On 08/06/16 21:55, Emilio G. Cota wrote:
> This is a fast, scalable chained hash table with optional auto-resizing,
> allowing
> reads that are concurrent with reads, and reads/writes that are concurrent
> with writes to separate buckets.
>
> A hash table with these features will be necessary for the scalability
> of the ongoing MTTCG work; before those changes arrive we can already
> benefit from the single-threaded speedup that qht also provides.
>
> Signed-off-by: Emilio G. Cota <c...@braap.org>
Reviewed-by: Sergey Fedorov <sergey.fedo...@linaro.org>
> ---
> include/qemu/qht.h | 183 ++++++++++++
> util/Makefile.objs | 1 +
> util/qht.c | 833
> +++++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 1017 insertions(+)
> create mode 100644 include/qemu/qht.h
> create mode 100644 util/qht.c
>
> diff --git a/include/qemu/qht.h b/include/qemu/qht.h
> new file mode 100644
> index 0000000..aec60aa
> --- /dev/null
> +++ b/include/qemu/qht.h
> @@ -0,0 +1,183 @@
> +/*
> + * Copyright (C) 2016, Emilio G. Cota <c...@braap.org>
> + *
> + * License: GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +#ifndef QEMU_QHT_H
> +#define QEMU_QHT_H
> +
> +#include "qemu/osdep.h"
> +#include "qemu/seqlock.h"
> +#include "qemu/thread.h"
> +#include "qemu/qdist.h"
> +
> +struct qht {
> + struct qht_map *map;
> + QemuMutex lock; /* serializes setters of ht->map */
> + unsigned int mode;
> +};
> +
> +/**
> + * struct qht_stats - Statistics of a QHT
> + * @head_buckets: number of head buckets
> + * @used_head_buckets: number of non-empty head buckets
> + * @entries: total number of entries
> + * @chain: frequency distribution representing the number of buckets in each
> + * chain, excluding empty chains.
> + * @occupancy: frequency distribution representing chain occupancy rate.
> + * Valid range: from 0.0 (empty) to 1.0 (full occupancy).
> + *
> + * An entry is a pointer-hash pair.
> + * Each bucket can host several entries.
> + * Chains are chains of buckets, whose first link is always a head bucket.
> + */
> +struct qht_stats {
> + size_t head_buckets;
> + size_t used_head_buckets;
> + size_t entries;
> + struct qdist chain;
> + struct qdist occupancy;
> +};
> +
> +typedef bool (*qht_lookup_func_t)(const void *obj, const void *userp);
> +typedef void (*qht_iter_func_t)(struct qht *ht, void *p, uint32_t h, void
> *up);
> +
> +#define QHT_MODE_AUTO_RESIZE 0x1 /* auto-resize when heavily loaded */
> +
> +/**
> + * qht_init - Initialize a QHT
> + * @ht: QHT to be initialized
> + * @n_elems: number of entries the hash table should be optimized for.
> + * @mode: bitmask with OR'ed QHT_MODE_*
> + */
> +void qht_init(struct qht *ht, size_t n_elems, unsigned int mode);
> +
> +/**
> + * qht_destroy - destroy a previously initialized QHT
> + * @ht: QHT to be destroyed
> + *
> + * Call only when there are no readers/writers left.
> + */
> +void qht_destroy(struct qht *ht);
> +
> +/**
> + * qht_insert - Insert a pointer into the hash table
> + * @ht: QHT to insert to
> + * @p: pointer to be inserted
> + * @hash: hash corresponding to @p
> + *
> + * Attempting to insert a NULL @p is a bug.
> + * Inserting the same pointer @p with different @hash values is a bug.
> + *
> + * Returns true on sucess.
> + * Returns false if the @p-@hash pair already exists in the hash table.
> + */
> +bool qht_insert(struct qht *ht, void *p, uint32_t hash);
> +
> +/**
> + * qht_lookup - Look up a pointer in a QHT
> + * @ht: QHT to be looked up
> + * @func: function to compare existing pointers against @userp
> + * @userp: pointer to pass to @func
> + * @hash: hash of the pointer to be looked up
> + *
> + * Needs to be called under an RCU read-critical section.
> + *
> + * The user-provided @func compares pointers in QHT against @userp.
> + * If the function returns true, a match has been found.
> + *
> + * Returns the corresponding pointer when a match is found.
> + * Returns NULL otherwise.
> + */
> +void *qht_lookup(struct qht *ht, qht_lookup_func_t func, const void *userp,
> + uint32_t hash);
> +
> +/**
> + * qht_remove - remove a pointer from the hash table
> + * @ht: QHT to remove from
> + * @p: pointer to be removed
> + * @hash: hash corresponding to @p
> + *
> + * Attempting to remove a NULL @p is a bug.
> + *
> + * Just-removed @p pointers cannot be immediately freed; they need to remain
> + * valid until the end of the RCU grace period in which qht_remove() is
> called.
> + * This guarantees that concurrent lookups will always compare against valid
> + * data.
> + *
> + * Returns true on success.
> + * Returns false if the @p-@hash pair was not found.
> + */
> +bool qht_remove(struct qht *ht, const void *p, uint32_t hash);
> +
> +/**
> + * qht_reset - reset a QHT
> + * @ht: QHT to be reset
> + *
> + * All entries in the hash table are reset. No resizing is performed.
> + *
> + * If concurrent readers may exist, the objects pointed to by the hash table
> + * must remain valid for the existing RCU grace period -- see qht_remove().
> + * See also: qht_reset_size()
> + */
> +void qht_reset(struct qht *ht);
> +
> +/**
> + * qht_reset_size - reset and resize a QHT
> + * @ht: QHT to be reset and resized
> + * @n_elems: number of entries the resized hash table should be optimized
> for.
> + *
> + * Returns true if the resize was necessary and therefore performed.
> + * Returns false otherwise.
> + *
> + * If concurrent readers may exist, the objects pointed to by the hash table
> + * must remain valid for the existing RCU grace period -- see qht_remove().
> + * See also: qht_reset(), qht_resize().
> + */
> +bool qht_reset_size(struct qht *ht, size_t n_elems);
> +
> +/**
> + * qht_resize - resize a QHT
> + * @ht: QHT to be resized
> + * @n_elems: number of entries the resized hash table should be optimized for
> + *
> + * Returns true on success.
> + * Returns false if the resize was not necessary and therefore not performed.
> + * See also: qht_reset_size().
> + */
> +bool qht_resize(struct qht *ht, size_t n_elems);
> +
> +/**
> + * qht_iter - Iterate over a QHT
> + * @ht: QHT to be iterated over
> + * @func: function to be called for each entry in QHT
> + * @userp: additional pointer to be passed to @func
> + *
> + * Each time it is called, user-provided @func is passed a pointer-hash pair,
> + * plus @userp.
> + */
> +void qht_iter(struct qht *ht, qht_iter_func_t func, void *userp);
> +
> +/**
> + * qht_statistics_init - Gather statistics from a QHT
> + * @ht: QHT to gather statistics from
> + * @stats: pointer to a struct qht_stats to be filled in
> + *
> + * Does NOT need to be called under an RCU read-critical section,
> + * since it does not dereference any pointers stored in the hash table.
> + *
> + * When done with @stats, pass the struct to qht_statistics_destroy().
> + * Failing to do this will leak memory.
> + */
> +void qht_statistics_init(struct qht *ht, struct qht_stats *stats);
> +
> +/**
> + * qht_statistics_destroy - Destroy a struct qht_stats
> + * @stats: stuct qht_stats to be destroyed
> + *
> + * See also: qht_statistics_init().
> + */
> +void qht_statistics_destroy(struct qht_stats *stats);
> +
> +#endif /* QEMU_QHT_H */
> diff --git a/util/Makefile.objs b/util/Makefile.objs
> index 702435e..45f8794 100644
> --- a/util/Makefile.objs
> +++ b/util/Makefile.objs
> @@ -33,3 +33,4 @@ util-obj-y += timed-average.o
> util-obj-y += base64.o
> util-obj-y += log.o
> util-obj-y += qdist.o
> +util-obj-y += qht.o
> diff --git a/util/qht.c b/util/qht.c
> new file mode 100644
> index 0000000..6f74909
> --- /dev/null
> +++ b/util/qht.c
> @@ -0,0 +1,833 @@
> +/*
> + * qht.c - QEMU Hash Table, designed to scale for read-mostly workloads.
> + *
> + * Copyright (C) 2016, Emilio G. Cota <c...@braap.org>
> + *
> + * License: GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + *
> + * Assumptions:
> + * - NULL cannot be inserted/removed as a pointer value.
> + * - Trying to insert an already-existing hash-pointer pair is OK. However,
> + * it is not OK to insert into the same hash table different hash-pointer
> + * pairs that have the same pointer value, but not the hashes.
> + * - Lookups are performed under an RCU read-critical section; removals
> + * must wait for a grace period to elapse before freeing removed objects.
> + *
> + * Features:
> + * - Reads (i.e. lookups and iterators) can be concurrent with other reads.
> + * Lookups that are concurrent with writes to the same bucket will retry
> + * via a seqlock; iterators acquire all bucket locks and therefore can be
> + * concurrent with lookups and are serialized wrt writers.
> + * - Writes (i.e. insertions/removals) can be concurrent with writes to
> + * different buckets; writes to the same bucket are serialized through a
> lock.
> + * - Optional auto-resizing: the hash table resizes up if the load surpasses
> + * a certain threshold. Resizing is done concurrently with readers; writes
> + * are serialized with the resize operation.
> + *
> + * The key structure is the bucket, which is cacheline-sized. Buckets
> + * contain a few hash values and pointers; the u32 hash values are stored in
> + * full so that resizing is fast. Having this structure instead of directly
> + * chaining items has two advantages:
> + * - Failed lookups fail fast, and touch a minimum number of cache lines.
> + * - Resizing the hash table with concurrent lookups is easy.
> + *
> + * There are two types of buckets:
> + * 1. "head" buckets are the ones allocated in the array of buckets in
> qht_map.
> + * 2. all "non-head" buckets (i.e. all others) are members of a chain that
> + * starts from a head bucket.
> + * Note that the seqlock and spinlock of a head bucket applies to all buckets
> + * chained to it; these two fields are unused in non-head buckets.
> + *
> + * On removals, we move the last valid item in the chain to the position of
> the
> + * just-removed entry. This makes lookups slightly faster, since the moment
> an
> + * invalid entry is found, the (failed) lookup is over.
> + *
> + * Resizing is done by taking all bucket spinlocks (so that no other writers
> can
> + * race with us) and then copying all entries into a new hash map. Then, the
> + * ht->map pointer is set, and the old map is freed once no RCU readers can
> see
> + * it anymore.
> + *
> + * Writers check for concurrent resizes by comparing ht->map before and after
> + * acquiring their bucket lock. If they don't match, a resize has occured
> + * while the bucket spinlock was being acquired.
> + *
> + * Related Work:
> + * - Idea of cacheline-sized buckets with full hashes taken from:
> + * David, Guerraoui & Trigonakis, "Asynchronized Concurrency:
> + * The Secret to Scaling Concurrent Search Data Structures", ASPLOS'15.
> + * - Why not RCU-based hash tables? They would allow us to get rid of the
> + * seqlock, but resizing would take forever since RCU read critical
> + * sections in QEMU take quite a long time.
> + * More info on relativistic hash tables:
> + * + Triplett, McKenney & Walpole, "Resizable, Scalable, Concurrent Hash
> + * Tables via Relativistic Programming", USENIX ATC'11.
> + * + Corbet, "Relativistic hash tables, part 1: Algorithms", @ lwn.net,
> 2014.
> + * https://lwn.net/Articles/612021/
> + */
> +#include "qemu/qht.h"
> +#include "qemu/atomic.h"
> +#include "qemu/rcu.h"
> +
> +//#define QHT_DEBUG
> +
> +/*
> + * We want to avoid false sharing of cache lines. Most systems have 64-byte
> + * cache lines so we go with it for simplicity.
> + *
> + * Note that systems with smaller cache lines will be fine (the struct is
> + * almost 64-bytes); systems with larger cache lines might suffer from
> + * some false sharing.
> + */
> +#define QHT_BUCKET_ALIGN 64
> +
> +/* define these to keep sizeof(qht_bucket) within QHT_BUCKET_ALIGN */
> +#if HOST_LONG_BITS == 32
> +#define QHT_BUCKET_ENTRIES 6
> +#else /* 64-bit */
> +#define QHT_BUCKET_ENTRIES 4
> +#endif
> +
> +/*
> + * Note: reading partially-updated pointers in @pointers could lead to
> + * segfaults. We thus access them with atomic_read/set; this guarantees
> + * that the compiler makes all those accesses atomic. We also need the
> + * volatile-like behavior in atomic_read, since otherwise the compiler
> + * might refetch the pointer.
> + * atomic_read's are of course not necessary when the bucket lock is held.
> + *
> + * If both ht->lock and b->lock are grabbed, ht->lock should always
> + * be grabbed first.
> + */
> +struct qht_bucket {
> + QemuSpin lock;
> + QemuSeqLock sequence;
> + uint32_t hashes[QHT_BUCKET_ENTRIES];
> + void *pointers[QHT_BUCKET_ENTRIES];
> + struct qht_bucket *next;
> +} QEMU_ALIGNED(QHT_BUCKET_ALIGN);
> +
> +QEMU_BUILD_BUG_ON(sizeof(struct qht_bucket) > QHT_BUCKET_ALIGN);
> +
> +/**
> + * struct qht_map - structure to track an array of buckets
> + * @rcu: used by RCU. Keep it as the top field in the struct to help valgrind
> + * find the whole struct.
> + * @buckets: array of head buckets. It is constant once the map is created.
> + * @n_buckets: number of head buckets. It is constant once the map is
> created.
> + * @n_added_buckets: number of added (i.e. "non-head") buckets
> + * @n_added_buckets_threshold: threshold to trigger an upward resize once the
> + * number of added buckets surpasses it.
> + *
> + * Buckets are tracked in what we call a "map", i.e. this structure.
> + */
> +struct qht_map {
> + struct rcu_head rcu;
> + struct qht_bucket *buckets;
> + size_t n_buckets;
> + size_t n_added_buckets;
> + size_t n_added_buckets_threshold;
> +};
> +
> +/* trigger a resize when n_added_buckets > n_buckets / div */
> +#define QHT_NR_ADDED_BUCKETS_THRESHOLD_DIV 8
> +
> +static void qht_do_resize(struct qht *ht, struct qht_map *new);
> +static void qht_grow_maybe(struct qht *ht);
> +
> +#ifdef QHT_DEBUG
> +
> +#define qht_debug_assert(X) do { assert(X); } while (0)
> +
> +static void qht_bucket_debug__locked(struct qht_bucket *b)
> +{
> + bool seen_empty = false;
> + bool corrupt = false;
> + int i;
> +
> + do {
> + for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> + if (b->pointers[i] == NULL) {
> + seen_empty = true;
> + continue;
> + }
> + if (seen_empty) {
> + fprintf(stderr, "%s: b: %p, pos: %i, hash: 0x%x, p: %p\n",
> + __func__, b, i, b->hashes[i], b->pointers[i]);
> + corrupt = true;
> + }
> + }
> + b = b->next;
> + } while (b);
> + qht_debug_assert(!corrupt);
> +}
> +
> +static void qht_map_debug__all_locked(struct qht_map *map)
> +{
> + int i;
> +
> + for (i = 0; i < map->n_buckets; i++) {
> + qht_bucket_debug__locked(&map->buckets[i]);
> + }
> +}
> +#else
> +
> +#define qht_debug_assert(X) do { (void)(X); } while (0)
> +
> +static inline void qht_bucket_debug__locked(struct qht_bucket *b)
> +{ }
> +
> +static inline void qht_map_debug__all_locked(struct qht_map *map)
> +{ }
> +#endif /* QHT_DEBUG */
> +
> +static inline size_t qht_elems_to_buckets(size_t n_elems)
> +{
> + return pow2ceil(n_elems / QHT_BUCKET_ENTRIES);
> +}
> +
> +static inline void qht_head_init(struct qht_bucket *b)
> +{
> + memset(b, 0, sizeof(*b));
> + qemu_spin_init(&b->lock);
> + seqlock_init(&b->sequence);
> +}
> +
> +static inline
> +struct qht_bucket *qht_map_to_bucket(struct qht_map *map, uint32_t hash)
> +{
> + return &map->buckets[hash & (map->n_buckets - 1)];
> +}
> +
> +/* acquire all bucket locks from a map */
> +static void qht_map_lock_buckets(struct qht_map *map)
> +{
> + size_t i;
> +
> + for (i = 0; i < map->n_buckets; i++) {
> + struct qht_bucket *b = &map->buckets[i];
> +
> + qemu_spin_lock(&b->lock);
> + }
> +}
> +
> +static void qht_map_unlock_buckets(struct qht_map *map)
> +{
> + size_t i;
> +
> + for (i = 0; i < map->n_buckets; i++) {
> + struct qht_bucket *b = &map->buckets[i];
> +
> + qemu_spin_unlock(&b->lock);
> + }
> +}
> +
> +/*
> + * Call with at least a bucket lock held.
> + * @map should be the value read before acquiring the lock (or locks).
> + */
> +static inline bool qht_map_is_stale__locked(struct qht *ht, struct qht_map
> *map)
> +{
> + return map != ht->map;
> +}
> +
> +/*
> + * Grab all bucket locks, and set @pmap after making sure the map isn't
> stale.
> + *
> + * Pairs with qht_map_unlock_buckets(), hence the pass-by-reference.
> + *
> + * Note: callers cannot have ht->lock held.
> + */
> +static inline
> +void qht_map_lock_buckets__no_stale(struct qht *ht, struct qht_map **pmap)
> +{
> + struct qht_map *map;
> +
> + map = atomic_rcu_read(&ht->map);
> + qht_map_lock_buckets(map);
> + if (likely(!qht_map_is_stale__locked(ht, map))) {
> + *pmap = map;
> + return;
> + }
> + qht_map_unlock_buckets(map);
> +
> + /* we raced with a resize; acquire ht->lock to see the updated ht->map */
> + qemu_mutex_lock(&ht->lock);
> + map = ht->map;
> + qht_map_lock_buckets(map);
> + qemu_mutex_unlock(&ht->lock);
> + *pmap = map;
> + return;
> +}
> +
> +/*
> + * Get a head bucket and lock it, making sure its parent map is not stale.
> + * @pmap is filled with a pointer to the bucket's parent map.
> + *
> + * Unlock with qemu_spin_unlock(&b->lock).
> + *
> + * Note: callers cannot have ht->lock held.
> + */
> +static inline
> +struct qht_bucket *qht_bucket_lock__no_stale(struct qht *ht, uint32_t hash,
> + struct qht_map **pmap)
> +{
> + struct qht_bucket *b;
> + struct qht_map *map;
> +
> + map = atomic_rcu_read(&ht->map);
> + b = qht_map_to_bucket(map, hash);
> +
> + qemu_spin_lock(&b->lock);
> + if (likely(!qht_map_is_stale__locked(ht, map))) {
> + *pmap = map;
> + return b;
> + }
> + qemu_spin_unlock(&b->lock);
> +
> + /* we raced with a resize; acquire ht->lock to see the updated ht->map */
> + qemu_mutex_lock(&ht->lock);
> + map = ht->map;
> + b = qht_map_to_bucket(map, hash);
> + qemu_spin_lock(&b->lock);
> + qemu_mutex_unlock(&ht->lock);
> + *pmap = map;
> + return b;
> +}
> +
> +static inline bool qht_map_needs_resize(struct qht_map *map)
> +{
> + return atomic_read(&map->n_added_buckets) >
> map->n_added_buckets_threshold;
> +}
> +
> +static inline void qht_chain_destroy(struct qht_bucket *head)
> +{
> + struct qht_bucket *curr = head->next;
> + struct qht_bucket *prev;
> +
> + while (curr) {
> + prev = curr;
> + curr = curr->next;
> + qemu_vfree(prev);
> + }
> +}
> +
> +/* pass only an orphan map */
> +static void qht_map_destroy(struct qht_map *map)
> +{
> + size_t i;
> +
> + for (i = 0; i < map->n_buckets; i++) {
> + qht_chain_destroy(&map->buckets[i]);
> + }
> + qemu_vfree(map->buckets);
> + g_free(map);
> +}
> +
> +static struct qht_map *qht_map_create(size_t n_buckets)
> +{
> + struct qht_map *map;
> + size_t i;
> +
> + map = g_malloc(sizeof(*map));
> + map->n_buckets = n_buckets;
> +
> + map->n_added_buckets = 0;
> + map->n_added_buckets_threshold = n_buckets /
> + QHT_NR_ADDED_BUCKETS_THRESHOLD_DIV;
> +
> + /* let tiny hash tables to at least add one non-head bucket */
> + if (unlikely(map->n_added_buckets_threshold == 0)) {
> + map->n_added_buckets_threshold = 1;
> + }
> +
> + map->buckets = qemu_memalign(QHT_BUCKET_ALIGN,
> + sizeof(*map->buckets) * n_buckets);
> + for (i = 0; i < n_buckets; i++) {
> + qht_head_init(&map->buckets[i]);
> + }
> + return map;
> +}
> +
> +void qht_init(struct qht *ht, size_t n_elems, unsigned int mode)
> +{
> + struct qht_map *map;
> + size_t n_buckets = qht_elems_to_buckets(n_elems);
> +
> + ht->mode = mode;
> + qemu_mutex_init(&ht->lock);
> + map = qht_map_create(n_buckets);
> + atomic_rcu_set(&ht->map, map);
> +}
> +
> +/* call only when there are no readers/writers left */
> +void qht_destroy(struct qht *ht)
> +{
> + qht_map_destroy(ht->map);
> + memset(ht, 0, sizeof(*ht));
> +}
> +
> +static void qht_bucket_reset__locked(struct qht_bucket *head)
> +{
> + struct qht_bucket *b = head;
> + int i;
> +
> + seqlock_write_begin(&head->sequence);
> + do {
> + for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> + if (b->pointers[i] == NULL) {
> + goto done;
> + }
> + b->hashes[i] = 0;
> + atomic_set(&b->pointers[i], NULL);
> + }
> + b = b->next;
> + } while (b);
> + done:
> + seqlock_write_end(&head->sequence);
> +}
> +
> +/* call with all bucket locks held */
> +static void qht_map_reset__all_locked(struct qht_map *map)
> +{
> + size_t i;
> +
> + for (i = 0; i < map->n_buckets; i++) {
> + qht_bucket_reset__locked(&map->buckets[i]);
> + }
> + qht_map_debug__all_locked(map);
> +}
> +
> +void qht_reset(struct qht *ht)
> +{
> + struct qht_map *map;
> +
> + qht_map_lock_buckets__no_stale(ht, &map);
> + qht_map_reset__all_locked(map);
> + qht_map_unlock_buckets(map);
> +}
> +
> +bool qht_reset_size(struct qht *ht, size_t n_elems)
> +{
> + struct qht_map *new;
> + struct qht_map *map;
> + size_t n_buckets;
> + bool resize = false;
> +
> + n_buckets = qht_elems_to_buckets(n_elems);
> +
> + qemu_mutex_lock(&ht->lock);
> + map = ht->map;
> + if (n_buckets != map->n_buckets) {
> + new = qht_map_create(n_buckets);
> + resize = true;
> + }
> +
> + qht_map_lock_buckets(map);
> + qht_map_reset__all_locked(map);
> + if (resize) {
> + qht_do_resize(ht, new);
> + }
> + qht_map_unlock_buckets(map);
> + qemu_mutex_unlock(&ht->lock);
> +
> + return resize;
> +}
> +
> +static inline
> +void *qht_do_lookup(struct qht_bucket *head, qht_lookup_func_t func,
> + const void *userp, uint32_t hash)
> +{
> + struct qht_bucket *b = head;
> + int i;
> +
> + do {
> + for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> + if (b->hashes[i] == hash) {
> + void *p = atomic_read(&b->pointers[i]);
> +
> + if (likely(p) && likely(func(p, userp))) {
> + return p;
> + }
> + }
> + }
> + b = atomic_rcu_read(&b->next);
> + } while (b);
> +
> + return NULL;
> +}
> +
> +static __attribute__((noinline))
> +void *qht_lookup__slowpath(struct qht_bucket *b, qht_lookup_func_t func,
> + const void *userp, uint32_t hash)
> +{
> + unsigned int version;
> + void *ret;
> +
> + do {
> + version = seqlock_read_begin(&b->sequence);
> + ret = qht_do_lookup(b, func, userp, hash);
> + } while (seqlock_read_retry(&b->sequence, version));
> + return ret;
> +}
> +
> +void *qht_lookup(struct qht *ht, qht_lookup_func_t func, const void *userp,
> + uint32_t hash)
> +{
> + struct qht_bucket *b;
> + struct qht_map *map;
> + unsigned int version;
> + void *ret;
> +
> + map = atomic_rcu_read(&ht->map);
> + b = qht_map_to_bucket(map, hash);
> +
> + version = seqlock_read_begin(&b->sequence);
> + ret = qht_do_lookup(b, func, userp, hash);
> + if (likely(!seqlock_read_retry(&b->sequence, version))) {
> + return ret;
> + }
> + /*
> + * Removing the do/while from the fastpath gives a 4% perf. increase when
> + * running a 100%-lookup microbenchmark.
> + */
> + return qht_lookup__slowpath(b, func, userp, hash);
> +}
> +
> +/* call with head->lock held */
> +static bool qht_insert__locked(struct qht *ht, struct qht_map *map,
> + struct qht_bucket *head, void *p, uint32_t
> hash,
> + bool *needs_resize)
> +{
> + struct qht_bucket *b = head;
> + struct qht_bucket *prev = NULL;
> + struct qht_bucket *new = NULL;
> + int i;
> +
> + do {
> + for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> + if (b->pointers[i]) {
> + if (unlikely(b->pointers[i] == p)) {
> + return false;
> + }
> + } else {
> + goto found;
> + }
> + }
> + prev = b;
> + b = b->next;
> + } while (b);
> +
> + b = qemu_memalign(QHT_BUCKET_ALIGN, sizeof(*b));
> + memset(b, 0, sizeof(*b));
> + new = b;
> + i = 0;
> + atomic_inc(&map->n_added_buckets);
> + if (unlikely(qht_map_needs_resize(map)) && needs_resize) {
> + *needs_resize = true;
> + }
> +
> + found:
> + /* found an empty key: acquire the seqlock and write */
> + seqlock_write_begin(&head->sequence);
> + if (new) {
> + atomic_rcu_set(&prev->next, b);
> + }
> + b->hashes[i] = hash;
> + atomic_set(&b->pointers[i], p);
> + seqlock_write_end(&head->sequence);
> + return true;
> +}
> +
> +static __attribute__((noinline)) void qht_grow_maybe(struct qht *ht)
> +{
> + struct qht_map *map;
> +
> + /*
> + * If the lock is taken it probably means there's an ongoing resize,
> + * so bail out.
> + */
> + if (qemu_mutex_trylock(&ht->lock)) {
> + return;
> + }
> + map = ht->map;
> + /* another thread might have just performed the resize we were after */
> + if (qht_map_needs_resize(map)) {
> + struct qht_map *new = qht_map_create(map->n_buckets * 2);
> +
> + qht_map_lock_buckets(map);
> + qht_do_resize(ht, new);
> + qht_map_unlock_buckets(map);
> + }
> + qemu_mutex_unlock(&ht->lock);
> +}
> +
> +bool qht_insert(struct qht *ht, void *p, uint32_t hash)
> +{
> + struct qht_bucket *b;
> + struct qht_map *map;
> + bool needs_resize = false;
> + bool ret;
> +
> + /* NULL pointers are not supported */
> + qht_debug_assert(p);
> +
> + b = qht_bucket_lock__no_stale(ht, hash, &map);
> + ret = qht_insert__locked(ht, map, b, p, hash, &needs_resize);
> + qht_bucket_debug__locked(b);
> + qemu_spin_unlock(&b->lock);
> +
> + if (unlikely(needs_resize) && ht->mode & QHT_MODE_AUTO_RESIZE) {
> + qht_grow_maybe(ht);
> + }
> + return ret;
> +}
> +
> +static inline bool qht_entry_is_last(struct qht_bucket *b, int pos)
> +{
> + if (pos == QHT_BUCKET_ENTRIES - 1) {
> + if (b->next == NULL) {
> + return true;
> + }
> + return b->next->pointers[0] == NULL;
> + }
> + return b->pointers[pos + 1] == NULL;
> +}
> +
> +static void
> +qht_entry_move(struct qht_bucket *to, int i, struct qht_bucket *from, int j)
> +{
> + qht_debug_assert(!(to == from && i == j));
> + qht_debug_assert(to->pointers[i]);
> + qht_debug_assert(from->pointers[j]);
> +
> + to->hashes[i] = from->hashes[j];
> + atomic_set(&to->pointers[i], from->pointers[j]);
> +
> + from->hashes[j] = 0;
> + atomic_set(&from->pointers[j], NULL);
> +}
> +
> +/*
> + * Find the last valid entry in @head, and swap it with @orig[pos], which has
> + * just been invalidated.
> + */
> +static inline void qht_bucket_remove_entry(struct qht_bucket *orig, int pos)
> +{
> + struct qht_bucket *b = orig;
> + struct qht_bucket *prev = NULL;
> + int i;
> +
> + if (qht_entry_is_last(orig, pos)) {
> + orig->hashes[pos] = 0;
> + atomic_set(&orig->pointers[pos], NULL);
> + return;
> + }
> + do {
> + for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> + if (b->pointers[i]) {
> + continue;
> + }
> + if (i > 0) {
> + return qht_entry_move(orig, pos, b, i - 1);
> + }
> + qht_debug_assert(prev);
> + return qht_entry_move(orig, pos, prev, QHT_BUCKET_ENTRIES - 1);
> + }
> + prev = b;
> + b = b->next;
> + } while (b);
> + /* no free entries other than orig[pos], so swap it with the last one */
> + qht_entry_move(orig, pos, prev, QHT_BUCKET_ENTRIES - 1);
> +}
> +
> +/* call with b->lock held */
> +static inline
> +bool qht_remove__locked(struct qht_map *map, struct qht_bucket *head,
> + const void *p, uint32_t hash)
> +{
> + struct qht_bucket *b = head;
> + int i;
> +
> + do {
> + for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> + void *q = b->pointers[i];
> +
> + if (unlikely(q == NULL)) {
> + return false;
> + }
> + if (q == p) {
> + qht_debug_assert(b->hashes[i] == hash);
> + seqlock_write_begin(&head->sequence);
> + qht_bucket_remove_entry(b, i);
> + seqlock_write_end(&head->sequence);
> + return true;
> + }
> + }
> + b = b->next;
> + } while (b);
> + return false;
> +}
> +
> +bool qht_remove(struct qht *ht, const void *p, uint32_t hash)
> +{
> + struct qht_bucket *b;
> + struct qht_map *map;
> + bool ret;
> +
> + /* NULL pointers are not supported */
> + qht_debug_assert(p);
> +
> + b = qht_bucket_lock__no_stale(ht, hash, &map);
> + ret = qht_remove__locked(map, b, p, hash);
> + qht_bucket_debug__locked(b);
> + qemu_spin_unlock(&b->lock);
> + return ret;
> +}
> +
> +static inline void qht_bucket_iter(struct qht *ht, struct qht_bucket *b,
> + qht_iter_func_t func, void *userp)
> +{
> + int i;
> +
> + do {
> + for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> + if (b->pointers[i] == NULL) {
> + return;
> + }
> + func(ht, b->pointers[i], b->hashes[i], userp);
> + }
> + b = b->next;
> + } while (b);
> +}
> +
> +/* call with all of the map's locks held */
> +static inline void qht_map_iter__all_locked(struct qht *ht, struct qht_map
> *map,
> + qht_iter_func_t func, void
> *userp)
> +{
> + size_t i;
> +
> + for (i = 0; i < map->n_buckets; i++) {
> + qht_bucket_iter(ht, &map->buckets[i], func, userp);
> + }
> +}
> +
> +void qht_iter(struct qht *ht, qht_iter_func_t func, void *userp)
> +{
> + struct qht_map *map;
> +
> + map = atomic_rcu_read(&ht->map);
> + qht_map_lock_buckets(map);
> + /* Note: ht here is merely for carrying ht->mode; ht->map won't be read
> */
> + qht_map_iter__all_locked(ht, map, func, userp);
> + qht_map_unlock_buckets(map);
> +}
> +
> +static void qht_map_copy(struct qht *ht, void *p, uint32_t hash, void *userp)
> +{
> + struct qht_map *new = userp;
> + struct qht_bucket *b = qht_map_to_bucket(new, hash);
> +
> + /* no need to acquire b->lock because no thread has seen this map yet */
> + qht_insert__locked(ht, new, b, p, hash, NULL);
> +}
> +
> +/*
> + * Call with ht->lock and all bucket locks held.
> + *
> + * Creating the @new map here would add unnecessary delay while all the locks
> + * are held--holding up the bucket locks is particularly bad, since no writes
> + * can occur while these are held. Thus, we let callers create the new map,
> + * hopefully without the bucket locks held.
> + */
> +static void qht_do_resize(struct qht *ht, struct qht_map *new)
> +{
> + struct qht_map *old;
> +
> + old = ht->map;
> + g_assert_cmpuint(new->n_buckets, !=, old->n_buckets);
> +
> + qht_map_iter__all_locked(ht, old, qht_map_copy, new);
> + qht_map_debug__all_locked(new);
> +
> + atomic_rcu_set(&ht->map, new);
> + call_rcu(old, qht_map_destroy, rcu);
> +}
> +
> +bool qht_resize(struct qht *ht, size_t n_elems)
> +{
> + size_t n_buckets = qht_elems_to_buckets(n_elems);
> + size_t ret = false;
> +
> + qemu_mutex_lock(&ht->lock);
> + if (n_buckets != ht->map->n_buckets) {
> + struct qht_map *new;
> + struct qht_map *old = ht->map;
> +
> + new = qht_map_create(n_buckets);
> + qht_map_lock_buckets(old);
> + qht_do_resize(ht, new);
> + qht_map_unlock_buckets(old);
> + ret = true;
> + }
> + qemu_mutex_unlock(&ht->lock);
> +
> + return ret;
> +}
> +
> +/* pass @stats to qht_statistics_destroy() when done */
> +void qht_statistics_init(struct qht *ht, struct qht_stats *stats)
> +{
> + struct qht_map *map;
> + int i;
> +
> + map = atomic_rcu_read(&ht->map);
> +
> + stats->head_buckets = map->n_buckets;
> + stats->used_head_buckets = 0;
> + stats->entries = 0;
> + qdist_init(&stats->chain);
> + qdist_init(&stats->occupancy);
> +
> + for (i = 0; i < map->n_buckets; i++) {
> + struct qht_bucket *head = &map->buckets[i];
> + struct qht_bucket *b;
> + unsigned int version;
> + size_t buckets;
> + size_t entries;
> + int j;
> +
> + do {
> + version = seqlock_read_begin(&head->sequence);
> + buckets = 0;
> + entries = 0;
> + b = head;
> + do {
> + for (j = 0; j < QHT_BUCKET_ENTRIES; j++) {
> + if (atomic_read(&b->pointers[j]) == NULL) {
> + break;
> + }
> + entries++;
> + }
> + buckets++;
> + b = atomic_rcu_read(&b->next);
> + } while (b);
> + } while (seqlock_read_retry(&head->sequence, version));
> +
> + if (entries) {
> + qdist_inc(&stats->chain, buckets);
> + qdist_inc(&stats->occupancy,
> + (double)entries / QHT_BUCKET_ENTRIES / buckets);
> + stats->used_head_buckets++;
> + stats->entries += entries;
> + } else {
> + qdist_inc(&stats->occupancy, 0);
> + }
> + }
> +}
> +
> +void qht_statistics_destroy(struct qht_stats *stats)
> +{
> + qdist_destroy(&stats->occupancy);
> + qdist_destroy(&stats->chain);
> +}
