rculfhash: Ensure future-proof memory barrier semantic consistency

[urcu.git] / rculfhash.c

/*
 * rculfhash.c
 *
 * Userspace RCU library - Lock-Free Resizable RCU Hash Table
 *
 * Copyright 2010-2011 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 * Copyright 2011 - Lai Jiangshan <laijs@cn.fujitsu.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, as well as traversals, and use the returned objects
 *   safely by allowing memory reclaim to take place only after 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 not to
 *   populate a duplicate key if the node key already exists in the hash
 *   table.
 * - The resize operation executes concurrently with
 *   add/add_unique/add_replace/remove/lookup/traversal.
 * - Hash table nodes are contained within a split-ordered list. This
 *   list is ordered by incrementing reversed-bits-hash value.
 * - An index of bucket nodes is kept. These bucket nodes are the hash
 *   table "buckets". These buckets are internal nodes that allow to
 *   perform a fast hash lookup, similarly to a skip list. These
 *   buckets are chained together in the split-ordered list, which
 *   allows recursive expansion by inserting new buckets between the
 *   existing buckets. The split-ordered list allows adding new buckets
 *   between existing buckets as the table needs to grow.
 * - 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.
 * - 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. After setting the "removal" flag,
 *   only the thread which removal is the first to set the "removal
 *   owner" flag (with an xchg) 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 bucket 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 it does not
 *   contain the "removed" node anymore, even if concurrent delete/add
 *   operations are changing the structure of the list concurrently.
 * - The add operations perform garbage collection of buckets if they
 *   encounter nodes with removed flag set in the bucket where they want
 *   to add their 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.
 * - There are three memory backends for the hash table buckets: the
 *   "order table", the "chunks", and the "mmap".
 * - These bucket containers contain a compact version of the hash table
 *   nodes.
 * - The RCU "order table":
 *   -  has a first level table indexed by log2(hash index) which is
 *      copied and expanded by the resize operation. This order table
 *      allows finding the "bucket node" tables.
 *   - There is one bucket node table per hash index order. The size of
 *     each bucket node table is half the number of hashes contained in
 *     this order (except for order 0).
 * - The RCU "chunks" is best suited for close interaction with a page
 *   allocator. It uses a linear array as index to "chunks" containing
 *   each the same number of buckets.
 * - The RCU "mmap" memory backend uses a single memory map to hold
 *   all buckets.
 * - synchronize_rcu is used to garbage-collect the old bucket node table.
 *
 * Ordering Guarantees:
 *
 * To discuss these guarantees, we first define "read" operation as any
 * of the the basic cds_lfht_lookup, cds_lfht_next_duplicate,
 * cds_lfht_first, cds_lfht_next operation, as well as
 * cds_lfht_add_unique (failure). 
 *
 * We define "read traversal" operation as any of the following
 * group of operations
 *  - cds_lfht_lookup followed by iteration with cds_lfht_next_duplicate
 *    (and/or cds_lfht_next, although less common).
 *  - cds_lfht_add_unique (failure) followed by iteration with
 *    cds_lfht_next_duplicate (and/or cds_lfht_next, although less
 *    common).
 *  - cds_lfht_first followed iteration with cds_lfht_next (and/or
 *    cds_lfht_next_duplicate, although less common).
 *
 * We define "write" operations as any of cds_lfht_add,
 * cds_lfht_add_unique (success), cds_lfht_add_replace, cds_lfht_del.
 *
 * When cds_lfht_add_unique succeeds (returns the node passed as
 * parameter), it acts as a "write" operation. When cds_lfht_add_unique
 * fails (returns a node different from the one passed as parameter), it
 * acts as a "read" operation. A cds_lfht_add_unique failure is a
 * cds_lfht_lookup "read" operation, therefore, any ordering guarantee
 * referring to "lookup" imply any of "lookup" or cds_lfht_add_unique
 * (failure).
 *
 * We define "prior" and "later" node as nodes observable by reads and
 * read traversals respectively before and after a write or sequence of
 * write operations.
 *
 * Hash-table operations are often cascaded, for example, the pointer
 * returned by a cds_lfht_lookup() might be passed to a cds_lfht_next(),
 * whose return value might in turn be passed to another hash-table
 * operation. This entire cascaded series of operations must be enclosed
 * by a pair of matching rcu_read_lock() and rcu_read_unlock()
 * operations.
 *
 * The following ordering guarantees are offered by this hash table:
 *
 * A.1) "read" after "write": if there is ordering between a write and a
 *      later read, then the read is guaranteed to see the write or some
 *      later write.
 * A.2) "read traversal" after "write": given that there is dependency
 *      ordering between reads in a "read traversal", if there is
 *      ordering between a write and the first read of the traversal,
 *      then the "read traversal" is guaranteed to see the write or
 *      some later write.
 * B.1) "write" after "read": if there is ordering between a read and a
 *      later write, then the read will never see the write.
 * B.2) "write" after "read traversal": given that there is dependency
 *      ordering between reads in a "read traversal", if there is
 *      ordering between the last read of the traversal and a later
 *      write, then the "read traversal" will never see the write.
 * C)   "write" while "read traversal": if a write occurs during a "read
 *      traversal", the traversal may, or may not, see the write.
 * D.1) "write" after "write": if there is ordering between a write and
 *      a later write, then the later write is guaranteed to see the
 *      effects of the first write.
 * D.2) Concurrent "write" pairs: The system will assign an arbitrary
 *      order to any pair of concurrent conflicting writes.
 *      Non-conflicting writes (for example, to different keys) are
 *      unordered.
 * E)   If a grace period separates a "del" or "replace" operation
 *      and a subsequent operation, then that subsequent operation is
 *      guaranteed not to see the removed item.
 * F)   Uniqueness guarantee: given a hash table that does not contain
 *      duplicate items for a given key, there will only be one item in
 *      the hash table after an arbitrary sequence of add_unique and/or
 *      add_replace operations. Note, however, that a pair of
 *      concurrent read operations might well access two different items
 *      with that key.
 * G.1) If a pair of lookups for a given key are ordered (e.g. by a
 *      memory barrier), then the second lookup will return the same
 *      node as the previous lookup, or some later node.
 * G.2) A "read traversal" that starts after the end of a prior "read
 *      traversal" (ordered by memory barriers) is guaranteed to see the
 *      same nodes as the previous traversal, or some later nodes.
 * G.3) Concurrent "read" pairs: concurrent reads are unordered. For
 *      example, if a pair of reads to the same key run concurrently
 *      with an insertion of that same key, the reads remain unordered
 *      regardless of their return values. In other words, you cannot
 *      rely on the values returned by the reads to deduce ordering.
 *
 * Progress guarantees:
 *
 * * Reads are wait-free. These operations always move forward in the
 *   hash table linked list, and this list has no loop.
 * * Writes are lock-free. Any retry loop performed by a write operation
 *   is triggered by progress made within another update operation.
 *
 * Bucket node tables:
 *
 * hash table   hash table      the last        all bucket node tables
 * order        size            bucket node     0   1   2   3   4   5   6(index)
 *                              table size
 * 0            1               1               1
 * 1            2               1               1   1
 * 2            4               2               1   1   2
 * 3            8               4               1   1   2   4
 * 4            16              8               1   1   2   4   8
 * 5            32              16              1   1   2   4   8  16
 * 6            64              32              1   1   2   4   8  16  32
 *
 * When growing/shrinking, we only focus on the last bucket node table
 * which size is (!order ? 1 : (1 << (order -1))).
 *
 * Example for growing/shrinking:
 * grow hash table from order 5 to 6: init the index=6 bucket node table
 * shrink hash table from order 6 to 5: fini the index=6 bucket node table
 *
 * A bit of ascii art explanation:
 * 
 * The order index is the off-by-one compared 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
#define _GNU_SOURCE
#include <stdlib.h>
#include <errno.h>
#include <assert.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <sched.h>

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

/*
 * 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 DEFAULT_SPLIT_COUNT_MASK        0xFUL
#define CHAIN_LEN_TARGET                1
#define CHAIN_LEN_RESIZE_THRESHOLD      3

/*
 * Define the minimum table size.
 */
#define MIN_TABLE_ORDER                 0
#define MIN_TABLE_SIZE                  (1UL << MIN_TABLE_ORDER)

/*
 * Minimum number of bucket nodes to touch per thread to parallelize grow/shrink.
 */
#define MIN_PARTITION_PER_THREAD_ORDER  12
#define MIN_PARTITION_PER_THREAD        (1UL << MIN_PARTITION_PER_THREAD_ORDER)

/*
 * The removed flag needs to be updated atomically with the pointer.
 * It indicates that no node must attach to the node scheduled for
 * removal, and that node garbage collection must be performed.
 * The bucket flag does not require to be updated atomically with the
 * pointer, but it is added as a pointer low bit flag to save space.
 * The "removal owner" flag is used to detect which of the "del"
 * operation that has set the "removed flag" gets to return the removed
 * node to its caller. Note that the replace operation does not need to
 * iteract with the "removal owner" flag, because it validates that
 * the "removed" flag is not set before performing its cmpxchg.
 */
#define REMOVED_FLAG            (1UL << 0)
#define BUCKET_FLAG             (1UL << 1)
#define REMOVAL_OWNER_FLAG      (1UL << 2)
#define FLAGS_MASK              ((1UL << 3) - 1)

/* Value of the end pointer. Should not interact with flags. */
#define END_VALUE               NULL

/*
 * ht_items_count: Split-counters counting the number of node addition
 * and removal in the table. Only used if the CDS_LFHT_ACCOUNTING flag
 * is set at hash table creation.
 *
 * These are free-running counters, never reset to zero. They count the
 * number of add/remove, and trigger every (1 << COUNT_COMMIT_ORDER)
 * operations to update the global counter. We choose a power-of-2 value
 * for the trigger to deal with 32 or 64-bit overflow of the counter.
 */
struct ht_items_count {
        unsigned long add, del;
} __attribute__((aligned(CAA_CACHE_LINE_SIZE)));

/*
 * rcu_resize_work: Contains arguments passed to RCU worker thread
 * responsible for performing lazy resize.
 */
struct rcu_resize_work {
        struct rcu_head head;
        struct cds_lfht *ht;
};

/*
 * partition_resize_work: Contains arguments passed to worker threads
 * executing the hash table resize on partitions of the hash table
 * assigned to each processor's worker thread.
 */
struct partition_resize_work {
        pthread_t thread_id;
        struct cds_lfht *ht;
        unsigned long i, start, len;
        void (*fct)(struct cds_lfht *ht, unsigned long i,
                    unsigned long start, unsigned long len);
};

/*
 * 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 cds_lfht_fls_ulong(unsigned long x)
{
#if (CAA_BITS_PER_LONG == 32)
        return fls_u32(x);
#else
        return fls_u64(x);
#endif
}

/*
 * Return the minimum order for which x <= (1UL << order).
 * Return -1 if x is 0.
 */
int cds_lfht_get_count_order_u32(uint32_t x)
{
        if (!x)
                return -1;

        return fls_u32(x - 1);
}

/*
 * Return the minimum order for which x <= (1UL << order).
 * Return -1 if x is 0.
 */
int cds_lfht_get_count_order_ulong(unsigned long x)
{
        if (!x)
                return -1;

        return cds_lfht_fls_ulong(x - 1);
}

static
void cds_lfht_resize_lazy_grow(struct cds_lfht *ht, unsigned long size, int growth);

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

static long nr_cpus_mask = -1;
static long split_count_mask = -1;

#if defined(HAVE_SYSCONF)
static void ht_init_nr_cpus_mask(void)
{
        long maxcpus;

        maxcpus = sysconf(_SC_NPROCESSORS_CONF);
        if (maxcpus <= 0) {
                nr_cpus_mask = -2;
                return;
        }
        /*
         * round up number of CPUs to next power of two, so we
         * can use & for modulo.
         */
        maxcpus = 1UL << cds_lfht_get_count_order_ulong(maxcpus);
        nr_cpus_mask = maxcpus - 1;
}
#else /* #if defined(HAVE_SYSCONF) */
static void ht_init_nr_cpus_mask(void)
{
        nr_cpus_mask = -2;
}
#endif /* #else #if defined(HAVE_SYSCONF) */

static
void alloc_split_items_count(struct cds_lfht *ht)
{
        struct ht_items_count *count;

        if (nr_cpus_mask == -1) {
                ht_init_nr_cpus_mask();
                if (nr_cpus_mask < 0)
                        split_count_mask = DEFAULT_SPLIT_COUNT_MASK;
                else
                        split_count_mask = nr_cpus_mask;
        }

        assert(split_count_mask >= 0);

        if (ht->flags & CDS_LFHT_ACCOUNTING) {
                ht->split_count = calloc(split_count_mask + 1, sizeof(*count));
                assert(ht->split_count);
        } else {
                ht->split_count = NULL;
        }
}

static
void free_split_items_count(struct cds_lfht *ht)
{
        poison_free(ht->split_count);
}

#if defined(HAVE_SCHED_GETCPU)
static
int ht_get_split_count_index(unsigned long hash)
{
        int cpu;

        assert(split_count_mask >= 0);
        cpu = sched_getcpu();
        if (caa_unlikely(cpu < 0))
                return hash & split_count_mask;
        else
                return cpu & split_count_mask;
}
#else /* #if defined(HAVE_SCHED_GETCPU) */
static
int ht_get_split_count_index(unsigned long hash)
{
        return hash & split_count_mask;
}
#endif /* #else #if defined(HAVE_SCHED_GETCPU) */

static
void ht_count_add(struct cds_lfht *ht, unsigned long size, unsigned long hash)
{
        unsigned long split_count;
        int index;
        long count;

        if (caa_unlikely(!ht->split_count))
                return;
        index = ht_get_split_count_index(hash);
        split_count = uatomic_add_return(&ht->split_count[index].add, 1);
        if (caa_likely(split_count & ((1UL << COUNT_COMMIT_ORDER) - 1)))
                return;
        /* Only if number of add multiple of 1UL << COUNT_COMMIT_ORDER */

        dbg_printf("add split count %lu\n", split_count);
        count = uatomic_add_return(&ht->count,
                                   1UL << COUNT_COMMIT_ORDER);
        if (caa_likely(count & (count - 1)))
                return;
        /* Only if global count is power of 2 */

        if ((count >> CHAIN_LEN_RESIZE_THRESHOLD) < size)
                return;
        dbg_printf("add set global %ld\n", count);
        cds_lfht_resize_lazy_count(ht, size,
                count >> (CHAIN_LEN_TARGET - 1));
}

static
void ht_count_del(struct cds_lfht *ht, unsigned long size, unsigned long hash)
{
        unsigned long split_count;
        int index;
        long count;

        if (caa_unlikely(!ht->split_count))
                return;
        index = ht_get_split_count_index(hash);
        split_count = uatomic_add_return(&ht->split_count[index].del, 1);
        if (caa_likely(split_count & ((1UL << COUNT_COMMIT_ORDER) - 1)))
                return;
        /* Only if number of deletes multiple of 1UL << COUNT_COMMIT_ORDER */

        dbg_printf("del split count %lu\n", split_count);
        count = uatomic_add_return(&ht->count,
                                   -(1UL << COUNT_COMMIT_ORDER));
        if (caa_likely(count & (count - 1)))
                return;
        /* Only if global count is power of 2 */

        if ((count >> CHAIN_LEN_RESIZE_THRESHOLD) >= size)
                return;
        dbg_printf("del set global %ld\n", count);
        /*
         * Don't shrink table if the number of nodes is below a
         * certain threshold.
         */
        if (count < (1UL << COUNT_COMMIT_ORDER) * (split_count_mask + 1))
                return;
        cds_lfht_resize_lazy_count(ht, size,
                count >> (CHAIN_LEN_TARGET - 1));
}

static
void check_resize(struct cds_lfht *ht, unsigned long size, 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_grow(ht, size,
                        cds_lfht_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
int is_bucket(struct cds_lfht_node *node)
{
        return ((unsigned long) node) & BUCKET_FLAG;
}

static
struct cds_lfht_node *flag_bucket(struct cds_lfht_node *node)
{
        return (struct cds_lfht_node *) (((unsigned long) node) | BUCKET_FLAG);
}

static
int is_removal_owner(struct cds_lfht_node *node)
{
        return ((unsigned long) node) & REMOVAL_OWNER_FLAG;
}

static
struct cds_lfht_node *flag_removal_owner(struct cds_lfht_node *node)
{
        return (struct cds_lfht_node *) (((unsigned long) node) | REMOVAL_OWNER_FLAG);
}

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

static
struct cds_lfht_node *get_end(void)
{
        return (struct cds_lfht_node *) END_VALUE;
}

static
int is_end(struct cds_lfht_node *node)
{
        return clear_flag(node) == (struct cds_lfht_node *) END_VALUE;
}

static
unsigned long _uatomic_xchg_monotonic_increase(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 old2;
}

static
void cds_lfht_alloc_bucket_table(struct cds_lfht *ht, unsigned long order)
{
        return ht->mm->alloc_bucket_table(ht, order);
}

/*
 * cds_lfht_free_bucket_table() should be called with decreasing order.
 * When cds_lfht_free_bucket_table(0) is called, it means the whole
 * lfht is destroyed.
 */
static
void cds_lfht_free_bucket_table(struct cds_lfht *ht, unsigned long order)
{
        return ht->mm->free_bucket_table(ht, order);
}

static inline
struct cds_lfht_node *bucket_at(struct cds_lfht *ht, unsigned long index)
{
        return ht->bucket_at(ht, index);
}

static inline
struct cds_lfht_node *lookup_bucket(struct cds_lfht *ht, unsigned long size,
                unsigned long hash)
{
        assert(size > 0);
        return bucket_at(ht, hash & (size - 1));
}

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

        assert(!is_bucket(bucket));
        assert(!is_removed(bucket));
        assert(!is_bucket(node));
        assert(!is_removed(node));
        for (;;) {
                iter_prev = bucket;
                /* We can always skip the bucket node initially */
                iter = rcu_dereference(iter_prev->next);
                assert(!is_removed(iter));
                assert(iter_prev->reverse_hash <= node->reverse_hash);
                /*
                 * We should never be called with bucket (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(bucket != node);
                for (;;) {
                        if (caa_unlikely(is_end(iter)))
                                return;
                        if (caa_likely(clear_flag(iter)->reverse_hash > node->reverse_hash))
                                return;
                        next = rcu_dereference(clear_flag(iter)->next);
                        if (caa_likely(is_removed(next)))
                                break;
                        iter_prev = clear_flag(iter);
                        iter = next;
                }
                assert(!is_removed(iter));
                if (is_bucket(iter))
                        new_next = flag_bucket(clear_flag(next));
                else
                        new_next = clear_flag(next);
                (void) uatomic_cmpxchg(&iter_prev->next, iter, new_next);
        }
}

static
int _cds_lfht_replace(struct cds_lfht *ht, unsigned long size,
                struct cds_lfht_node *old_node,
                struct cds_lfht_node *old_next,
                struct cds_lfht_node *new_node)
{
        struct cds_lfht_node *bucket, *ret_next;

        if (!old_node)  /* Return -ENOENT if asked to replace NULL node */
                return -ENOENT;

        assert(!is_removed(old_node));
        assert(!is_bucket(old_node));
        assert(!is_removed(new_node));
        assert(!is_bucket(new_node));
        assert(new_node != old_node);
        for (;;) {
                /* Insert after node to be replaced */
                if (is_removed(old_next)) {
                        /*
                         * Too late, the old node has been removed under us
                         * between lookup and replace. Fail.
                         */
                        return -ENOENT;
                }
                assert(old_next == clear_flag(old_next));
                assert(new_node != old_next);
                /*
                 * REMOVAL_OWNER flag is _NEVER_ set before the REMOVED
                 * flag. It is either set atomically at the same time
                 * (replace) or after (del).
                 */
                assert(!is_removal_owner(old_next));
                new_node->next = old_next;
                /*
                 * Here is the whole trick for lock-free replace: we add
                 * the replacement node _after_ the node we want to
                 * replace by atomically setting its next pointer at the
                 * same time we set its removal flag. Given that
                 * the lookups/get next use an iterator aware of the
                 * next pointer, they will either skip the old node due
                 * to the removal flag and see the new node, or use
                 * the old node, but will not see the new one.
                 * This is a replacement of a node with another node
                 * that has the same value: we are therefore not
                 * removing a value from the hash table. We set both the
                 * REMOVED and REMOVAL_OWNER flags atomically so we own
                 * the node after successful cmpxchg.
                 */
                ret_next = uatomic_cmpxchg(&old_node->next,
                        old_next, flag_removed_or_removal_owner(new_node));
                if (ret_next == old_next)
                        break;          /* We performed the replacement. */
                old_next = ret_next;
        }

        /*
         * Ensure that the old node is not visible to readers anymore:
         * lookup for the node, and remove it (along with any other
         * logically removed node) if found.
         */
        bucket = lookup_bucket(ht, size, bit_reverse_ulong(old_node->reverse_hash));
        _cds_lfht_gc_bucket(bucket, new_node);

        assert(is_removed(CMM_LOAD_SHARED(old_node->next)));
        return 0;
}

/*
 * A non-NULL unique_ret pointer uses the "add unique" (or uniquify) add
 * mode. A NULL unique_ret allows creation of duplicate keys.
 */
static
void _cds_lfht_add(struct cds_lfht *ht,
                unsigned long hash,
                cds_lfht_match_fct match,
                const void *key,
                unsigned long size,
                struct cds_lfht_node *node,
                struct cds_lfht_iter *unique_ret,
                int bucket_flag)
{
        struct cds_lfht_node *iter_prev, *iter, *next, *new_node, *new_next,
                        *return_node;
        struct cds_lfht_node *bucket;

        assert(!is_bucket(node));
        assert(!is_removed(node));
        bucket = lookup_bucket(ht, size, hash);
        for (;;) {
                uint32_t chain_len = 0;

                /*
                 * iter_prev points to the non-removed node prior to the
                 * insert location.
                 */
                iter_prev = bucket;
                /* We can always skip the bucket node initially */
                iter = rcu_dereference(iter_prev->next);
                assert(iter_prev->reverse_hash <= node->reverse_hash);
                for (;;) {
                        if (caa_unlikely(is_end(iter)))
                                goto insert;
                        if (caa_likely(clear_flag(iter)->reverse_hash > node->reverse_hash))
                                goto insert;

                        /* bucket node is the first node of the identical-hash-value chain */
                        if (bucket_flag && clear_flag(iter)->reverse_hash == node->reverse_hash)
                                goto insert;

                        next = rcu_dereference(clear_flag(iter)->next);
                        if (caa_unlikely(is_removed(next)))
                                goto gc_node;

                        /* uniquely add */
                        if (unique_ret
                            && !is_bucket(next)
                            && clear_flag(iter)->reverse_hash == node->reverse_hash) {
                                struct cds_lfht_iter d_iter = { .node = node, .next = iter, };

                                /*
                                 * uniquely adding inserts the node as the first
                                 * node of the identical-hash-value node chain.
                                 *
                                 * This semantic ensures no duplicated keys
                                 * should ever be observable in the table
                                 * (including traversing the table node by
                                 * node by forward iterations)
                                 */
                                cds_lfht_next_duplicate(ht, match, key, &d_iter);
                                if (!d_iter.node)
                                        goto insert;

                                *unique_ret = d_iter;
                                return;
                        }

                        /* Only account for identical reverse hash once */
                        if (iter_prev->reverse_hash != clear_flag(iter)->reverse_hash
                            && !is_bucket(next))
                                check_resize(ht, size, ++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 (!bucket_flag)
                        node->next = clear_flag(iter);
                else
                        node->next = flag_bucket(clear_flag(iter));
                if (is_bucket(iter))
                        new_node = flag_bucket(node);
                else
                        new_node = node;
                if (uatomic_cmpxchg(&iter_prev->next, iter,
                                    new_node) != iter) {
                        continue;       /* retry */
                } else {
                        return_node = node;
                        goto end;
                }

        gc_node:
                assert(!is_removed(iter));
                if (is_bucket(iter))
                        new_next = flag_bucket(clear_flag(next));
                else
                        new_next = clear_flag(next);
                (void) uatomic_cmpxchg(&iter_prev->next, iter, new_next);
                /* retry */
        }
end:
        if (unique_ret) {
                unique_ret->node = return_node;
                /* unique_ret->next left unset, never used. */
        }
}

static
int _cds_lfht_del(struct cds_lfht *ht, unsigned long size,
                struct cds_lfht_node *node)
{
        struct cds_lfht_node *bucket, *next;

        if (!node)      /* Return -ENOENT if asked to delete NULL node */
                return -ENOENT;

        /* logically delete the node */
        assert(!is_bucket(node));
        assert(!is_removed(node));
        assert(!is_removal_owner(node));

        /*
         * We are first checking if the node had previously been
         * logically removed (this check is not atomic with setting the
         * logical removal flag). Return -ENOENT if the node had
         * previously been removed.
         */
        next = CMM_LOAD_SHARED(node->next);     /* next is not dereferenced */
        if (caa_unlikely(is_removed(next)))
                return -ENOENT;
        assert(!is_bucket(next));
        /*
         * The del operation semantic guarantees a full memory barrier
         * before the uatomic_or atomic commit of the deletion flag.
         */
        cmm_smp_mb__before_uatomic_or();
        /*
         * We set the REMOVED_FLAG unconditionally. Note that there may
         * be more than one concurrent thread setting this flag.
         * Knowing which wins the race will be known after the garbage
         * collection phase, stay tuned!
         */
        uatomic_or(&node->next, REMOVED_FLAG);
        /* We performed the (logical) deletion. */

        /*
         * 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.
         */
        bucket = lookup_bucket(ht, size, bit_reverse_ulong(node->reverse_hash));
        _cds_lfht_gc_bucket(bucket, node);

        assert(is_removed(CMM_LOAD_SHARED(node->next)));
        /*
         * Last phase: atomically exchange node->next with a version
         * having "REMOVAL_OWNER_FLAG" set. If the returned node->next
         * pointer did _not_ have "REMOVAL_OWNER_FLAG" set, we now own
         * the node and win the removal race.
         * It is interesting to note that all "add" paths are forbidden
         * to change the next pointer starting from the point where the
         * REMOVED_FLAG is set, so here using a read, followed by a
         * xchg() suffice to guarantee that the xchg() will ever only
         * set the "REMOVAL_OWNER_FLAG" (or change nothing if the flag
         * was already set).
         */
        if (!is_removal_owner(uatomic_xchg(&node->next,
                        flag_removal_owner(node->next))))
                return 0;
        else
                return -ENOENT;
}

static
void *partition_resize_thread(void *arg)
{
        struct partition_resize_work *work = arg;

        work->ht->flavor->register_thread();
        work->fct(work->ht, work->i, work->start, work->len);
        work->ht->flavor->unregister_thread();
        return NULL;
}

static
void partition_resize_helper(struct cds_lfht *ht, unsigned long i,
                unsigned long len,
                void (*fct)(struct cds_lfht *ht, unsigned long i,
                        unsigned long start, unsigned long len))
{
        unsigned long partition_len;
        struct partition_resize_work *work;
        int thread, ret;
        unsigned long nr_threads;

        /*
         * Note: nr_cpus_mask + 1 is always power of 2.
         * We spawn just the number of threads we need to satisfy the minimum
         * partition size, up to the number of CPUs in the system.
         */
        if (nr_cpus_mask > 0) {
                nr_threads = min(nr_cpus_mask + 1,
                                 len >> MIN_PARTITION_PER_THREAD_ORDER);
        } else {
                nr_threads = 1;
        }
        partition_len = len >> cds_lfht_get_count_order_ulong(nr_threads);
        work = calloc(nr_threads, sizeof(*work));
        assert(work);
        for (thread = 0; thread < nr_threads; thread++) {
                work[thread].ht = ht;
                work[thread].i = i;
                work[thread].len = partition_len;
                work[thread].start = thread * partition_len;
                work[thread].fct = fct;
                ret = pthread_create(&(work[thread].thread_id), ht->resize_attr,
                        partition_resize_thread, &work[thread]);
                assert(!ret);
        }
        for (thread = 0; thread < nr_threads; thread++) {
                ret = pthread_join(work[thread].thread_id, NULL);
                assert(!ret);
        }
        free(work);
}

/*
 * Holding RCU read lock to protect _cds_lfht_add against memory
 * reclaim that could be performed by other call_rcu worker threads (ABA
 * problem).
 *
 * When we reach a certain length, we can split this population phase over
 * many worker threads, based on the number of CPUs available in the system.
 * This should therefore take care of not having the expand lagging behind too
 * many concurrent insertion threads by using the scheduler's ability to
 * schedule bucket node population fairly with insertions.
 */
static
void init_table_populate_partition(struct cds_lfht *ht, unsigned long i,
                                   unsigned long start, unsigned long len)
{
        unsigned long j, size = 1UL << (i - 1);

        assert(i > MIN_TABLE_ORDER);
        ht->flavor->read_lock();
        for (j = size + start; j < size + start + len; j++) {
                struct cds_lfht_node *new_node = bucket_at(ht, j);

                assert(j >= size && j < (size << 1));
                dbg_printf("init populate: order %lu index %lu hash %lu\n",
                           i, j, j);
                new_node->reverse_hash = bit_reverse_ulong(j);
                _cds_lfht_add(ht, j, NULL, NULL, size, new_node, NULL, 1);
        }
        ht->flavor->read_unlock();
}

static
void init_table_populate(struct cds_lfht *ht, unsigned long i,
                         unsigned long len)
{
        assert(nr_cpus_mask != -1);
        if (nr_cpus_mask < 0 || len < 2 * MIN_PARTITION_PER_THREAD) {
                ht->flavor->thread_online();
                init_table_populate_partition(ht, i, 0, len);
                ht->flavor->thread_offline();
                return;
        }
        partition_resize_helper(ht, i, len, init_table_populate_partition);
}

static
void init_table(struct cds_lfht *ht,
                unsigned long first_order, unsigned long last_order)
{
        unsigned long i;

        dbg_printf("init table: first_order %lu last_order %lu\n",
                   first_order, last_order);
        assert(first_order > MIN_TABLE_ORDER);
        for (i = first_order; i <= last_order; i++) {
                unsigned long len;

                len = 1UL << (i - 1);
                dbg_printf("init order %lu len: %lu\n", i, len);

                /* Stop expand if the resize target changes under us */
                if (CMM_LOAD_SHARED(ht->resize_target) < (1UL << i))
                        break;

                cds_lfht_alloc_bucket_table(ht, i);

                /*
                 * Set all bucket nodes reverse hash values for a level and
                 * link all bucket nodes into the table.
                 */
                init_table_populate(ht, i, len);

                /*
                 * Update table size.
                 */
                cmm_smp_wmb();  /* populate data before RCU size */
                CMM_STORE_SHARED(ht->size, 1UL << i);

                dbg_printf("init new size: %lu\n", 1UL << i);
                if (CMM_LOAD_SHARED(ht->in_progress_destroy))
                        break;
        }
}

/*
 * Holding RCU read lock to protect _cds_lfht_remove against memory
 * reclaim that could be performed by other call_rcu worker threads (ABA
 * problem).
 * For a single level, we logically remove and garbage collect each node.
 *
 * As a design choice, we perform logical removal and garbage collection on a
 * node-per-node basis to simplify this algorithm. We also assume keeping good
 * cache locality of the operation would overweight possible performance gain
 * that could be achieved by batching garbage collection for multiple levels.
 * However, this would have to be justified by benchmarks.
 *
 * Concurrent removal and add operations are helping us perform garbage
 * collection of logically removed nodes. We guarantee that all logically
 * removed nodes have been garbage-collected (unlinked) before call_rcu is
 * invoked to free a hole level of bucket nodes (after a grace period).
 *
 * Logical removal and garbage collection can therefore be done in batch
 * or on a node-per-node basis, as long as the guarantee above holds.
 *
 * When we reach a certain length, we can split this removal over many worker
 * threads, based on the number of CPUs available in the system. This should
 * take care of not letting resize process lag behind too many concurrent
 * updater threads actively inserting into the hash table.
 */
static
void remove_table_partition(struct cds_lfht *ht, unsigned long i,
                            unsigned long start, unsigned long len)
{
        unsigned long j, size = 1UL << (i - 1);

        assert(i > MIN_TABLE_ORDER);
        ht->flavor->read_lock();
        for (j = size + start; j < size + start + len; j++) {
                struct cds_lfht_node *fini_bucket = bucket_at(ht, j);
                struct cds_lfht_node *parent_bucket = bucket_at(ht, j - size);

                assert(j >= size && j < (size << 1));
                dbg_printf("remove entry: order %lu index %lu hash %lu\n",
                           i, j, j);
                /* Set the REMOVED_FLAG to freeze the ->next for gc */
                uatomic_or(&fini_bucket->next, REMOVED_FLAG);
                _cds_lfht_gc_bucket(parent_bucket, fini_bucket);
        }
        ht->flavor->read_unlock();
}

static
void remove_table(struct cds_lfht *ht, unsigned long i, unsigned long len)
{

        assert(nr_cpus_mask != -1);
        if (nr_cpus_mask < 0 || len < 2 * MIN_PARTITION_PER_THREAD) {
                ht->flavor->thread_online();
                remove_table_partition(ht, i, 0, len);
                ht->flavor->thread_offline();
                return;
        }
        partition_resize_helper(ht, i, len, remove_table_partition);
}

/*
 * fini_table() is never called for first_order == 0, which is why
 * free_by_rcu_order == 0 can be used as criterion to know if free must
 * be called.
 */
static
void fini_table(struct cds_lfht *ht,
                unsigned long first_order, unsigned long last_order)
{
        long i;
        unsigned long free_by_rcu_order = 0;

        dbg_printf("fini table: first_order %lu last_order %lu\n",
                   first_order, last_order);
        assert(first_order > MIN_TABLE_ORDER);
        for (i = last_order; i >= first_order; i--) {
                unsigned long len;

                len = 1UL << (i - 1);
                dbg_printf("fini order %lu len: %lu\n", i, len);

                /* Stop shrink if the resize target changes under us */
                if (CMM_LOAD_SHARED(ht->resize_target) > (1UL << (i - 1)))
                        break;

                cmm_smp_wmb();  /* populate data before RCU size */
                CMM_STORE_SHARED(ht->size, 1UL << (i - 1));

                /*
                 * 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 bucket nodes. Otherwise their lookup will
                 * return a logically removed node as insert position.
                 */
                ht->flavor->update_synchronize_rcu();
                if (free_by_rcu_order)
                        cds_lfht_free_bucket_table(ht, free_by_rcu_order);

                /*
                 * Set "removed" flag in bucket nodes about to be removed.
                 * Unlink all now-logically-removed bucket node pointers.
                 * Concurrent add/remove operation are helping us doing
                 * the gc.
                 */
                remove_table(ht, i, len);

                free_by_rcu_order = i;

                dbg_printf("fini new size: %lu\n", 1UL << i);
                if (CMM_LOAD_SHARED(ht->in_progress_destroy))
                        break;
        }

        if (free_by_rcu_order) {
                ht->flavor->update_synchronize_rcu();
                cds_lfht_free_bucket_table(ht, free_by_rcu_order);
        }
}

static
void cds_lfht_create_bucket(struct cds_lfht *ht, unsigned long size)
{
        struct cds_lfht_node *prev, *node;
        unsigned long order, len, i;

        cds_lfht_alloc_bucket_table(ht, 0);

        dbg_printf("create bucket: order 0 index 0 hash 0\n");
        node = bucket_at(ht, 0);
        node->next = flag_bucket(get_end());
        node->reverse_hash = 0;

        for (order = 1; order < cds_lfht_get_count_order_ulong(size) + 1; order++) {
                len = 1UL << (order - 1);
                cds_lfht_alloc_bucket_table(ht, order);

                for (i = 0; i < len; i++) {
                        /*
                         * Now, we are trying to init the node with the
                         * hash=(len+i) (which is also a bucket with the
                         * index=(len+i)) and insert it into the hash table,
                         * so this node has to be inserted after the bucket
                         * with the index=(len+i)&(len-1)=i. And because there
                         * is no other non-bucket node nor bucket node with
                         * larger index/hash inserted, so the bucket node
                         * being inserted should be inserted directly linked
                         * after the bucket node with index=i.
                         */
                        prev = bucket_at(ht, i);
                        node = bucket_at(ht, len + i);

                        dbg_printf("create bucket: order %lu index %lu hash %lu\n",
                                   order, len + i, len + i);
                        node->reverse_hash = bit_reverse_ulong(len + i);

                        /* insert after prev */
                        assert(is_bucket(prev->next));
                        node->next = prev->next;
                        prev->next = flag_bucket(node);
                }
        }
}

struct cds_lfht *_cds_lfht_new(unsigned long init_size,
                        unsigned long min_nr_alloc_buckets,
                        unsigned long max_nr_buckets,
                        int flags,
                        const struct cds_lfht_mm_type *mm,
                        const struct rcu_flavor_struct *flavor,
                        pthread_attr_t *attr)
{
        struct cds_lfht *ht;
        unsigned long order;

        /* min_nr_alloc_buckets must be power of two */
        if (!min_nr_alloc_buckets || (min_nr_alloc_buckets & (min_nr_alloc_buckets - 1)))
                return NULL;

        /* init_size must be power of two */
        if (!init_size || (init_size & (init_size - 1)))
                return NULL;

        /*
         * Memory management plugin default.
         */
        if (!mm) {
                if (CAA_BITS_PER_LONG > 32
                                && max_nr_buckets
                                && max_nr_buckets <= (1ULL << 32)) {
                        /*
                         * For 64-bit architectures, with max number of
                         * buckets small enough not to use the entire
                         * 64-bit memory mapping space (and allowing a
                         * fair number of hash table instances), use the
                         * mmap allocator, which is faster than the
                         * order allocator.
                         */
                        mm = &cds_lfht_mm_mmap;
                } else {
                        /*
                         * The fallback is to use the order allocator.
                         */
                        mm = &cds_lfht_mm_order;
                }
        }

        /* max_nr_buckets == 0 for order based mm means infinite */
        if (mm == &cds_lfht_mm_order && !max_nr_buckets)
                max_nr_buckets = 1UL << (MAX_TABLE_ORDER - 1);

        /* max_nr_buckets must be power of two */
        if (!max_nr_buckets || (max_nr_buckets & (max_nr_buckets - 1)))
                return NULL;

        min_nr_alloc_buckets = max(min_nr_alloc_buckets, MIN_TABLE_SIZE);
        init_size = max(init_size, MIN_TABLE_SIZE);
        max_nr_buckets = max(max_nr_buckets, min_nr_alloc_buckets);
        init_size = min(init_size, max_nr_buckets);

        ht = mm->alloc_cds_lfht(min_nr_alloc_buckets, max_nr_buckets);
        assert(ht);
        assert(ht->mm == mm);
        assert(ht->bucket_at == mm->bucket_at);

        ht->flags = flags;
        ht->flavor = flavor;
        ht->resize_attr = attr;
        alloc_split_items_count(ht);
        /* this mutex should not nest in read-side C.S. */
        pthread_mutex_init(&ht->resize_mutex, NULL);
        order = cds_lfht_get_count_order_ulong(init_size);
        ht->resize_target = 1UL << order;
        cds_lfht_create_bucket(ht, 1UL << order);
        ht->size = 1UL << order;
        return ht;
}

void cds_lfht_lookup(struct cds_lfht *ht, unsigned long hash,
                cds_lfht_match_fct match, const void *key,
                struct cds_lfht_iter *iter)
{
        struct cds_lfht_node *node, *next, *bucket;
        unsigned long reverse_hash, size;

        reverse_hash = bit_reverse_ulong(hash);

        size = rcu_dereference(ht->size);
        bucket = lookup_bucket(ht, size, hash);
        /* We can always skip the bucket node initially */
        node = rcu_dereference(bucket->next);
        node = clear_flag(node);
        for (;;) {
                if (caa_unlikely(is_end(node))) {
                        node = next = NULL;
                        break;
                }
                if (caa_unlikely(node->reverse_hash > reverse_hash)) {
                        node = next = NULL;
                        break;
                }
                next = rcu_dereference(node->next);
                assert(node == clear_flag(node));
                if (caa_likely(!is_removed(next))
                    && !is_bucket(next)
                    && node->reverse_hash == reverse_hash
                    && caa_likely(match(node, key))) {
                                break;
                }
                node = clear_flag(next);
        }
        assert(!node || !is_bucket(CMM_LOAD_SHARED(node->next)));
        iter->node = node;
        iter->next = next;
}

void cds_lfht_next_duplicate(struct cds_lfht *ht, cds_lfht_match_fct match,
                const void *key, struct cds_lfht_iter *iter)
{
        struct cds_lfht_node *node, *next;
        unsigned long reverse_hash;

        node = iter->node;
        reverse_hash = node->reverse_hash;
        next = iter->next;
        node = clear_flag(next);

        for (;;) {
                if (caa_unlikely(is_end(node))) {
                        node = next = NULL;
                        break;
                }
                if (caa_unlikely(node->reverse_hash > reverse_hash)) {
                        node = next = NULL;
                        break;
                }
                next = rcu_dereference(node->next);
                if (caa_likely(!is_removed(next))
                    && !is_bucket(next)
                    && caa_likely(match(node, key))) {
                                break;
                }
                node = clear_flag(next);
        }
        assert(!node || !is_bucket(CMM_LOAD_SHARED(node->next)));
        iter->node = node;
        iter->next = next;
}

void cds_lfht_next(struct cds_lfht *ht, struct cds_lfht_iter *iter)
{
        struct cds_lfht_node *node, *next;

        node = clear_flag(iter->next);
        for (;;) {
                if (caa_unlikely(is_end(node))) {
                        node = next = NULL;
                        break;
                }
                next = rcu_dereference(node->next);
                if (caa_likely(!is_removed(next))
                    && !is_bucket(next)) {
                                break;
                }
                node = clear_flag(next);
        }
        assert(!node || !is_bucket(CMM_LOAD_SHARED(node->next)));
        iter->node = node;
        iter->next = next;
}

void cds_lfht_first(struct cds_lfht *ht, struct cds_lfht_iter *iter)
{
        /*
         * Get next after first bucket node. The first bucket node is the
         * first node of the linked list.
         */
        iter->next = bucket_at(ht, 0)->next;
        cds_lfht_next(ht, iter);
}

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

        node->reverse_hash = bit_reverse_ulong(hash);
        size = rcu_dereference(ht->size);
        _cds_lfht_add(ht, hash, NULL, NULL, size, node, NULL, 0);
        ht_count_add(ht, size, hash);
}

struct cds_lfht_node *cds_lfht_add_unique(struct cds_lfht *ht,
                                unsigned long hash,
                                cds_lfht_match_fct match,
                                const void *key,
                                struct cds_lfht_node *node)
{
        unsigned long size;
        struct cds_lfht_iter iter;

        node->reverse_hash = bit_reverse_ulong(hash);
        size = rcu_dereference(ht->size);
        _cds_lfht_add(ht, hash, match, key, size, node, &iter, 0);
        if (iter.node == node)
                ht_count_add(ht, size, hash);
        return iter.node;
}

struct cds_lfht_node *cds_lfht_add_replace(struct cds_lfht *ht,
                                unsigned long hash,
                                cds_lfht_match_fct match,
                                const void *key,
                                struct cds_lfht_node *node)
{
        unsigned long size;
        struct cds_lfht_iter iter;

        node->reverse_hash = bit_reverse_ulong(hash);
        size = rcu_dereference(ht->size);
        for (;;) {
                _cds_lfht_add(ht, hash, match, key, size, node, &iter, 0);
                if (iter.node == node) {
                        ht_count_add(ht, size, hash);
                        return NULL;
                }

                if (!_cds_lfht_replace(ht, size, iter.node, iter.next, node))
                        return iter.node;
        }
}

int cds_lfht_replace(struct cds_lfht *ht,
                struct cds_lfht_iter *old_iter,
                unsigned long hash,
                cds_lfht_match_fct match,
                const void *key,
                struct cds_lfht_node *new_node)
{
        unsigned long size;

        new_node->reverse_hash = bit_reverse_ulong(hash);
        if (!old_iter->node)
                return -ENOENT;
        if (caa_unlikely(old_iter->node->reverse_hash != new_node->reverse_hash))
                return -EINVAL;
        if (caa_unlikely(!match(old_iter->node, key)))
                return -EINVAL;
        size = rcu_dereference(ht->size);
        return _cds_lfht_replace(ht, size, old_iter->node, old_iter->next,
                        new_node);
}

int cds_lfht_del(struct cds_lfht *ht, struct cds_lfht_node *node)
{
        unsigned long size, hash;
        int ret;

        size = rcu_dereference(ht->size);
        ret = _cds_lfht_del(ht, size, node);
        if (!ret) {
                hash = bit_reverse_ulong(node->reverse_hash);
                ht_count_del(ht, size, hash);
        }
        return ret;
}

int cds_lfht_is_node_deleted(struct cds_lfht_node *node)
{
        return is_removed(CMM_LOAD_SHARED(node->next));
}

static
int cds_lfht_delete_bucket(struct cds_lfht *ht)
{
        struct cds_lfht_node *node;
        unsigned long order, i, size;

        /* Check that the table is empty */
        node = bucket_at(ht, 0);
        do {
                node = clear_flag(node)->next;
                if (!is_bucket(node))
                        return -EPERM;
                assert(!is_removed(node));
        } while (!is_end(node));
        /*
         * size accessed without rcu_dereference because hash table is
         * being destroyed.
         */
        size = ht->size;
        /* Internal sanity check: all nodes left should be buckets */
        for (i = 0; i < size; i++) {
                node = bucket_at(ht, i);
                dbg_printf("delete bucket: index %lu expected hash %lu hash %lu\n",
                        i, i, bit_reverse_ulong(node->reverse_hash));
                assert(is_bucket(node->next));
        }

        for (order = cds_lfht_get_count_order_ulong(size); (long)order >= 0; order--)
                cds_lfht_free_bucket_table(ht, 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, pthread_attr_t **attr)
{
        int ret;

        /* Wait for in-flight resize operations to complete */
        _CMM_STORE_SHARED(ht->in_progress_destroy, 1);
        cmm_smp_mb();   /* Store destroy before load resize */
        while (uatomic_read(&ht->in_progress_resize))
                poll(NULL, 0, 100);     /* wait for 100ms */
        ret = cds_lfht_delete_bucket(ht);
        if (ret)
                return ret;
        free_split_items_count(ht);
        if (attr)
                *attr = ht->resize_attr;
        poison_free(ht);
        return ret;
}

void cds_lfht_count_nodes(struct cds_lfht *ht,
                long *approx_before,
                unsigned long *count,
                long *approx_after)
{
        struct cds_lfht_node *node, *next;
        unsigned long nr_bucket = 0, nr_removed = 0;

        *approx_before = 0;
        if (ht->split_count) {
                int i;

                for (i = 0; i < split_count_mask + 1; i++) {
                        *approx_before += uatomic_read(&ht->split_count[i].add);
                        *approx_before -= uatomic_read(&ht->split_count[i].del);
                }
        }

        *count = 0;

        /* Count non-bucket nodes in the table */
        node = bucket_at(ht, 0);
        do {
                next = rcu_dereference(node->next);
                if (is_removed(next)) {
                        if (!is_bucket(next))
                                (nr_removed)++;
                        else
                                (nr_bucket)++;
                } else if (!is_bucket(next))
                        (*count)++;
                else
                        (nr_bucket)++;
                node = clear_flag(next);
        } while (!is_end(node));
        dbg_printf("number of logically removed nodes: %lu\n", nr_removed);
        dbg_printf("number of bucket nodes: %lu\n", nr_bucket);
        *approx_after = 0;
        if (ht->split_count) {
                int i;

                for (i = 0; i < split_count_mask + 1; i++) {
                        *approx_after += uatomic_read(&ht->split_count[i].add);
                        *approx_after -= uatomic_read(&ht->split_count[i].del);
                }
        }
}

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

        old_order = cds_lfht_get_count_order_ulong(old_size);
        new_order = cds_lfht_get_count_order_ulong(new_size);
        dbg_printf("resize from %lu (order %lu) to %lu (order %lu) buckets\n",
                   old_size, old_order, new_size, new_order);
        assert(new_size > old_size);
        init_table(ht, old_order + 1, new_order);
}

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

        new_size = max(new_size, MIN_TABLE_SIZE);
        old_order = cds_lfht_get_count_order_ulong(old_size);
        new_order = cds_lfht_get_count_order_ulong(new_size);
        dbg_printf("resize from %lu (order %lu) to %lu (order %lu) buckets\n",
                   old_size, old_order, new_size, new_order);
        assert(new_size < old_size);

        /* Remove and unlink all bucket nodes to remove. */
        fini_table(ht, new_order + 1, old_order);
}


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

        /*
         * Resize table, re-do if the target size has changed under us.
         */
        do {
                assert(uatomic_read(&ht->in_progress_resize));
                if (CMM_LOAD_SHARED(ht->in_progress_destroy))
                        break;
                ht->resize_initiated = 1;
                old_size = ht->size;
                new_size = CMM_LOAD_SHARED(ht->resize_target);
                if (old_size < new_size)
                        _do_cds_lfht_grow(ht, old_size, new_size);
                else if (old_size > new_size)
                        _do_cds_lfht_shrink(ht, old_size, new_size);
                ht->resize_initiated = 0;
                /* write resize_initiated before read resize_target */
                cmm_smp_mb();
        } while (ht->size != CMM_LOAD_SHARED(ht->resize_target));
}

static
unsigned long resize_target_grow(struct cds_lfht *ht, unsigned long new_size)
{
        return _uatomic_xchg_monotonic_increase(&ht->resize_target, new_size);
}

static
void resize_target_update_count(struct cds_lfht *ht,
                                unsigned long count)
{
        count = max(count, MIN_TABLE_SIZE);
        count = min(count, ht->max_nr_buckets);
        uatomic_set(&ht->resize_target, count);
}

void cds_lfht_resize(struct cds_lfht *ht, unsigned long new_size)
{
        resize_target_update_count(ht, new_size);
        CMM_STORE_SHARED(ht->resize_initiated, 1);
        ht->flavor->thread_offline();
        pthread_mutex_lock(&ht->resize_mutex);
        _do_cds_lfht_resize(ht);
        pthread_mutex_unlock(&ht->resize_mutex);
        ht->flavor->thread_online();
}

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;

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

static
void __cds_lfht_resize_lazy_launch(struct cds_lfht *ht)
{
        struct rcu_resize_work *work;

        /* Store resize_target before read resize_initiated */
        cmm_smp_mb();
        if (!CMM_LOAD_SHARED(ht->resize_initiated)) {
                uatomic_inc(&ht->in_progress_resize);
                cmm_smp_mb();   /* increment resize count before load destroy */
                if (CMM_LOAD_SHARED(ht->in_progress_destroy)) {
                        uatomic_dec(&ht->in_progress_resize);
                        return;
                }
                work = malloc(sizeof(*work));
                if (work == NULL) {
                        dbg_printf("error allocating resize work, bailing out\n");
                        uatomic_dec(&ht->in_progress_resize);
                        return;
                }
                work->ht = ht;
                ht->flavor->update_call_rcu(&work->head, do_resize_cb);
                CMM_STORE_SHARED(ht->resize_initiated, 1);
        }
}

static
void cds_lfht_resize_lazy_grow(struct cds_lfht *ht, unsigned long size, int growth)
{
        unsigned long target_size = size << growth;

        target_size = min(target_size, ht->max_nr_buckets);
        if (resize_target_grow(ht, target_size) >= target_size)
                return;

        __cds_lfht_resize_lazy_launch(ht);
}

/*
 * We favor grow operations over shrink. A shrink operation never occurs
 * if a grow operation is queued for lazy execution. A grow operation
 * cancels any pending shrink lazy execution.
 */
static
void cds_lfht_resize_lazy_count(struct cds_lfht *ht, unsigned long size,
                                unsigned long count)
{
        if (!(ht->flags & CDS_LFHT_AUTO_RESIZE))
                return;
        count = max(count, MIN_TABLE_SIZE);
        count = min(count, ht->max_nr_buckets);
        if (count == size)
                return;         /* Already the right size, no resize needed */
        if (count > size) {     /* lazy grow */
                if (resize_target_grow(ht, count) >= count)
                        return;
        } else {                /* lazy shrink */
                for (;;) {
                        unsigned long s;

                        s = uatomic_cmpxchg(&ht->resize_target, size, count);
                        if (s == size)
                                break;  /* no resize needed */
                        if (s > size)
                                return; /* growing is/(was just) in progress */
                        if (s <= count)
                                return; /* some other thread do shrink */
                        size = s;
                }
        }
        __cds_lfht_resize_lazy_launch(ht);
}