Commit ccd57b1b authored by Herbert Xu's avatar Herbert Xu Committed by David S. Miller

rhashtable: Add immediate rehash during insertion

This patch reintroduces immediate rehash during insertion.  If
we find during insertion that the table is full or the chain
length exceeds a set limit (currently 16 but may be disabled
with insecure_elasticity) then we will force an immediate rehash.
The rehash will contain an expansion if the table utilisation
exceeds 75%.

If this rehash fails then the insertion will fail.  Otherwise the
insertion will be reattempted in the new hash table.
Signed-off-by: default avatarHerbert Xu <>
Acked-by: default avatarThomas Graf <>
Signed-off-by: default avatarDavid S. Miller <>
parent b9ecfdaa
......@@ -103,6 +103,7 @@ struct rhashtable;
* @max_size: Maximum size while expanding
* @min_size: Minimum size while shrinking
* @nulls_base: Base value to generate nulls marker
* @insecure_elasticity: Set to true to disable chain length checks
* @locks_mul: Number of bucket locks to allocate per cpu (default: 128)
* @hashfn: Hash function (default: jhash2 if !(key_len % 4), or jhash)
* @obj_hashfn: Function to hash object
......@@ -116,6 +117,7 @@ struct rhashtable_params {
unsigned int max_size;
unsigned int min_size;
u32 nulls_base;
bool insecure_elasticity;
size_t locks_mul;
rht_hashfn_t hashfn;
rht_obj_hashfn_t obj_hashfn;
......@@ -127,6 +129,7 @@ struct rhashtable_params {
* @tbl: Bucket table
* @nelems: Number of elements in table
* @key_len: Key length for hashfn
* @elasticity: Maximum chain length before rehash
* @p: Configuration parameters
* @run_work: Deferred worker to expand/shrink asynchronously
* @mutex: Mutex to protect current/future table swapping
......@@ -137,6 +140,7 @@ struct rhashtable {
atomic_t nelems;
bool being_destroyed;
unsigned int key_len;
unsigned int elasticity;
struct rhashtable_params p;
struct work_struct run_work;
struct mutex mutex;
......@@ -266,6 +270,17 @@ static inline bool rht_shrink_below_30(const struct rhashtable *ht,
tbl->size > ht->p.min_size;
* rht_grow_above_100 - returns true if nelems > table-size
* @ht: hash table
* @tbl: current table
static inline bool rht_grow_above_100(const struct rhashtable *ht,
const struct bucket_table *tbl)
return atomic_read(&ht->nelems) > tbl->size;
/* The bucket lock is selected based on the hash and protects mutations
* on a group of hash buckets.
......@@ -307,6 +322,7 @@ int rhashtable_init(struct rhashtable *ht,
int rhashtable_insert_slow(struct rhashtable *ht, const void *key,
struct rhash_head *obj,
struct bucket_table *old_tbl);
int rhashtable_insert_rehash(struct rhashtable *ht);
int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter);
void rhashtable_walk_exit(struct rhashtable_iter *iter);
......@@ -529,12 +545,14 @@ static inline int __rhashtable_insert_fast(
.ht = ht,
.key = key,
int err = -EEXIST;
struct bucket_table *tbl, *new_tbl;
struct rhash_head *head;
spinlock_t *lock;
unsigned elasticity;
unsigned hash;
int err;
tbl = rht_dereference_rcu(ht->tbl, ht);
......@@ -557,20 +575,34 @@ static inline int __rhashtable_insert_fast(
new_tbl = rht_dereference_rcu(tbl->future_tbl, ht);
if (unlikely(new_tbl)) {
err = rhashtable_insert_slow(ht, key, obj, new_tbl);
if (err == -EAGAIN)
goto slow_path;
goto out;
if (!key)
goto skip_lookup;
if (unlikely(rht_grow_above_100(ht, tbl))) {
err = rhashtable_insert_rehash(ht);
if (err)
return err;
goto restart;
err = -EEXIST;
elasticity = ht->elasticity;
rht_for_each(head, tbl, hash) {
if (unlikely(!(params.obj_cmpfn ?
if (key &&
unlikely(!(params.obj_cmpfn ?
params.obj_cmpfn(&arg, rht_obj(ht, head)) :
rhashtable_compare(&arg, rht_obj(ht, head)))))
goto out;
if (!--elasticity)
goto slow_path;
err = 0;
head = rht_dereference_bucket(tbl->buckets[hash], tbl, hash);
......@@ -375,21 +375,76 @@ unlock:
static bool rhashtable_check_elasticity(struct rhashtable *ht,
struct bucket_table *tbl,
unsigned hash)
unsigned elasticity = ht->elasticity;
struct rhash_head *head;
rht_for_each(head, tbl, hash)
if (!--elasticity)
return true;
return false;
int rhashtable_insert_rehash(struct rhashtable *ht)
struct bucket_table *old_tbl;
struct bucket_table *new_tbl;
struct bucket_table *tbl;
unsigned int size;
int err;
old_tbl = rht_dereference_rcu(ht->tbl, ht);
tbl = rhashtable_last_table(ht, old_tbl);
size = tbl->size;
if (rht_grow_above_75(ht, tbl))
size *= 2;
/* More than two rehashes (not resizes) detected. */
else if (WARN_ON(old_tbl != tbl && old_tbl->size == size))
return -EBUSY;
new_tbl = bucket_table_alloc(ht, size, GFP_ATOMIC);
if (new_tbl == NULL)
return -ENOMEM;
err = rhashtable_rehash_attach(ht, tbl, new_tbl);
if (err) {
if (err == -EEXIST)
err = 0;
} else
return err;
int rhashtable_insert_slow(struct rhashtable *ht, const void *key,
struct rhash_head *obj,
struct bucket_table *tbl)
struct rhash_head *head;
unsigned hash;
int err = -EEXIST;
int err;
tbl = rhashtable_last_table(ht, tbl);
hash = head_hashfn(ht, tbl, obj);
spin_lock_nested(rht_bucket_lock(tbl, hash), SINGLE_DEPTH_NESTING);
err = -EEXIST;
if (key && rhashtable_lookup_fast(ht, key, ht->p))
goto exit;
err = -EAGAIN;
if (rhashtable_check_elasticity(ht, tbl, hash) ||
rht_grow_above_100(ht, tbl))
goto exit;
err = 0;
head = rht_dereference_bucket(tbl->buckets[hash], tbl, hash);
......@@ -678,6 +733,9 @@ int rhashtable_init(struct rhashtable *ht,
ht->p.min_size = max(ht->p.min_size, HASH_MIN_SIZE);
if (!params->insecure_elasticity)
ht->elasticity = 16;
if (params->locks_mul)
ht->p.locks_mul = roundup_pow_of_two(params->locks_mul);
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