Re: [Qemu-devel] [PATCH v3 02/17] qht: return existing entry when qht_insert fails

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

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

> The meaning of "existing" is now changed to "matches in hash and
> ht->cmp result". This is saner than just checking the pointer value.
>
> Suggested-by: Richard Henderson <richard.henderson@linaro.org>
> Reviewed-by:  Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Emilio G. Cota <cota@braap.org>

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

> ---
>  include/qemu/qht.h        |  7 +++++--
>  accel/tcg/translate-all.c |  2 +-
>  tests/qht-bench.c         |  4 ++--
>  tests/test-qht.c          |  8 +++++++-
>  util/qht.c                | 27 +++++++++++++++++----------
>  5 files changed, 32 insertions(+), 16 deletions(-)
>
> diff --git a/include/qemu/qht.h b/include/qemu/qht.h
> index 5f03a0f..1fb9116 100644
> --- a/include/qemu/qht.h
> +++ b/include/qemu/qht.h
> @@ -70,6 +70,7 @@ void qht_destroy(struct qht *ht);
>   * @ht: QHT to insert to
>   * @p: pointer to be inserted
>   * @hash: hash corresponding to @p
> + * @existing: address where the pointer to an existing entry can be copied to
>   *
>   * Attempting to insert a NULL @p is a bug.
>   * Inserting the same pointer @p with different @hash values is a bug.
> @@ -78,9 +79,11 @@ void qht_destroy(struct qht *ht);
>   * inserted into the hash table.
>   *
>   * Returns true on success.
> - * Returns false if the @p-@hash pair already exists in the hash table.
> + * Returns false if there is an existing entry in the table that is equivalent
> + * (i.e. ht->cmp matches and the hash is the same) to @p-@h. If @existing
> + * is !NULL, a pointer to this existing entry is copied to it.
>   */
> -bool qht_insert(struct qht *ht, void *p, uint32_t hash);
> +bool qht_insert(struct qht *ht, void *p, uint32_t hash, void **existing);
>
>  /**
>   * qht_lookup_custom - Look up a pointer using a custom comparison function.
> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> index 5b7b91d..8080eb7 100644
> --- a/accel/tcg/translate-all.c
> +++ b/accel/tcg/translate-all.c
> @@ -1242,7 +1242,7 @@ static void tb_link_page(TranslationBlock *tb, tb_page_addr_t phys_pc,
>      /* add in the hash table */
>      h = tb_hash_func(phys_pc, tb->pc, tb->flags, tb->cflags & CF_HASH_MASK,
>                       tb->trace_vcpu_dstate);
> -    qht_insert(&tb_ctx.htable, tb, h);
> +    qht_insert(&tb_ctx.htable, tb, h, NULL);
>
>  #ifdef CONFIG_USER_ONLY
>      if (DEBUG_TB_CHECK_GATE) {
> diff --git a/tests/qht-bench.c b/tests/qht-bench.c
> index c94ac25..f492b3a 100644
> --- a/tests/qht-bench.c
> +++ b/tests/qht-bench.c
> @@ -163,7 +163,7 @@ static void do_rw(struct thread_info *info)
>              bool written = false;
>
>              if (qht_lookup(&ht, p, hash) == NULL) {
> -                written = qht_insert(&ht, p, hash);
> +                written = qht_insert(&ht, p, hash, NULL);
>              }
>              if (written) {
>                  stats->in++;
> @@ -322,7 +322,7 @@ static void htable_init(void)
>              r = xorshift64star(r);
>              p = &keys[r & (init_range - 1)];
>              hash = h(*p);
> -            if (qht_insert(&ht, p, hash)) {
> +            if (qht_insert(&ht, p, hash, NULL)) {
>                  break;
>              }
>              retries++;
> diff --git a/tests/test-qht.c b/tests/test-qht.c
> index b069881..dda6a06 100644
> --- a/tests/test-qht.c
> +++ b/tests/test-qht.c
> @@ -27,11 +27,17 @@ static void insert(int a, int b)
>
>      for (i = a; i < b; i++) {
>          uint32_t hash;
> +        void *existing;
> +        bool inserted;
>
>          arr[i] = i;
>          hash = i;
>
> -        qht_insert(&ht, &arr[i], hash);
> +        inserted = qht_insert(&ht, &arr[i], hash, NULL);
> +        g_assert_true(inserted);
> +        inserted = qht_insert(&ht, &arr[i], hash, &existing);
> +        g_assert_false(inserted);
> +        g_assert_true(existing == &arr[i]);
>      }
>  }
>
> diff --git a/util/qht.c b/util/qht.c
> index 8610ce3..9d030e7 100644
> --- a/util/qht.c
> +++ b/util/qht.c
> @@ -511,9 +511,9 @@ void *qht_lookup(struct qht *ht, const void *userp, uint32_t 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)
> +static void *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;
> @@ -523,8 +523,9 @@ static bool qht_insert__locked(struct qht *ht, struct qht_map *map,
>      do {
>          for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
>              if (b->pointers[i]) {
> -                if (unlikely(b->pointers[i] == p)) {
> -                    return false;
> +                if (unlikely(b->hashes[i] == hash &&
> +                             ht->cmp(b->pointers[i], p))) {
> +                    return b->pointers[i];
>                  }
>              } else {
>                  goto found;
> @@ -553,7 +554,7 @@ static bool qht_insert__locked(struct qht *ht, struct qht_map *map,
>      atomic_set(&b->hashes[i], hash);
>      atomic_set(&b->pointers[i], p);
>      seqlock_write_end(&head->sequence);
> -    return true;
> +    return NULL;
>  }
>
>  static __attribute__((noinline)) void qht_grow_maybe(struct qht *ht)
> @@ -577,25 +578,31 @@ static __attribute__((noinline)) void qht_grow_maybe(struct qht *ht)
>      qemu_mutex_unlock(&ht->lock);
>  }
>
> -bool qht_insert(struct qht *ht, void *p, uint32_t hash)
> +bool qht_insert(struct qht *ht, void *p, uint32_t hash, void **existing)
>  {
>      struct qht_bucket *b;
>      struct qht_map *map;
>      bool needs_resize = false;
> -    bool ret;
> +    void *prev;
>
>      /* 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);
> +    prev = 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;
> +    if (likely(prev == NULL)) {
> +        return true;
> +    }
> +    if (existing) {
> +        *existing = prev;
> +    }
> +    return false;
>  }
>
>  static inline bool qht_entry_is_last(struct qht_bucket *b, int pos)


--
Alex Bennée