_tbl, hash);
        __rhashtable_insert(ht, obj, tbl, hash);
        unlock_buckets(tbl, old_tbl, hash);

        rcu_read_unlock();
}
EXPORT_SYMBOL_GPL(rhashtable_insert);

/**
 * rhashtable_remove - remove object from hash table
 * @ht:         hash table
 * @obj:        pointer to hash head inside object
 *
 * Since the hash chain is single linked, the removal operation needs to
 * walk the bucket chain upon removal. The removal operation is thus
 * considerable slow if the hash table is not correctly sized.
 *
 * Will automatically shrink the table via rhashtable_expand() if the
 * shrink_decision function specified at rhashtable_init() returns true.
 *
 * The caller must ensure that no concurrent table mutations occur. It is
 * however valid to have concurrent lookups if they are RCU protected.
 */
bool rhashtable_remove(struct rhashtable *ht, struct rhash_head *obj)
{
        struct bucket_table *tbl, *new_tbl, *old_tbl;
        struct rhash_head __rcu **pprev;
        struct rhash_head *he, *he2;
        unsigned int hash, new_hash;
        bool ret = false;

        rcu_read_lock();
        old_tbl = rht_dereference_rcu(ht->tbl, ht);
        tbl = new_tbl = rht_dereference_rcu(ht->future_tbl, ht);
        new_hash = obj_raw_hashfn(ht, rht_obj(ht, obj));

        lock_buckets(new_tbl, old_tbl, new_hash);
restart:
        hash = rht_bucket_index(tbl, new_hash);
        pprev = &tbl->buckets[hash];
        rht_for_each(he, tbl, hash) {
                if (he != obj) {
                        pprev = &he->next;
                        continue;
                }

                ASSERT_BUCKET_LOCK(ht, tbl, hash);

                if (old_tbl->size > new_tbl->size && tbl == old_tbl &&
                    !rht_is_a_nulls(obj->next) &&
                    head_hashfn(ht, tbl, obj->next) != hash) {
                        rcu_assign_pointer(*pprev, (struct rhash_head *) rht_marker(ht, hash));
                } else if (unlikely(old_tbl->size < new_tbl->size && tbl == new_tbl)) {
                        rht_for_each_continue(he2, obj->next, tbl, hash) {
                                if (head_hashfn(ht, tbl, he2) == hash) {
                                        rcu_assign_pointer(*pprev, he2);
                                        goto found;
                                }
                        }

                        rcu_assign_pointer(*pprev, (struct rhash_head *) rht_marker(ht, hash));
                } else {
                        rcu_assign_pointer(*pprev, obj->next);
                }

found:
                ret = true;
                break;
        }

        /* The entry may be linked in either 'tbl', 'future_tbl', or both.
         * 'future_tbl' only exists for a short period of time during
         * resizing. Thus traversing both is fine and the added cost is
         * very rare.
         */
        if (tbl != old_tbl) {
                tbl = old_tbl;
                goto restart;
        }

        unlock_buckets(new_tbl, old_tbl, new_hash);

        if (ret) {
                atomic_dec(&ht->nelems);
                rhashtable_probe_shrink(ht);
        }

        rcu_read_unlock();

        return ret;
}
EXPORT_SYMBOL_GPL(rhashtable_remove);

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

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

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

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

        BUG_ON(!ht->p.key_len);

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

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

        rcu_read_lock();

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

        if (unlikely(tbl != old_tbl)) {
                tbl = old_tbl;
                goto restart;
        }
        rcu_read_unlock();

        return NULL;
}
EXPORT_SYMBOL_GPL(rhashtable_lookup_compare);

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

        BUG_ON(!ht->p.key_len);

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

/**
 * rhashtable_lookup_compare_insert - search and insert object to hash table
 *                                    with compare function
 * @ht:         hash table
 * @obj:        pointer to hash head inside object
 * @compare:    compare function, must return true on match
 * @arg:        argument passed on to compare function
 *
 * Locks down the bucket chain in both the old and new table if a resize
 * is in progress to ensure that writers can't remove from the old table
 * and can't insert to the new table during the atomic operation of search
 * and insertion. Searches for duplicates in both the old and new table if
 * a resize is in progress.
 *
 * Lookups may occur in parallel with hashtable mutations and resizing.
 *
 * Will trigger an automatic deferred table resizing if the size grows
 * beyond the watermark indicated by grow_decision() which can be passed
 * to rhashtable_init().
 */
bool rhashtable_lookup_compare_insert(struct rhashtable *ht,
                                      struct rhash_head *obj,
                                      bool (*compare)(void *, void *),
                                      void *arg)
{
        struct bucket_table *new_tbl, *old_tbl;
        u32 new_hash;
        bool success = true;

        BUG_ON(!ht->p.key_len);

        rcu_read_lock();
        old_tbl = rht_dereference_rcu(ht->tbl, ht);
        new_tbl = rht_dereference_rcu(ht->future_tbl, ht);
        new_hash = obj_raw_hashfn(ht, rht_obj(ht, obj));

        lock_buckets(new_tbl, old_tbl, new_hash);

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

        __rhashtable_insert(ht, obj, new_tbl, new_hash);

exit:
        unlock_buckets(new_tbl, old_tbl, new_hash);
        rcu_read_unlock();

        return success;
}
EXPORT_SYMBOL_GPL(rhashtable_lookup_compare_insert);

/**
 * rhashtable_walk_init - Initialise an iterator
 * @ht:         Table to walk over
 * @iter:       Hash table Iterator
 *
 * This function prepares a hash table walk.
 *
 * Note that if you restart a walk after rhashtable_walk_stop you
 * may see the same object twice.  Also, you may miss objects if
 * there are removals in between rhashtable_walk_stop and the next
 * call to rhashtable_walk_start.
 *
 * For a completely stable walk you should construct your own data
 * structure outside the hash table.
 *
 * This function may sleep so you must not call it from interrupt
 * context or with spin locks held.
 *
 * You must call rhashtable_walk_exit if this function returns
 * successfully.
 */
int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter)
{
        iter->ht = ht;
        iter->p = NULL;
        iter->slot = 0;
        iter->skip = 0;

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

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

        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_init);

/**
 * rhashtable_walk_exit - Free an iterator
 * @iter:       Hash table Iterator
 *
 * This function frees resources allocated by rhashtable_walk_init.
 */
void rhashtable_walk_exit(struct rhashtable_iter *iter)
{
        mutex_lock(&iter->ht->mutex);
        list_del(&iter->walker->list);
        mutex_unlock(&iter->ht->mutex);
        kfree(iter->walker);
}
EXPORT_SYMBOL_GPL(rhashtable_walk_exit);

/**
 * rhashtable_walk_start - Start a hash table walk
 * @iter:       Hash table iterator
 *
 * Start a hash table walk.  Note that we take the RCU lock in all
 * cases including when we return an error.  So you must always call
 * rhashtable_walk_stop to clean up.
 *
 * Returns zero if successful.
 *
 * Returns -EAGAIN if resize event occured.  Note that the iterator
 * will rewind back to the beginning and you may use it immediately
 * by calling rhashtable_walk_next.
 */
int rhashtable_walk_start(struct rhashtable_iter *iter)
{
        rcu_read_lock();

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

        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_start);

/**
 * rhashtable_walk_next - Return the next object and advance the iterator
 * @iter:       Hash table iterator
 *
 * Note that you must call rhashtable_walk_stop when you are finished
 * with the walk.
 *
 * Returns the next object or NULL when the end of the table is reached.
 *
 * Returns -EAGAIN if resize event occured.  Note that the iterator
 * will rewind back to the beginning and you may continue to use it.
 */
void *rhashtable_walk_next(struct rhashtable_iter *iter)
{
        const struct bucket_table *tbl;
        struct rhashtable *ht = iter->ht;
        struct rhash_head *p = iter->p;
        void *obj = NULL;

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

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

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

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

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

                iter->skip = 0;
        }

        iter->p = NULL;

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

        return obj;
}
EXPORT_SYMBOL_GPL(rhashtable_walk_next);

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

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

/**
 * rhashtable_init - initialize a new hash table
 * @ht:         hash table to be initialized
 * @params:     configuration parameters
 *
 * Initializes a new hash table based on the provided configuration
 * parameters. A table can be configured either with a variable or
 * fixed length key:
 *
 * Configuration Example 1: Fixed length keys
 * struct test_obj {
 *      int                     key;
 *      void *                  my_member;
 *      struct rhash_head       node;
 * };
 *
 * struct rhashtable_params params = {
 *      .head_offset = offsetof(struct test_obj, node),
 *      .key_offset = offsetof(struct test_obj, key),
 *      .key_len = sizeof(int),
 *      .hashfn = jhash,
 *      .nulls_base = (1U << RHT_BASE_SHIFT),
 * };
 *
 * Configuration Example 2: Variable length keys
 * struct test_obj {
 *      [...]
 *      struct rhash_head       node;
 * };
 *
 * u32 my_hash_fn(const void *data, u32 seed)
 * {
 *      struct test_obj *obj = data;
 *
 *      return [... hash ...];
 * }
 *
 * struct rhashtable_params params = {
 *      .head_offset = offsetof(struct test_obj, node),
 *      .hashfn = jhash,
 *      .obj_hashfn = my_hash_fn,
 * };
 */
int rhashtable_init(struct rhashtable *ht, struct rhashtable_params *params)
{
        struct bucket_table *tbl;
        size_t size;

        size = HASH_DEFAULT_SIZE;

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

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

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

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

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

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

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

        atomic_set(&ht->nelems, 0);
        atomic_set(&ht->shift, ilog2(tbl->size));
        RCU_INIT_POINTER(ht->tbl, tbl);
        RCU_INIT_POINTER(ht->future_tbl, tbl);

        if (!ht->p.hash_rnd)
                get_random_bytes(&ht->p.hash_rnd, sizeof(ht->p.hash_rnd));

        if (ht->p.grow_decision || ht->p.shrink_decision)
                INIT_WORK(&ht->run_work, rht_deferred_worker);

        return 0;
}
EXPORT_SYMBOL_GPL(rhashtable_init);

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

        if (ht->p.grow_decision || ht->p.shrink_decision)
                cancel_work_sync(&ht->run_work);

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