On 14/05/16 06:34, Emilio G. Cota wrote:
> +/*
> + * 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).
> + */
> +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;
> +
> + for (;;) {
> + map = qht_map__atomic_mb(ht);
> + b = qht_map_to_bucket(map, hash);
> +
> + qemu_spin_lock(&b->lock);
> + if (likely(!map->stale)) {
> + *pmap = map;
> + return b;
> + }
> + qemu_spin_unlock(&b->lock);
> +
> + /* resize in progress; wait until it completes */
> + while (qemu_spin_locked(&ht->lock)) {
> + cpu_relax();
> + }
> + }
> +}
What if we turn qht::lock into a mutex and change the function as follows:
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);
/* 'ht->map' access is serialized by 'b->lock' here */
if (likely(map == ht->map)) {
/* no resize in progress; we're done */
*pmap = map;
return b;
}
qemu_spin_unlock(&b->lock);
/* resize in progress; retry grabbing 'ht->lock' */
qemu_mutex_lock(&ht->lock);
b = qht_map_to_bucket(ht->map, hash);
*pmap = ht->map;
qemu_spin_lock(&b->lock);
qemu_mutex_unlock(&ht->lock);
return b;
}
With this implementation we could:
(1) get rid of qht_map::stale
(2) don't waste cycles waiting for resize to complete
Kind regards,
Sergey
