[urcu.git] / rculfhash.c

/*
 * rculfhash.c
 *
 * Userspace RCU library - Lock-Free Resizable RCU Hash Table
 *
 * Copyright 2010-2011 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/*
 * Based on the following articles:
 * - Ori Shalev and Nir Shavit. Split-ordered lists: Lock-free
 *   extensible hash tables. J. ACM 53, 3 (May 2006), 379-405.
 * - Michael, M. M. High performance dynamic lock-free hash tables
 *   and list-based sets. In Proceedings of the fourteenth annual ACM
 *   symposium on Parallel algorithms and architectures, ACM Press,
 *   (2002), 73-82.
 *
 * Some specificities of this Lock-Free Resizable RCU Hash Table
 * implementation:
 *
 * - RCU read-side critical section allows readers to perform hash
 *   table lookups and use the returned objects safely by delaying
 *   memory reclaim of a grace period.
 * - Add and remove operations are lock-free, and do not need to
 *   allocate memory. They need to be executed within RCU read-side
 *   critical section to ensure the objects they read are valid and to
 *   deal with the cmpxchg ABA problem.
 * - add and add_unique operations are supported. add_unique checks if
 *   the node key already exists in the hash table. It ensures no key
 *   duplicata exists.
 * - The resize operation executes concurrently with add/remove/lookup.
 * - Hash table nodes are contained within a split-ordered list. This
 *   list is ordered by incrementing reversed-bits-hash value.
 * - An index of dummy nodes is kept. These dummy nodes are the hash
 *   table "buckets", and they are also chained together in the
 *   split-ordered list, which allows recursive expansion.
 * - The resize operation for small tables only allows expanding the hash table.
 *   It is triggered automatically by detecting long chains in the add
 *   operation.
 * - The resize operation for larger tables (and available through an
 *   API) allows both expanding and shrinking the hash table.
 * - Per-CPU Split-counters are used to keep track of the number of
 *   nodes within the hash table for automatic resize triggering.
 * - Resize operation initiated by long chain detection is executed by a
 *   call_rcu thread, which keeps lock-freedom of add and remove.
 * - Resize operations are protected by a mutex.
 * - The removal operation is split in two parts: first, a "removed"
 *   flag is set in the next pointer within the node to remove. Then,
 *   a "garbage collection" is performed in the bucket containing the
 *   removed node (from the start of the bucket up to the removed node).
 *   All encountered nodes with "removed" flag set in their next
 *   pointers are removed from the linked-list. If the cmpxchg used for
 *   removal fails (due to concurrent garbage-collection or concurrent
 *   add), we retry from the beginning of the bucket. This ensures that
 *   the node with "removed" flag set is removed from the hash table
 *   (not visible to lookups anymore) before the RCU read-side critical
 *   section held across removal ends. Furthermore, this ensures that
 *   the node with "removed" flag set is removed from the linked-list
 *   before its memory is reclaimed. Only the thread which removal
 *   successfully set the "removed" flag (with a cmpxchg) into a node's
 *   next pointer is considered to have succeeded its removal (and thus
 *   owns the node to reclaim). Because we garbage-collect starting from
 *   an invariant node (the start-of-bucket dummy node) up to the
 *   "removed" node (or find a reverse-hash that is higher), we are sure
 *   that a successful traversal of the chain leads to a chain that is
 *   present in the linked-list (the start node is never removed) and
 *   that is does not contain the "removed" node anymore, even if
 *   concurrent delete/add operations are changing the structure of the
 *   list concurrently.
 * - The add operation performs gargage collection of buckets if it
 *   encounters nodes with removed flag set in the bucket where it wants
 *   to add its new node. This ensures lock-freedom of add operation by
 *   helping the remover unlink nodes from the list rather than to wait
 *   for it do to so.
 * - A RCU "order table" indexed by log2(hash index) is copied and
 *   expanded by the resize operation. This order table allows finding
 *   the "dummy node" tables.
 * - There is one dummy node table per hash index order. The size of
 *   each dummy node table is half the number of hashes contained in
 *   this order.
 * - call_rcu is used to garbage-collect the old order table.
 * - The per-order dummy node tables contain a compact version of the
 *   hash table nodes. These tables are invariant after they are
 *   populated into the hash table.
 * 
 * A bit of ascii art explanation:
 * 
 * Order index is the off-by-one compare to the actual power of 2 because 
 * we use index 0 to deal with the 0 special-case.
 * 
 * This shows the nodes for a small table ordered by reversed bits:
 * 
 *    bits   reverse
 * 0  000        000
 * 4  100        001
 * 2  010        010
 * 6  110        011
 * 1  001        100
 * 5  101        101
 * 3  011        110
 * 7  111        111
 * 
 * This shows the nodes in order of non-reversed bits, linked by 
 * reversed-bit order.
 * 
 * order              bits       reverse
 * 0               0  000        000
 *                 |
 * 1               |  1  001        100       <-    <-
 *                 |  |                        |     |
 * 2               |  |  2  010        010     |     |
 *                 |  |  |  3  011        110  | <-  |
 *                 |  |  |  |                  |  |  |
 * 3               -> |  |  |  4  100        001  |  |
 *                    -> |  |     5  101        101  |
 *                       -> |        6  110        011
 *                          ->          7  111        111
 */

#define _LGPL_SOURCE
#include <stdlib.h>
#include <errno.h>
#include <assert.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>

#include "config.h"
#include <urcu.h>
#include <urcu-call-rcu.h>
#include <urcu/arch.h>
#include <urcu/uatomic.h>
#include <urcu/jhash.h>
#include <urcu/compiler.h>
#include <urcu/rculfhash.h>
#include <stdio.h>
#include <pthread.h>

#ifdef DEBUG
#define dbg_printf(fmt, args...)     printf("[debug rculfhash] " fmt, ## args)
#else
#define dbg_printf(fmt, args...)
#endif

/* For testing */
#define POISON_FREE

/*
 * Per-CPU split-counters lazily update the global counter each 1024
 * addition/removal. It automatically keeps track of resize required.
 * We use the bucket length as indicator for need to expand for small
 * tables and machines lacking per-cpu data suppport.
 */
#define COUNT_COMMIT_ORDER              10
#define CHAIN_LEN_TARGET                1
#define CHAIN_LEN_RESIZE_THRESHOLD      3

/*
 * Define the minimum table size. Protects against hash table resize overload
 * when too many entries are added quickly before the resize can complete.
 * This is especially the case if the table could be shrinked to a size of 1.
 * TODO: we might want to make the add/remove operations help the resize to
 * add or remove dummy nodes when a resize is ongoing to ensure upper-bound on
 * chain length.
 */
#define MIN_TABLE_SIZE                  128

#ifndef max
#define max(a, b)       ((a) > (b) ? (a) : (b))
#endif

/*
 * The removed flag needs to be updated atomically with the pointer.
 * The dummy flag does not require to be updated atomically with the
 * pointer, but it is added as a pointer low bit flag to save space.
 */
#define REMOVED_FLAG            (1UL << 0)
#define DUMMY_FLAG              (1UL << 1)
#define FLAGS_MASK              ((1UL << 2) - 1)

struct ht_items_count {
        unsigned long add, remove;
} __attribute__((aligned(CAA_CACHE_LINE_SIZE)));

struct rcu_level {
        struct rcu_head head;
        struct _cds_lfht_node nodes[0];
};

struct rcu_table {
        unsigned long size;     /* always a power of 2 */
        unsigned long resize_target;
        int resize_initiated;
        struct rcu_head head;
        struct rcu_level *tbl[0];
};

struct cds_lfht {
        struct rcu_table *t;            /* shared */
        cds_lfht_hash_fct hash_fct;
        cds_lfht_compare_fct compare_fct;
        unsigned long hash_seed;
        int flags;
        pthread_mutex_t resize_mutex;   /* resize mutex: add/del mutex */
        unsigned int in_progress_resize, in_progress_destroy;
        void (*cds_lfht_call_rcu)(struct rcu_head *head,
                      void (*func)(struct rcu_head *head));
        void (*cds_lfht_synchronize_rcu)(void);
        unsigned long count;            /* global approximate item count */
        struct ht_items_count *percpu_count;    /* per-cpu item count */
};

struct rcu_resize_work {
        struct rcu_head head;
        struct cds_lfht *ht;
};

/*
 * Algorithm to reverse bits in a word by lookup table, extended to
 * 64-bit words.
 * Source:
 * http://graphics.stanford.edu/~seander/bithacks.html#BitReverseTable
 * Originally from Public Domain.
 */

static const uint8_t BitReverseTable256[256] = 
{
#define R2(n) (n),   (n) + 2*64,     (n) + 1*64,     (n) + 3*64
#define R4(n) R2(n), R2((n) + 2*16), R2((n) + 1*16), R2((n) + 3*16)
#define R6(n) R4(n), R4((n) + 2*4 ), R4((n) + 1*4 ), R4((n) + 3*4 )
        R6(0), R6(2), R6(1), R6(3)
};
#undef R2
#undef R4
#undef R6

static
uint8_t bit_reverse_u8(uint8_t v)
{
        return BitReverseTable256[v];
}

static __attribute__((unused))
uint32_t bit_reverse_u32(uint32_t v)
{
        return ((uint32_t) bit_reverse_u8(v) << 24) | 
                ((uint32_t) bit_reverse_u8(v >> 8) << 16) | 
                ((uint32_t) bit_reverse_u8(v >> 16) << 8) | 
                ((uint32_t) bit_reverse_u8(v >> 24));
}

static __attribute__((unused))
uint64_t bit_reverse_u64(uint64_t v)
{
        return ((uint64_t) bit_reverse_u8(v) << 56) | 
                ((uint64_t) bit_reverse_u8(v >> 8)  << 48) | 
                ((uint64_t) bit_reverse_u8(v >> 16) << 40) |
                ((uint64_t) bit_reverse_u8(v >> 24) << 32) |
                ((uint64_t) bit_reverse_u8(v >> 32) << 24) | 
                ((uint64_t) bit_reverse_u8(v >> 40) << 16) | 
                ((uint64_t) bit_reverse_u8(v >> 48) << 8) |
                ((uint64_t) bit_reverse_u8(v >> 56));
}

static
unsigned long bit_reverse_ulong(unsigned long v)
{
#if (CAA_BITS_PER_LONG == 32)
        return bit_reverse_u32(v);
#else
        return bit_reverse_u64(v);
#endif
}

/*
 * fls: returns the position of the most significant bit.
 * Returns 0 if no bit is set, else returns the position of the most
 * significant bit (from 1 to 32 on 32-bit, from 1 to 64 on 64-bit).
 */
#if defined(__i386) || defined(__x86_64)
static inline
unsigned int fls_u32(uint32_t x)
{
        int r;

        asm("bsrl %1,%0\n\t"
            "jnz 1f\n\t"
            "movl $-1,%0\n\t"
            "1:\n\t"
            : "=r" (r) : "rm" (x));
        return r + 1;
}
#define HAS_FLS_U32
#endif

#if defined(__x86_64)
static inline
unsigned int fls_u64(uint64_t x)
{
        long r;

        asm("bsrq %1,%0\n\t"
            "jnz 1f\n\t"
            "movq $-1,%0\n\t"
            "1:\n\t"
            : "=r" (r) : "rm" (x));
        return r + 1;
}
#define HAS_FLS_U64
#endif

#ifndef HAS_FLS_U64
static __attribute__((unused))
unsigned int fls_u64(uint64_t x)
{
        unsigned int r = 64;

        if (!x)
                return 0;

        if (!(x & 0xFFFFFFFF00000000ULL)) {
                x <<= 32;
                r -= 32;
        }
        if (!(x & 0xFFFF000000000000ULL)) {
                x <<= 16;
                r -= 16;
        }
        if (!(x & 0xFF00000000000000ULL)) {
                x <<= 8;
                r -= 8;
        }
        if (!(x & 0xF000000000000000ULL)) {
                x <<= 4;
                r -= 4;
        }
        if (!(x & 0xC000000000000000ULL)) {
                x <<= 2;
                r -= 2;
        }
        if (!(x & 0x8000000000000000ULL)) {
                x <<= 1;
                r -= 1;
        }
        return r;
}
#endif

#ifndef HAS_FLS_U32
static __attribute__((unused))
unsigned int fls_u32(uint32_t x)
{
        unsigned int r = 32;

        if (!x)
                return 0;
        if (!(x & 0xFFFF0000U)) {
                x <<= 16;
                r -= 16;
        }
        if (!(x & 0xFF000000U)) {
                x <<= 8;
                r -= 8;
        }
        if (!(x & 0xF0000000U)) {
                x <<= 4;
                r -= 4;
        }
        if (!(x & 0xC0000000U)) {
                x <<= 2;
                r -= 2;
        }
        if (!(x & 0x80000000U)) {
                x <<= 1;
                r -= 1;
        }
        return r;
}
#endif

unsigned int fls_ulong(unsigned long x)
{
#if (CAA_BITS_PER_lONG == 32)
        return fls_u32(x);
#else
        return fls_u64(x);
#endif
}

int get_count_order_u32(uint32_t x)
{
        int order;

        order = fls_u32(x) - 1;
        if (x & (x - 1))
                order++;
        return order;
}

int get_count_order_ulong(unsigned long x)
{
        int order;

        order = fls_ulong(x) - 1;
        if (x & (x - 1))
                order++;
        return order;
}

#ifdef POISON_FREE
#define poison_free(ptr)                                \
        do {                                            \
                memset(ptr, 0x42, sizeof(*(ptr)));      \
                free(ptr);                              \
        } while (0)
#else
#define poison_free(ptr)        free(ptr)
#endif

static
void cds_lfht_resize_lazy(struct cds_lfht *ht, struct rcu_table *t, int growth);

/*
 * If the sched_getcpu() and sysconf(_SC_NPROCESSORS_CONF) calls are
 * available, then we support hash table item accounting.
 * In the unfortunate event the number of CPUs reported would be
 * inaccurate, we use modulo arithmetic on the number of CPUs we got.
 */
#if defined(HAVE_SCHED_GETCPU) && defined(HAVE_SYSCONF)

static
void cds_lfht_resize_lazy_count(struct cds_lfht *ht, struct rcu_table *t,
                                unsigned long count);

static long nr_cpus_mask = -1;

static
struct ht_items_count *alloc_per_cpu_items_count(void)
{
        struct ht_items_count *count;

        switch (nr_cpus_mask) {
        case -2:
                return NULL;
        case -1:
        {
                long maxcpus;

                maxcpus = sysconf(_SC_NPROCESSORS_CONF);
                if (maxcpus <= 0) {
                        nr_cpus_mask = -2;
                        return NULL;
                }
                /*
                 * round up number of CPUs to next power of two, so we
                 * can use & for modulo.
                 */
                maxcpus = 1UL << get_count_order_ulong(maxcpus);
                nr_cpus_mask = maxcpus - 1;
        }
                /* Fall-through */
        default:
                return calloc(nr_cpus_mask + 1, sizeof(*count));
        }
}

static
void free_per_cpu_items_count(struct ht_items_count *count)
{
        poison_free(count);
}

static
int ht_get_cpu(void)
{
        int cpu;

        assert(nr_cpus_mask >= 0);
        cpu = sched_getcpu();
        if (unlikely(cpu < 0))
                return cpu;
        else
                return cpu & nr_cpus_mask;
}

static
void ht_count_add(struct cds_lfht *ht, struct rcu_table *t)
{
        unsigned long percpu_count;
        int cpu;

        if (unlikely(!ht->percpu_count))
                return;
        cpu = ht_get_cpu();
        if (unlikely(cpu < 0))
                return;
        percpu_count = uatomic_add_return(&ht->percpu_count[cpu].add, 1);
        if (unlikely(!(percpu_count & ((1UL << COUNT_COMMIT_ORDER) - 1)))) {
                unsigned long count;

                dbg_printf("add percpu %lu\n", percpu_count);
                count = uatomic_add_return(&ht->count,
                                           1UL << COUNT_COMMIT_ORDER);
                /* If power of 2 */
                if (!(count & (count - 1))) {
                        if ((count >> CHAIN_LEN_RESIZE_THRESHOLD)
                                        < t->size)
                                return;
                        dbg_printf("add set global %lu\n", count);
                        cds_lfht_resize_lazy_count(ht, t,
                                count >> (CHAIN_LEN_TARGET - 1));
                }
        }
}

static
void ht_count_remove(struct cds_lfht *ht, struct rcu_table *t)
{
        unsigned long percpu_count;
        int cpu;

        if (unlikely(!ht->percpu_count))
                return;
        cpu = ht_get_cpu();
        if (unlikely(cpu < 0))
                return;
        percpu_count = uatomic_add_return(&ht->percpu_count[cpu].remove, -1);
        if (unlikely(!(percpu_count & ((1UL << COUNT_COMMIT_ORDER) - 1)))) {
                unsigned long count;

                dbg_printf("remove percpu %lu\n", percpu_count);
                count = uatomic_add_return(&ht->count,
                                           -(1UL << COUNT_COMMIT_ORDER));
                /* If power of 2 */
                if (!(count & (count - 1))) {
                        if ((count >> CHAIN_LEN_RESIZE_THRESHOLD)
                                        >= t->size)
                                return;
                        dbg_printf("remove set global %lu\n", count);
                        cds_lfht_resize_lazy_count(ht, t,
                                count >> (CHAIN_LEN_TARGET - 1));
                }
        }
}

#else /* #if defined(HAVE_SCHED_GETCPU) && defined(HAVE_SYSCONF) */

static const long nr_cpus_mask = -1;

static
struct ht_items_count *alloc_per_cpu_items_count(void)
{
        return NULL;
}

static
void free_per_cpu_items_count(struct ht_items_count *count)
{
}

static
void ht_count_add(struct cds_lfht *ht, struct rcu_table *t)
{
}

static
void ht_count_remove(struct cds_lfht *ht, struct rcu_table *t)
{
}

#endif /* #else #if defined(HAVE_SCHED_GETCPU) && defined(HAVE_SYSCONF) */


static
void check_resize(struct cds_lfht *ht, struct rcu_table *t,
                  uint32_t chain_len)
{
        unsigned long count;

        if (!(ht->flags & CDS_LFHT_AUTO_RESIZE))
                return;
        count = uatomic_read(&ht->count);
        /*
         * Use bucket-local length for small table expand and for
         * environments lacking per-cpu data support.
         */
        if (count >= (1UL << COUNT_COMMIT_ORDER))
                return;
        if (chain_len > 100)
                dbg_printf("WARNING: large chain length: %u.\n",
                           chain_len);
        if (chain_len >= CHAIN_LEN_RESIZE_THRESHOLD)
                cds_lfht_resize_lazy(ht, t,
                        get_count_order_u32(chain_len - (CHAIN_LEN_TARGET - 1)));
}

static
struct cds_lfht_node *clear_flag(struct cds_lfht_node *node)
{
        return (struct cds_lfht_node *) (((unsigned long) node) & ~FLAGS_MASK);
}

static
int is_removed(struct cds_lfht_node *node)
{
        return ((unsigned long) node) & REMOVED_FLAG;
}

static
struct cds_lfht_node *flag_removed(struct cds_lfht_node *node)
{
        return (struct cds_lfht_node *) (((unsigned long) node) | REMOVED_FLAG);
}

static
int is_dummy(struct cds_lfht_node *node)
{
        return ((unsigned long) node) & DUMMY_FLAG;
}

static
struct cds_lfht_node *flag_dummy(struct cds_lfht_node *node)
{
        return (struct cds_lfht_node *) (((unsigned long) node) | DUMMY_FLAG);
}
 
static
unsigned long _uatomic_max(unsigned long *ptr, unsigned long v)
{
        unsigned long old1, old2;

        old1 = uatomic_read(ptr);
        do {
                old2 = old1;
                if (old2 >= v)
                        return old2;
        } while ((old1 = uatomic_cmpxchg(ptr, old2, v)) != old2);
        return v;
}

static
void cds_lfht_free_table_cb(struct rcu_head *head)
{
        struct rcu_table *t =
                caa_container_of(head, struct rcu_table, head);
        poison_free(t);
}

static
void cds_lfht_free_level(struct rcu_head *head)
{
        struct rcu_level *l =
                caa_container_of(head, struct rcu_level, head);
        poison_free(l);
}

/*
 * Remove all logically deleted nodes from a bucket up to a certain node key.
 */
static
void _cds_lfht_gc_bucket(struct cds_lfht_node *dummy, struct cds_lfht_node *node)
{
        struct cds_lfht_node *iter_prev, *iter, *next, *new_next;

        assert(!is_dummy(dummy));
        assert(!is_removed(dummy));
        assert(!is_dummy(node));
        assert(!is_removed(node));
        for (;;) {
                iter_prev = dummy;
                /* We can always skip the dummy node initially */
                iter = rcu_dereference(iter_prev->p.next);
                assert(iter_prev->p.reverse_hash <= node->p.reverse_hash);
                /*
                 * We should never be called with dummy (start of chain)
                 * and logically removed node (end of path compression
                 * marker) being the actual same node. This would be a
                 * bug in the algorithm implementation.
                 */
                assert(dummy != node);
                for (;;) {
                        if (unlikely(!clear_flag(iter)))
                                return;
                        if (likely(clear_flag(iter)->p.reverse_hash > node->p.reverse_hash))
                                return;
                        next = rcu_dereference(clear_flag(iter)->p.next);
                        if (likely(is_removed(next)))
                                break;
                        iter_prev = clear_flag(iter);
                        iter = next;
                }
                assert(!is_removed(iter));
                if (is_dummy(iter))
                        new_next = flag_dummy(clear_flag(next));
                else
                        new_next = clear_flag(next);
                (void) uatomic_cmpxchg(&iter_prev->p.next, iter, new_next);
        }
}

static
struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht, struct rcu_table *t,
                                struct cds_lfht_node *node, int unique, int dummy)
{
        struct cds_lfht_node *iter_prev, *iter, *next, *new_node, *new_next,
                        *dummy_node;
        struct _cds_lfht_node *lookup;
        unsigned long hash, index, order;

        assert(!is_dummy(node));
        assert(!is_removed(node));
        if (!t->size) {
                assert(dummy);
                node->p.next = flag_dummy(NULL);
                return node;    /* Initial first add (head) */
        }
        hash = bit_reverse_ulong(node->p.reverse_hash);
        for (;;) {
                uint32_t chain_len = 0;

                /*
                 * iter_prev points to the non-removed node prior to the
                 * insert location.
                 */
                index = hash & (t->size - 1);
                order = get_count_order_ulong(index + 1);
                lookup = &t->tbl[order]->nodes[index & ((!order ? 0 : (1UL << (order - 1))) - 1)];
                iter_prev = (struct cds_lfht_node *) lookup;
                /* We can always skip the dummy node initially */
                iter = rcu_dereference(iter_prev->p.next);
                assert(iter_prev->p.reverse_hash <= node->p.reverse_hash);
                for (;;) {
                        /* TODO: check if removed */
                        if (unlikely(!clear_flag(iter)))
                                goto insert;
                        /* TODO: check if removed */
                        if (likely(clear_flag(iter)->p.reverse_hash > node->p.reverse_hash))
                                goto insert;
                        next = rcu_dereference(clear_flag(iter)->p.next);
                        if (unlikely(is_removed(next)))
                                goto gc_node;
                        if (unique
                            && !is_dummy(next)
                            && !ht->compare_fct(node->key, node->key_len,
                                                clear_flag(iter)->key,
                                                clear_flag(iter)->key_len))
                                return clear_flag(iter);
                        /* Only account for identical reverse hash once */
                        if (iter_prev->p.reverse_hash != clear_flag(iter)->p.reverse_hash
                            && !is_dummy(next))
                                check_resize(ht, t, ++chain_len);
                        iter_prev = clear_flag(iter);
                        iter = next;
                }
        insert:
                assert(node != clear_flag(iter));
                assert(!is_removed(iter_prev));
                assert(!is_removed(iter));
                assert(iter_prev != node);
                if (!dummy)
                        node->p.next = clear_flag(iter);
                else
                        node->p.next = flag_dummy(clear_flag(iter));
                if (is_dummy(iter))
                        new_node = flag_dummy(node);
                else
                        new_node = node;
                if (uatomic_cmpxchg(&iter_prev->p.next, iter,
                                    new_node) != iter)
                        continue;       /* retry */
                else
                        goto gc_end;
        gc_node:
                assert(!is_removed(iter));
                if (is_dummy(iter))
                        new_next = flag_dummy(clear_flag(next));
                else
                        new_next = clear_flag(next);
                (void) uatomic_cmpxchg(&iter_prev->p.next, iter, new_next);
                /* retry */
        }
gc_end:
        /* Garbage collect logically removed nodes in the bucket */
        index = hash & (t->size - 1);
        order = get_count_order_ulong(index + 1);
        lookup = &t->tbl[order]->nodes[index & (!order ? 0 : ((1UL << (order - 1)) - 1))];
        dummy_node = (struct cds_lfht_node *) lookup;
        _cds_lfht_gc_bucket(dummy_node, node);
        return node;
}

static
int _cds_lfht_remove(struct cds_lfht *ht, struct rcu_table *t,
                struct cds_lfht_node *node, int dummy_removal)
{
        struct cds_lfht_node *dummy, *next, *old;
        struct _cds_lfht_node *lookup;
        int flagged = 0;
        unsigned long hash, index, order;

        /* logically delete the node */
        assert(!is_dummy(node));
        assert(!is_removed(node));
        old = rcu_dereference(node->p.next);
        do {
                next = old;
                if (unlikely(is_removed(next)))
                        goto end;
                if (dummy_removal)
                        assert(is_dummy(next));
                else
                        assert(!is_dummy(next));
                old = uatomic_cmpxchg(&node->p.next, next,
                                      flag_removed(next));
        } while (old != next);

        /* We performed the (logical) deletion. */
        flagged = 1;

        /*
         * Ensure that the node is not visible to readers anymore: lookup for
         * the node, and remove it (along with any other logically removed node)
         * if found.
         */
        hash = bit_reverse_ulong(node->p.reverse_hash);
        assert(t->size > 0);
        index = hash & (t->size - 1);
        order = get_count_order_ulong(index + 1);
        lookup = &t->tbl[order]->nodes[index & (!order ? 0 : ((1UL << (order - 1)) - 1))];
        dummy = (struct cds_lfht_node *) lookup;
        _cds_lfht_gc_bucket(dummy, node);
end:
        /*
         * Only the flagging action indicated that we (and no other)
         * removed the node from the hash.
         */
        if (flagged) {
                assert(is_removed(rcu_dereference(node->p.next)));
                return 0;
        } else
                return -ENOENT;
}

static
void init_table(struct cds_lfht *ht, struct rcu_table *t,
                unsigned long first_order, unsigned long len_order)
{
        unsigned long i, end_order;

        dbg_printf("init table: first_order %lu end_order %lu\n",
                   first_order, first_order + len_order);
        end_order = first_order + len_order;
        t->size = !first_order ? 0 : (1UL << (first_order - 1));
        for (i = first_order; i < end_order; i++) {
                unsigned long j, len;

                len = !i ? 1 : 1UL << (i - 1);
                dbg_printf("init order %lu len: %lu\n", i, len);
                t->tbl[i] = calloc(1, sizeof(struct rcu_level)
                                + (len * sizeof(struct _cds_lfht_node)));
                for (j = 0; j < len; j++) {
                        struct cds_lfht_node *new_node =
                                (struct cds_lfht_node *) &t->tbl[i]->nodes[j];

                        dbg_printf("init entry: i %lu j %lu hash %lu\n",
                                   i, j, !i ? 0 : (1UL << (i - 1)) + j);
                        new_node->p.reverse_hash =
                                bit_reverse_ulong(!i ? 0 : (1UL << (i - 1)) + j);
                        (void) _cds_lfht_add(ht, t, new_node, 0, 1);
                        if (CMM_LOAD_SHARED(ht->in_progress_destroy))
                                break;
                }
                /* Update table size */
                t->size = !i ? 1 : (1UL << i);
                dbg_printf("init new size: %lu\n", t->size);
                if (CMM_LOAD_SHARED(ht->in_progress_destroy))
                        break;
        }
        t->resize_target = t->size;
        t->resize_initiated = 0;
}

static
void fini_table(struct cds_lfht *ht, struct rcu_table *t,
                unsigned long first_order, unsigned long len_order)
{
        long i, end_order;

        dbg_printf("fini table: first_order %lu end_order %lu\n",
                   first_order, first_order + len_order);
        end_order = first_order + len_order;
        assert(first_order > 0);
        assert(t->size == (1UL << (end_order - 1)));
        for (i = end_order - 1; i >= first_order; i--) {
                unsigned long j, len;

                len = !i ? 1 : 1UL << (i - 1);
                dbg_printf("fini order %lu len: %lu\n", i, len);
                /*
                 * Update table size. Need to shrink this table prior to
                 * removal so gc lookups use non-logically-removed dummy
                 * nodes.
                 */
                t->size = 1UL << (i - 1);
                /* Unlink */
                for (j = 0; j < len; j++) {
                        struct cds_lfht_node *fini_node =
                                (struct cds_lfht_node *) &t->tbl[i]->nodes[j];

                        dbg_printf("fini entry: i %lu j %lu hash %lu\n",
                                   i, j, !i ? 0 : (1UL << (i - 1)) + j);
                        fini_node->p.reverse_hash =
                                bit_reverse_ulong(!i ? 0 : (1UL << (i - 1)) + j);
                        (void) _cds_lfht_remove(ht, t, fini_node, 1);
                        if (CMM_LOAD_SHARED(ht->in_progress_destroy))
                                break;
                }
                ht->cds_lfht_call_rcu(&t->tbl[i]->head, cds_lfht_free_level);
                dbg_printf("fini new size: %lu\n", t->size);
                if (CMM_LOAD_SHARED(ht->in_progress_destroy))
                        break;
        }
        t->resize_target = t->size;
        t->resize_initiated = 0;
}

struct cds_lfht *cds_lfht_new(cds_lfht_hash_fct hash_fct,
                        cds_lfht_compare_fct compare_fct,
                        unsigned long hash_seed,
                        unsigned long init_size,
                        int flags,
                        void (*cds_lfht_call_rcu)(struct rcu_head *head,
                                        void (*func)(struct rcu_head *head)),
                        void (*cds_lfht_synchronize_rcu)(void))
{
        struct cds_lfht *ht;
        unsigned long order;

        /* init_size must be power of two */
        if (init_size && (init_size & (init_size - 1)))
                return NULL;
        ht = calloc(1, sizeof(struct cds_lfht));
        ht->hash_fct = hash_fct;
        ht->compare_fct = compare_fct;
        ht->hash_seed = hash_seed;
        ht->cds_lfht_call_rcu = cds_lfht_call_rcu;
        ht->cds_lfht_synchronize_rcu = cds_lfht_synchronize_rcu;
        ht->in_progress_resize = 0;
        ht->percpu_count = alloc_per_cpu_items_count();
        /* this mutex should not nest in read-side C.S. */
        pthread_mutex_init(&ht->resize_mutex, NULL);
        order = get_count_order_ulong(max(init_size, MIN_TABLE_SIZE)) + 1;
        ht->t = calloc(1, sizeof(struct cds_lfht)
                       + (order * sizeof(struct rcu_level *)));
        ht->t->size = 0;
        ht->flags = flags;
        pthread_mutex_lock(&ht->resize_mutex);
        init_table(ht, ht->t, 0, order);
        pthread_mutex_unlock(&ht->resize_mutex);
        return ht;
}

struct cds_lfht_node *cds_lfht_lookup(struct cds_lfht *ht, void *key, size_t key_len)
{
        struct rcu_table *t;
        struct cds_lfht_node *node, *next;
        struct _cds_lfht_node *lookup;
        unsigned long hash, reverse_hash, index, order;

        hash = ht->hash_fct(key, key_len, ht->hash_seed);
        reverse_hash = bit_reverse_ulong(hash);

        t = rcu_dereference(ht->t);
        index = hash & (t->size - 1);
        order = get_count_order_ulong(index + 1);
        lookup = &t->tbl[order]->nodes[index & (!order ? 0 : ((1UL << (order - 1))) - 1)];
        dbg_printf("lookup hash %lu index %lu order %lu aridx %lu\n",
                   hash, index, order, index & (!order ? 0 : ((1UL << (order - 1)) - 1)));
        node = (struct cds_lfht_node *) lookup;
        for (;;) {
                if (unlikely(!node))
                        break;
                if (unlikely(node->p.reverse_hash > reverse_hash)) {
                        node = NULL;
                        break;
                }
                next = rcu_dereference(node->p.next);
                if (likely(!is_removed(next))
                    && !is_dummy(next)
                    && likely(!ht->compare_fct(node->key, node->key_len, key, key_len))) {
                                break;
                }
                node = clear_flag(next);
        }
        assert(!node || !is_dummy(rcu_dereference(node->p.next)));
        return node;
}

struct cds_lfht_node *cds_lfht_next(struct cds_lfht *ht,
                                struct cds_lfht_node *node)
{
        struct cds_lfht_node *next;
        unsigned long reverse_hash;
        void *key;
        size_t key_len;

        reverse_hash = node->p.reverse_hash;
        key = node->key;
        key_len = node->key_len;
        next = rcu_dereference(node->p.next);
        node = clear_flag(next);

        for (;;) {
                if (unlikely(!node))
                        break;
                if (unlikely(node->p.reverse_hash > reverse_hash)) {
                        node = NULL;
                        break;
                }
                next = rcu_dereference(node->p.next);
                if (likely(!is_removed(next))
                    && !is_dummy(next)
                    && likely(!ht->compare_fct(node->key, node->key_len, key, key_len))) {
                                break;
                }
                node = clear_flag(next);
        }
        assert(!node || !is_dummy(rcu_dereference(node->p.next)));
        return node;
}

void cds_lfht_add(struct cds_lfht *ht, struct cds_lfht_node *node)
{
        struct rcu_table *t;
        unsigned long hash;

        hash = ht->hash_fct(node->key, node->key_len, ht->hash_seed);
        node->p.reverse_hash = bit_reverse_ulong((unsigned long) hash);

        t = rcu_dereference(ht->t);
        (void) _cds_lfht_add(ht, t, node, 0, 0);
        ht_count_add(ht, t);
}

struct cds_lfht_node *cds_lfht_add_unique(struct cds_lfht *ht,
                                        struct cds_lfht_node *node)
{
        struct rcu_table *t;
        unsigned long hash;
        struct cds_lfht_node *ret;

        hash = ht->hash_fct(node->key, node->key_len, ht->hash_seed);
        node->p.reverse_hash = bit_reverse_ulong((unsigned long) hash);

        t = rcu_dereference(ht->t);
        ret = _cds_lfht_add(ht, t, node, 1, 0);
        if (ret != node)
                ht_count_add(ht, t);
        return ret;
}

int cds_lfht_remove(struct cds_lfht *ht, struct cds_lfht_node *node)
{
        struct rcu_table *t;
        int ret;

        t = rcu_dereference(ht->t);
        ret = _cds_lfht_remove(ht, t, node, 0);
        if (!ret)
                ht_count_remove(ht, t);
        return ret;
}

static
int cds_lfht_delete_dummy(struct cds_lfht *ht)
{
        struct rcu_table *t;
        struct cds_lfht_node *node;
        struct _cds_lfht_node *lookup;
        unsigned long order, i;

        t = ht->t;
        /* Check that the table is empty */
        lookup = &t->tbl[0]->nodes[0];
        node = (struct cds_lfht_node *) lookup;
        do {
                node = clear_flag(node)->p.next;
                if (!is_dummy(node))
                        return -EPERM;
                assert(!is_removed(node));
        } while (clear_flag(node));
        /* Internal sanity check: all nodes left should be dummy */
        for (order = 0; order < get_count_order_ulong(t->size) + 1; order++) {
                unsigned long len;

                len = !order ? 1 : 1UL << (order - 1);
                for (i = 0; i < len; i++) {
                        dbg_printf("delete order %lu i %lu hash %lu\n",
                                order, i,
                                bit_reverse_ulong(t->tbl[order]->nodes[i].reverse_hash));
                        assert(is_dummy(t->tbl[order]->nodes[i].next));
                }
                poison_free(t->tbl[order]);
        }
        return 0;
}

/*
 * Should only be called when no more concurrent readers nor writers can
 * possibly access the table.
 */
int cds_lfht_destroy(struct cds_lfht *ht)
{
        int ret;

        /* Wait for in-flight resize operations to complete */
        CMM_STORE_SHARED(ht->in_progress_destroy, 1);
        while (uatomic_read(&ht->in_progress_resize))
                poll(NULL, 0, 100);     /* wait for 100ms */
        ret = cds_lfht_delete_dummy(ht);
        if (ret)
                return ret;
        poison_free(ht->t);
        free_per_cpu_items_count(ht->percpu_count);
        poison_free(ht);
        return ret;
}

void cds_lfht_count_nodes(struct cds_lfht *ht,
                unsigned long *count,
                unsigned long *removed)
{
        struct rcu_table *t;
        struct cds_lfht_node *node, *next;
        struct _cds_lfht_node *lookup;
        unsigned long nr_dummy = 0;

        *count = 0;
        *removed = 0;

        t = rcu_dereference(ht->t);
        /* Count non-dummy nodes in the table */
        lookup = &t->tbl[0]->nodes[0];
        node = (struct cds_lfht_node *) lookup;
        do {
                next = rcu_dereference(node->p.next);
                if (is_removed(next)) {
                        assert(!is_dummy(next));
                        (*removed)++;
                } else if (!is_dummy(next))
                        (*count)++;
                else
                        (nr_dummy)++;
                node = clear_flag(next);
        } while (node);
        dbg_printf("number of dummy nodes: %lu\n", nr_dummy);
}

/* called with resize mutex held */
static
void _do_cds_lfht_grow(struct cds_lfht *ht, struct rcu_table *old_t,
                unsigned long old_size, unsigned long new_size)
{
        unsigned long old_order, new_order;
        struct rcu_table *new_t;

        old_order = get_count_order_ulong(old_size) + 1;
        new_order = get_count_order_ulong(new_size) + 1;
        printf("resize from %lu (order %lu) to %lu (order %lu) buckets\n",
               old_size, old_order, new_size, new_order);
        new_t = malloc(sizeof(struct cds_lfht)
                        + (new_order * sizeof(struct rcu_level *)));
        assert(new_size > old_size);
        memcpy(&new_t->tbl, &old_t->tbl,
               old_order * sizeof(struct rcu_level *));
        init_table(ht, new_t, old_order, new_order - old_order);
        /* Changing table and size atomically wrt lookups */
        rcu_assign_pointer(ht->t, new_t);
        ht->cds_lfht_call_rcu(&old_t->head, cds_lfht_free_table_cb);
}

/* called with resize mutex held */
static
void _do_cds_lfht_shrink(struct cds_lfht *ht, struct rcu_table *old_t,
                unsigned long old_size, unsigned long new_size)
{
        unsigned long old_order, new_order;
        struct rcu_table *new_t;

        new_size = max(new_size, MIN_TABLE_SIZE);
        old_order = get_count_order_ulong(old_size) + 1;
        new_order = get_count_order_ulong(new_size) + 1;
        printf("resize from %lu (order %lu) to %lu (order %lu) buckets\n",
               old_size, old_order, new_size, new_order);
        new_t = malloc(sizeof(struct cds_lfht)
                        + (new_order * sizeof(struct rcu_level *)));
        assert(new_size < old_size);
        memcpy(&new_t->tbl, &old_t->tbl,
               new_order * sizeof(struct rcu_level *));
        new_t->size = !new_order ? 1 : (1UL << (new_order - 1));
        assert(new_t->size == new_size);
        new_t->resize_target = new_t->size;
        new_t->resize_initiated = 0;

        /* Changing table and size atomically wrt lookups */
        rcu_assign_pointer(ht->t, new_t);

        /*
         * We need to wait for all add operations to reach Q.S. (and
         * thus use the new table for lookups) before we can start
         * releasing the old dummy nodes. Otherwise their lookup will
         * return a logically removed node as insert position.
         */
        ht->cds_lfht_synchronize_rcu();

        /* Unlink and remove all now-unused dummy node pointers. */
        fini_table(ht, old_t, new_order, old_order - new_order);
        ht->cds_lfht_call_rcu(&old_t->head, cds_lfht_free_table_cb);
}


/* called with resize mutex held */
static
void _do_cds_lfht_resize(struct cds_lfht *ht)
{
        unsigned long new_size, old_size;
        struct rcu_table *old_t;

        old_t = ht->t;
        old_size = old_t->size;
        new_size = CMM_LOAD_SHARED(old_t->resize_target);
        if (old_size < new_size)
                _do_cds_lfht_grow(ht, old_t, old_size, new_size);
        else if (old_size > new_size)
                _do_cds_lfht_shrink(ht, old_t, old_size, new_size);
        else
                CMM_STORE_SHARED(old_t->resize_initiated, 0);
}

static
unsigned long resize_target_update(struct rcu_table *t,
                                   int growth_order)
{
        return _uatomic_max(&t->resize_target,
                            t->size << growth_order);
}

static
void resize_target_update_count(struct rcu_table *t,
                                unsigned long count)
{
        count = max(count, MIN_TABLE_SIZE);
        uatomic_set(&t->resize_target, count);
}

void cds_lfht_resize(struct cds_lfht *ht, unsigned long new_size)
{
        struct rcu_table *t = rcu_dereference(ht->t);

        resize_target_update_count(t, new_size);
        CMM_STORE_SHARED(t->resize_initiated, 1);
        pthread_mutex_lock(&ht->resize_mutex);
        _do_cds_lfht_resize(ht);
        pthread_mutex_unlock(&ht->resize_mutex);
}

static
void do_resize_cb(struct rcu_head *head)
{
        struct rcu_resize_work *work =
                caa_container_of(head, struct rcu_resize_work, head);
        struct cds_lfht *ht = work->ht;

        pthread_mutex_lock(&ht->resize_mutex);
        _do_cds_lfht_resize(ht);
        pthread_mutex_unlock(&ht->resize_mutex);
        poison_free(work);
        cmm_smp_mb();   /* finish resize before decrement */
        uatomic_dec(&ht->in_progress_resize);
}

static
void cds_lfht_resize_lazy(struct cds_lfht *ht, struct rcu_table *t, int growth)
{
        struct rcu_resize_work *work;
        unsigned long target_size;

        target_size = resize_target_update(t, growth);
        if (!CMM_LOAD_SHARED(t->resize_initiated) && t->size < target_size) {
                uatomic_inc(&ht->in_progress_resize);
                cmm_smp_mb();   /* increment resize count before calling it */
                work = malloc(sizeof(*work));
                work->ht = ht;
                ht->cds_lfht_call_rcu(&work->head, do_resize_cb);
                CMM_STORE_SHARED(t->resize_initiated, 1);
        }
}

#if defined(HAVE_SCHED_GETCPU) && defined(HAVE_SYSCONF)

static
void cds_lfht_resize_lazy_count(struct cds_lfht *ht, struct rcu_table *t,
                                unsigned long count)
{
        struct rcu_resize_work *work;

        if (!(ht->flags & CDS_LFHT_AUTO_RESIZE))
                return;
        resize_target_update_count(t, count);
        if (!CMM_LOAD_SHARED(t->resize_initiated)) {
                uatomic_inc(&ht->in_progress_resize);
                cmm_smp_mb();   /* increment resize count before calling it */
                work = malloc(sizeof(*work));
                work->ht = ht;
                ht->cds_lfht_call_rcu(&work->head, do_resize_cb);
                CMM_STORE_SHARED(t->resize_initiated, 1);
        }
}

#endif