Re: [Qemu-devel] [PATCH v3 08/11] qht: QEMU's fast, resizable and scalable Hash Table

["Alex Bennée" <alex.bennee@linaro.org>]

Emilio G. Cota <cota@braap.org> writes:

> This is a hash table with optional auto-resizing and MRU promotion for
> reads and writes. Its implementation goal is to stay fast while
> scaling for read-mostly workloads.
>
> A hash table with these features will be necessary for the scalability
> of the ongoing MTTCG work; before those changes arrive we can already
> benefit from the single-threaded speedup that qht also provides.
>
> Signed-off-by: Emilio G. Cota <cota@braap.org>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>


> ---
>  include/qemu/qht.h |  54 +++++
>  util/Makefile.objs |   2 +-
>  util/qht.c         | 590 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 645 insertions(+), 1 deletion(-)
>  create mode 100644 include/qemu/qht.h
>  create mode 100644 util/qht.c
>
> diff --git a/include/qemu/qht.h b/include/qemu/qht.h
> new file mode 100644
> index 0000000..a0a1aa8
> --- /dev/null
> +++ b/include/qemu/qht.h
> @@ -0,0 +1,54 @@
> +/*
> + * Copyright (C) 2016, Emilio G. Cota <cota@braap.org>
> + *
> + * License: GNU GPL, version 2 or later.
> + *   See the COPYING file in the top-level directory.
> + */
> +#ifndef QHT_H
> +#define QHT_H
> +
> +#include "qemu/osdep.h"
> +#include "qemu-common.h"
> +#include "qemu/seqlock.h"
> +#include "qemu/rcu.h"
> +
> +struct qht {
> +    struct qht_map *map;
> +    unsigned int mode;
> +};
> +
> +typedef bool (*qht_lookup_func_t)(const void *obj, const void *userp);
> +typedef void (*qht_iter_func_t)(struct qht *ht, void *p, uint32_t h, void *up);
> +
> +#define QHT_MODE_MRU_LOOKUP  0x1 /* move looked-up items to head */
> +#define QHT_MODE_MRU_INSERT  0x2 /* insert new elements at the head */
> +#define QHT_MODE_AUTO_RESIZE 0x4 /* auto-resize when heavily loaded */
> +
> +void qht_init(struct qht *ht, size_t n_elems, unsigned int mode);
> +
> +/* call only when there are no readers left */
> +void qht_destroy(struct qht *ht);
> +
> +/* call with an external lock held */
> +void qht_reset(struct qht *ht);
> +
> +/* call with an external lock held */
> +void qht_reset_size(struct qht *ht, size_t n_elems);
> +
> +/* call with an external lock held */
> +void qht_insert(struct qht *ht, void *p, uint32_t hash);
> +
> +/* call with an external lock held */
> +bool qht_remove(struct qht *ht, const void *p, uint32_t hash);
> +
> +/* call with an external lock held */
> +void qht_iter(struct qht *ht, qht_iter_func_t func, void *userp);
> +
> +/* call with an external lock held */
> +void qht_grow(struct qht *ht);
> +
> +void *qht_lookup(struct qht *ht, qht_lookup_func_t func, const void *userp,
> +                 uint32_t hash);
> +double qht_avg_bucket_chain_length(struct qht *ht, size_t *n_head_buckets);
> +
> +#endif /* QHT_H */
> diff --git a/util/Makefile.objs b/util/Makefile.objs
> index a8a777e..ae893dc 100644
> --- a/util/Makefile.objs
> +++ b/util/Makefile.objs
> @@ -1,4 +1,4 @@
> -util-obj-y = osdep.o cutils.o unicode.o qemu-timer-common.o
> +util-obj-y = osdep.o cutils.o unicode.o qemu-timer-common.o qht.o
>  util-obj-$(CONFIG_POSIX) += compatfd.o
>  util-obj-$(CONFIG_POSIX) += event_notifier-posix.o
>  util-obj-$(CONFIG_POSIX) += mmap-alloc.o
> diff --git a/util/qht.c b/util/qht.c
> new file mode 100644
> index 0000000..05ea5e8
> --- /dev/null
> +++ b/util/qht.c
> @@ -0,0 +1,590 @@
> +/*
> + * qht.c - QEMU Hash Table, designed to scale for read-mostly workloads.
> + *
> + * Copyright (C) 2016, Emilio G. Cota <cota@braap.org>
> + *
> + * License: GNU GPL, version 2 or later.
> + *   See the COPYING file in the top-level directory.
> + *
> + * Assumptions:
> + * - Writers and iterators must take an external lock.
> + * - A hash value of 0 is invalid.
> + * - NULL cannot be inserted as a pointer value.
> + *
> + * Features:
> + * - Optional auto-resizing: the hash table resizes up if the load surpasses
> + *   a certain threshold. Resizing is done concurrently with readers.
> + * - Optional bucket MRU promotion policy.
> + *
> + * The key structure is the bucket, which is cacheline-sized. Buckets
> + * contain a few hash values and pointers; the u32 hash values are stored in
> + * full so that resizing is fast. Having this structure instead of directly
> + * chaining items has three advantages:
> + * - Failed lookups fail fast, and touch a minimum number of cache lines.
> + * - Resizing the hash table with concurrent lookups is easy.
> + * - We can have a few Most-Recently-Used (MRU) hash-pointer pairs in the same
> + *   head bucket. This helps scalability, since MRU promotions (i.e. writes to
> + *   the bucket) become less common.
> + *
> + * For concurrent lookups we use a per-bucket seqlock; per-bucket spinlocks
> + * allow readers (lookups) to upgrade to writers and thus implement an MRU
> + * promotion policy; these MRU-induced writes do not touch the cache lines of
> + * other head buckets.
> + *
> + * Note that there are two types of buckets:
> + * 1. "head" buckets are the ones allocated in the array of buckets in qht_map.
> + * 2. all "non-head" buckets (i.e. all others) are members of a chain that
> + *    starts from a head bucket.
> + * Note that the seqlock and spinlock of a head bucket applies to all buckets
> + * chained to it; these two fields are unused in non-head buckets.
> + *
> + * Resizing is done by taking all spinlocks (so that no readers-turned-writers
> + * can race with us) and then placing all elements into a new hash table. Last,
> + * the ht->map pointer is set, and the old map is freed once no RCU readers can
> + * see it anymore.
> + *
> + * Related Work:
> + * - Idea of cacheline-sized buckets with full hashes taken from:
> + *   David, Guerraoui & Trigonakis, "Asynchronized Concurrency:
> + *   The Secret to Scaling Concurrent Search Data Structures", ASPLOS'15.
> + * - Why not RCU-based hash tables? They would allow us to get rid of the
> + *   seqlock, but resizing would take forever since RCU read critical
> + *   sections in QEMU take quite a long time.
> + *   More info on relativistic hash tables:
> + *   + Triplett, McKenney & Walpole, "Resizable, Scalable, Concurrent Hash
> + *     Tables via Relativistic Programming", USENIX ATC'11.
> + *   + Corbet, "Relativistic hash tables, part 1: Algorithms", @ lwn.net, 2014.
> + *     https://lwn.net/Articles/612021/
> + */
> +#include "qemu/qht.h"
> +#include "qemu/atomic.h"
> +
> +/*
> + * We want to avoid false sharing of cache lines. Most systems have 64-byte
> + * cache lines so we go with it for simplicity.
> + *
> + * Note that systems with smaller cache lines will be fine (the struct is
> + * almost 64-bytes); systems with larger cache lines might suffer from
> + * some false sharing.
> + */
> +#define QHT_BUCKET_ALIGN 64
> +
> +/* define these to keep sizeof(qht_bucket) within QHT_BUCKET_ALIGN */
> +#if HOST_LONG_BITS == 32
> +#define QHT_BUCKET_ENTRIES 6
> +#else /* 64-bit */
> +#define QHT_BUCKET_ENTRIES 4
> +#endif
> +
> +struct qht_bucket {
> +    QemuSpin lock;
> +    QemuSeqLock sequence;
> +    uint32_t hashes[QHT_BUCKET_ENTRIES];
> +    void *pointers[QHT_BUCKET_ENTRIES];
> +    struct qht_bucket *next;
> +} QEMU_ALIGNED(QHT_BUCKET_ALIGN);
> +
> +QEMU_BUILD_BUG_ON(sizeof(struct qht_bucket) > QHT_BUCKET_ALIGN);
> +
> +struct qht_map {
> +    struct qht_bucket *buckets;
> +    size_t n;
> +    size_t n_items;
> +    size_t n_items_threshold;
> +    struct rcu_head rcu;
> +};
> +
> +static inline size_t qht_elems_to_buckets(size_t n_elems)
> +{
> +    return pow2ceil(n_elems / QHT_BUCKET_ENTRIES);
> +}
> +
> +static inline void qht_head_init(struct qht_bucket *b)
> +{
> +    memset(b, 0, sizeof(*b));
> +    qemu_spin_init(&b->lock);
> +    seqlock_init(&b->sequence);
> +}
> +
> +static inline
> +struct qht_bucket *qht_map_to_bucket(struct qht_map *map, uint32_t hash)
> +{
> +    return &map->buckets[hash & (map->n - 1)];
> +}
> +
> +static inline void qht_chain_destroy(struct qht_bucket *head)
> +{
> +    struct qht_bucket *curr = head->next;
> +    struct qht_bucket *prev;
> +
> +    while (curr) {
> +        prev = curr;
> +        curr = curr->next;
> +        qemu_vfree(prev);
> +    }
> +}
> +
> +/* pass only an orphan map */
> +static void qht_map_destroy(struct qht_map *map)
> +{
> +    size_t i;
> +
> +    for (i = 0; i < map->n; i++) {
> +        qht_chain_destroy(&map->buckets[i]);
> +    }
> +    qemu_vfree(map->buckets);
> +    g_free(map);
> +}
> +
> +static void qht_map_reclaim(struct rcu_head *rcu)
> +{
> +    struct qht_map *map = container_of(rcu, struct qht_map, rcu);
> +
> +    qht_map_destroy(map);
> +}
> +
> +static struct qht_map *qht_map_create(size_t n)
> +{
> +    struct qht_map *map;
> +    size_t i;
> +
> +    map = g_malloc(sizeof(*map));
> +    map->n = n;
> +    map->n_items = 0;
> +    map->n_items_threshold = n * QHT_BUCKET_ENTRIES / 2;
> +    map->buckets = qemu_memalign(QHT_BUCKET_ALIGN, sizeof(*map->buckets) * n);
> +    for (i = 0; i < n; i++) {
> +        qht_head_init(&map->buckets[i]);
> +    }
> +    return map;
> +}
> +
> +static inline void qht_publish(struct qht *ht, struct qht_map *new)
> +{
> +    /* Readers should see a properly initialized map; pair with smp_rmb() */
> +    smp_wmb();
> +    atomic_set(&ht->map, new);
> +}
> +
> +void qht_init(struct qht *ht, size_t n_elems, unsigned int mode)
> +{
> +    struct qht_map *map;
> +    size_t n = qht_elems_to_buckets(n_elems);
> +
> +    map = qht_map_create(n);
> +    ht->mode = mode;
> +    qht_publish(ht, map);
> +}
> +
> +/* call only when there are no readers left */
> +void qht_destroy(struct qht *ht)
> +{
> +    qht_map_destroy(ht->map);
> +    memset(ht, 0, sizeof(*ht));
> +}
> +
> +static void qht_bucket_reset(struct qht_bucket *head)
> +{
> +    struct qht_bucket *b = head;
> +    int i;
> +
> +    qemu_spin_lock(&head->lock);
> +    seqlock_write_begin(&head->sequence);
> +    do {
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +            atomic_set(&b->hashes[i], 0);
> +            atomic_set(&b->pointers[i], NULL);
> +        }
> +        b = b->next;
> +    } while (b);
> +    seqlock_write_end(&head->sequence);
> +    qemu_spin_unlock(&head->lock);
> +}
> +
> +/* call with an external lock held */
> +void qht_reset(struct qht *ht)
> +{
> +    struct qht_map *map = ht->map;
> +    size_t i;
> +
> +    for (i = 0; i < map->n; i++) {
> +        qht_bucket_reset(&map->buckets[i]);
> +    }
> +}
> +
> +/* call with an external lock held */
> +void qht_reset_size(struct qht *ht, size_t n_elems)
> +{
> +    struct qht_map *old = ht->map;
> +
> +    qht_reset(ht);
> +    if (old->n == qht_elems_to_buckets(n_elems)) {
> +        return;
> +    }
> +    qht_init(ht, n_elems, ht->mode);
> +    call_rcu1(&old->rcu, qht_map_reclaim);
> +}
> +
> +/* Can only be called when at least orig is the 3rd link i.e. head->2nd->orig */
> +static void qht_move_bucket_to_front__locked(struct qht_bucket *head,
> +                                             struct qht_bucket *orig)
> +{
> +    struct qht_bucket *b = head->next;
> +
> +    for (;;) {
> +        if (b->next == orig) {
> +            atomic_set(&b->next, orig->next);
> +            atomic_set(&orig->next, head->next);
> +            atomic_set(&head->next, orig);
> +            return;
> +        }
> +        b = b->next;
> +    }
> +}
> +
> +/*
> + * Call with head->lock held.
> + *
> + * This function moves the item in @orig[@pos] to @head[0]. In order to
> + * do this, the item i in @head is moved to @head's i+1 position,
> + * with the last item being moved to @orig[@pos].
> + * Further, @orig is brought to the front of the bucket chain, i.e.
> + * @head->next is set to point to @orig. This is done to make sure that
> + * the last item in @head is not moved to a too-distant place. An
> + * alternative would be to perform full MRU for the entire bucket
> + * chain, but that would cause too much churn.
> + *
> + * Note: @orig == @head is a bug.
> + */
> +static inline void qht_bucket_mru__locked(struct qht_bucket *head,
> +                                          struct qht_bucket *orig, int pos)
> +{
> +    uint32_t *dest_hash;
> +    void **dest_p;
> +    void *p;
> +    uint32_t hash;
> +    int i;
> +
> +    assert(orig != head);
> +    for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +        if (i == QHT_BUCKET_ENTRIES - 1) {
> +            dest_hash = &orig->hashes[pos];
> +            dest_p = &orig->pointers[pos];
> +        } else {
> +            dest_hash = &head->hashes[i + 1];
> +            dest_p = &head->pointers[i + 1];
> +        }
> +        hash = *dest_hash;
> +        p = *dest_p;
> +
> +        atomic_set(dest_hash, head->hashes[i]);
> +        atomic_set(dest_p, head->pointers[i]);
> +
> +        atomic_set(&head->hashes[i], hash);
> +        atomic_set(&head->pointers[i], p);
> +    }
> +    if (head->next != orig) {
> +        qht_move_bucket_to_front__locked(head, orig);
> +    }
> +}
> +
> +static void qht_bucket_mru(struct qht_bucket *head, struct qht_bucket *orig,
> +                           const void *p, int pos)
> +{
> +    qemu_spin_lock(&head->lock);
> +    if (unlikely(orig->pointers[pos] != p)) {
> +        /* while we acquired the lock, the bucket was updated, so bail out */
> +        goto out;
> +    }
> +    seqlock_write_begin(&head->sequence);
> +    qht_bucket_mru__locked(head, orig, pos);
> +    seqlock_write_end(&head->sequence);
> + out:
> +    qemu_spin_unlock(&head->lock);
> +}
> +
> +/*
> + * @far_bucket and @pos are only filled in if the match is found in a bucket
> + * that is not the head bucket.
> + */
> +static inline
> +void *qht_do_lookup(struct qht_bucket *head, struct qht_bucket **far_bucket,
> +                    int *pos, qht_lookup_func_t func, const void *userp,
> +                    uint32_t hash)
> +{
> +    struct qht_bucket *b = head;
> +    int i;
> +
> +    do {
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +            if (b->hashes[i] == hash) {
> +                void *p = atomic_read(&b->pointers[i]);
> +
> +                /*
> +                 * p could be NULL if we're racing with a writer. We could use
> +                 * barriers here but for performance we only issue the ones
> +                 * in the seqlock.
> +                 */
> +                if (likely(p && func(p, userp))) {
> +                    if (unlikely(b != head)) {
> +                        *far_bucket = b;
> +                        *pos = i;
> +                    }
> +                    return p;
> +                }
> +            }
> +        }
> +        b = atomic_read(&b->next);
> +        /*
> +         * This barrier guarantees that we will read a properly initialized
> +         * b->next; it is paired with an smp_wmb() before setting b->next.
> +         */
> +        smp_rmb();
> +    } while (b);
> +    return NULL;
> +}
> +
> +void *qht_lookup(struct qht *ht, qht_lookup_func_t func, const void *userp,
> +                 uint32_t hash)
> +{
> +    struct qht_bucket *far_bucket = NULL;
> +    struct qht_bucket *b;
> +    struct qht_map *map;
> +    uint32_t version;
> +    int pos = 0;
> +    void *ret;
> +
> +    map = atomic_read(&ht->map);
> +    /* paired with smp_wmb() before setting ht->map */
> +    smp_rmb();
> +    b = qht_map_to_bucket(map, hash);
> +
> +    do {
> +        version = seqlock_read_begin(&b->sequence);
> +        ret = qht_do_lookup(b, &far_bucket, &pos, func, userp, hash);
> +    } while (seqlock_read_retry(&b->sequence, version));
> +    if ((ht->mode & QHT_MODE_MRU_LOOKUP) && unlikely(far_bucket)) {
> +        qht_bucket_mru(b, far_bucket, ret, pos);
> +    }
> +    return ret;
> +}
> +
> +/* call with b->lock held */
> +static void qht_insert__locked(struct qht *ht, struct qht_map *map,
> +                               struct qht_bucket *head, void *p, uint32_t hash)
> +{
> +    struct qht_bucket *b = head;
> +    struct qht_bucket *prev = NULL;
> +    struct qht_bucket *new = NULL;
> +    int i;
> +
> +    for (;;) {
> +        if (b == NULL) {
> +            b = qemu_memalign(QHT_BUCKET_ALIGN, sizeof(*b));
> +            memset(b, 0, sizeof(*b));
> +            new = b;
> +        }
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +            if (b->hashes[i]) {
> +                continue;
> +            }
> +            /* found an empty key: acquire the seqlock and write */
> +            seqlock_write_begin(&head->sequence);
> +            if (new) {
> +                /*
> +                 * This barrier is paired with smp_rmb() after reading
> +                 * b->next when not holding b->lock.
> +                 */
> +                smp_wmb();
> +                atomic_set(&prev->next, b);
> +            }
> +            atomic_set(&b->hashes[i], hash);
> +            atomic_set(&b->pointers[i], p);
> +            if ((ht->mode & QHT_MODE_MRU_INSERT) && b != head) {
> +                qht_bucket_mru__locked(head, b, i);
> +            }
> +            seqlock_write_end(&head->sequence);
> +            map->n_items++;
> +            return;
> +        }
> +        prev = b;
> +        b = b->next;
> +    }
> +}
> +
> +/* call with an external lock held */
> +void qht_insert(struct qht *ht, void *p, uint32_t hash)
> +{
> +    struct qht_map *map = ht->map;
> +    struct qht_bucket *b = qht_map_to_bucket(map, hash);
> +
> +    qemu_spin_lock(&b->lock);
> +    qht_insert__locked(ht, map, b, p, hash);
> +    qemu_spin_unlock(&b->lock);
> +
> +    if ((ht->mode & QHT_MODE_AUTO_RESIZE) &&
> +        unlikely(map->n_items > map->n_items_threshold)) {
> +        qht_grow(ht);
> +    }
> +}
> +
> +/* call with b->lock held */
> +static inline bool qht_remove__locked(struct qht_map *map, struct qht_bucket *b,
> +                                      const void *p, uint32_t hash)
> +{
> +    int i;
> +
> +    do {
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +            if (b->hashes[i] == hash && b->pointers[i] == p) {
> +                atomic_set(&b->hashes[i], 0);
> +                atomic_set(&b->pointers[i], NULL);
> +                map->n_items--;
> +                return true;
> +            }
> +        }
> +        b = b->next;
> +    } while (b);
> +    return false;
> +}
> +
> +/* call with an external lock held */
> +bool qht_remove(struct qht *ht, const void *p, uint32_t hash)
> +{
> +    struct qht_map *map = ht->map;
> +    struct qht_bucket *b = qht_map_to_bucket(map, hash);
> +    bool ret;
> +
> +    qemu_spin_lock(&b->lock);
> +    seqlock_write_begin(&b->sequence);
> +    ret = qht_remove__locked(map, b, p, hash);
> +    seqlock_write_end(&b->sequence);
> +    qemu_spin_unlock(&b->lock);
> +    return ret;
> +}
> +
> +/*
> + * acquire all spinlocks from a map, so that writers cannot race with
> + * readers-turned-writers.
> + */
> +static void qht_lock(struct qht *ht)
> +{
> +    struct qht_map *map = ht->map;
> +    size_t i;
> +
> +    /* if readers cannot upgrade, do nothing */
> +    if (!(ht->mode & QHT_MODE_MRU_LOOKUP)) {
> +        return;
> +    }
> +    for (i = 0; i < map->n; i++) {
> +        struct qht_bucket *b = &map->buckets[i];
> +
> +        qemu_spin_lock(&b->lock);
> +    }
> +}
> +
> +static void qht_unlock(struct qht *ht)
> +{
> +    struct qht_map *map = ht->map;
> +    size_t i;
> +
> +    if (!(ht->mode & QHT_MODE_MRU_LOOKUP)) {
> +        return;
> +    }
> +    for (i = 0; i < map->n; i++) {
> +        struct qht_bucket *b = &map->buckets[i];
> +
> +        qemu_spin_unlock(&b->lock);
> +    }
> +}
> +
> +/* external lock + all of the map's locks held (if !MRU_LOOKUP) */
> +static inline void qht_map_iter__locked(struct qht *ht, struct qht_map *map,
> +                                        qht_iter_func_t func, void *userp)
> +{
> +    size_t i;
> +
> +    for (i = 0; i < map->n; i++) {
> +        struct qht_bucket *b = &map->buckets[i];
> +        int j;
> +
> +        do {
> +            for (j = 0; j < QHT_BUCKET_ENTRIES; j++) {
> +                if (b->hashes[j]) {
> +                    func(ht, b->pointers[j], b->hashes[j], userp);
> +                }
> +            }
> +            b = b->next;
> +        } while (b);
> +    }
> +}
> +
> +/* call with an external lock held */
> +void qht_iter(struct qht *ht, qht_iter_func_t func, void *userp)
> +{
> +    qht_lock(ht);
> +    qht_map_iter__locked(ht, ht->map, func, userp);
> +    qht_unlock(ht);
> +}
> +
> +static void qht_map_copy(struct qht *ht, void *p, uint32_t hash, void *userp)
> +{
> +    struct qht_map *new = userp;
> +    struct qht_bucket *b = qht_map_to_bucket(new, hash);
> +
> +    /* no need to acquire b->lock because no thread has seen this map yet */
> +    qht_insert__locked(ht, new, b, p, hash);
> +}
> +
> +/* call with an external lock held */
> +void qht_grow(struct qht *ht)
> +{
> +    struct qht_map *old = ht->map;
> +    struct qht_map *new;
> +    size_t n = old->n * 2;
> +
> +    new = qht_map_create(n);
> +    qht_iter(ht, qht_map_copy, new);
> +
> +    qht_publish(ht, new);
> +    call_rcu1(&old->rcu, qht_map_reclaim);
> +}
> +
> +/*
> + * Returns the number of buckets that an average lookup will traverse.
> + * The return value is always >= 1. With a good hashing function, this
> + * value should be close to 1.
> + * Note that each bucket tracks up to QHT_BUCKET_ENTRIES items.
> + */
> +double qht_avg_bucket_chain_length(struct qht *ht, size_t *n_head_buckets)
> +{
> +    struct qht_map *map;
> +    size_t count = 0;
> +    size_t i;
> +
> +    map = atomic_read(&ht->map);
> +    /* paired with smp_wmb() before setting ht->map */
> +    smp_rmb();
> +
> +    for (i = 0; i < map->n; i++) {
> +        struct qht_bucket *head = &map->buckets[i];
> +        struct qht_bucket *b;
> +        size_t bucket_count;
> +        uint32_t version;
> +
> +        do {
> +            version = seqlock_read_begin(&head->sequence);
> +            bucket_count = 0;
> +            b = head;
> +            do {
> +                bucket_count++;
> +                b = b->next;
> +            } while (b);
> +        } while (seqlock_read_retry(&head->sequence, version));
> +        count += bucket_count;
> +    }
> +    if (n_head_buckets) {
> +        *n_head_buckets = map->n;
> +    }
> +    return (double)count / map->n;
> +}


--
Alex Bennée