[v2 PATCH 0/10] rhashtable: Multiple rehashing

@@ -477,6 +477,9 @@ next:
 		iter->skip = 0;
 	}
 
+	/* Ensure we see any new tables. */
+	smp_rmb();
+
 	iter->walker->tbl = rht_dereference_rcu(tbl->future_tbl, ht);
 	if (iter->walker->tbl) {
 		iter->slot = 0;

@@ -19,6 +19,7 @@
 
 #include <linux/compiler.h>
 #include <linux/errno.h>
+#include <linux/jhash.h>
 #include <linux/list_nulls.h>
 #include <linux/workqueue.h>
 #include <linux/mutex.h>

@@ -103,7 +104,7 @@ struct rhashtable;
  * @min_size: Minimum size while shrinking
  * @nulls_base: Base value to generate nulls marker
  * @locks_mul: Number of bucket locks to allocate per cpu (default: 128)
- * @hashfn: Function to hash key
+ * @hashfn: Hash function (default: jhash2 if !(key_len % 4), or jhash)
  * @obj_hashfn: Function to hash object
  * @obj_cmpfn: Function to compare key with object
  */

@@ -125,6 +126,7 @@ struct rhashtable_params {
  * struct rhashtable - Hash table handle
  * @tbl: Bucket table
  * @nelems: Number of elements in table
+ * @key_len: Key length for hashfn
  * @p: Configuration parameters
  * @run_work: Deferred worker to expand/shrink asynchronously
  * @mutex: Mutex to protect current/future table swapping

@@ -134,6 +136,7 @@ struct rhashtable {
 	struct bucket_table __rcu	*tbl;
 	atomic_t			nelems;
 	bool                            being_destroyed;
+	unsigned int			key_len;
 	struct rhashtable_params	p;
 	struct work_struct		run_work;
 	struct mutex                    mutex;

@@ -199,12 +202,30 @@ static inline unsigned int rht_key_hashfn(
 	struct rhashtable *ht, const struct bucket_table *tbl,
 	const void *key, const struct rhashtable_params params)
 {
-	unsigned key_len = __builtin_constant_p(params.key_len) ?
-			   (params.key_len ?: ht->p.key_len) :
-			   params.key_len;
+	unsigned hash;
+
+	if (!__builtin_constant_p(params.key_len))
+		hash = ht->p.hashfn(key, ht->key_len, tbl->hash_rnd);
+	else if (params.key_len) {
+		unsigned key_len = params.key_len;
+
+		if (params.hashfn)
+			hash = params.hashfn(key, key_len, tbl->hash_rnd);
+		else if (key_len & (sizeof(u32) - 1))
+			hash = jhash(key, key_len, tbl->hash_rnd);
+		else
+			hash = jhash2(key, key_len / sizeof(u32),
+				      tbl->hash_rnd);
+	} else {
+		unsigned key_len = ht->p.key_len;
+
+		if (params.hashfn)
+			hash = params.hashfn(key, key_len, tbl->hash_rnd);
+		else
+			hash = jhash(key, key_len, tbl->hash_rnd);
+	}
 
-	return rht_bucket_index(tbl, params.hashfn(key, key_len,
-						   tbl->hash_rnd));
+	return rht_bucket_index(tbl, hash);
 }
 
 static inline unsigned int rht_head_hashfn(

@@ -532,6 +532,11 @@ static size_t rounded_hashtable_size(const struct rhashtable_params *params)
 		   (unsigned long)params->min_size);
 }
 
+static u32 rhashtable_jhash2(const void *key, u32 length, u32 seed)
+{
+	return jhash2(key, length, seed);
+}
+
 /**
  * rhashtable_init - initialize a new hash table
  * @ht:		hash table to be initialized

@@ -583,7 +588,7 @@ int rhashtable_init(struct rhashtable *ht,
 
 	size = HASH_DEFAULT_SIZE;
 
-	if ((!(params->key_len && params->hashfn) && !params->obj_hashfn) ||
+	if ((!params->key_len && !params->obj_hashfn) ||
 	    (params->obj_hashfn && !params->obj_cmpfn))
 		return -EINVAL;
 

@@ -610,6 +615,16 @@ int rhashtable_init(struct rhashtable *ht,
 	else
 		ht->p.locks_mul = BUCKET_LOCKS_PER_CPU;
 
+	ht->key_len = ht->p.key_len;
+	if (!params->hashfn) {
+		ht->p.hashfn = jhash;
+
+		if (!(ht->key_len & (sizeof(u32) - 1))) {
+			ht->key_len /= sizeof(u32);
+			ht->p.hashfn = rhashtable_jhash2;
+		}
+	}
+
 	tbl = bucket_table_alloc(ht, size);
 	if (tbl == NULL)
 		return -ENOMEM;

@@ -199,8 +199,11 @@ static inline unsigned int rht_key_hashfn(
 	struct rhashtable *ht, const struct bucket_table *tbl,
 	const void *key, const struct rhashtable_params params)
 {
-	return rht_bucket_index(tbl, params.hashfn(key, params.key_len ?:
-							ht->p.key_len,
+	unsigned key_len = __builtin_constant_p(params.key_len) ?
+			   (params.key_len ?: ht->p.key_len) :
+			   params.key_len;
+
+	return rht_bucket_index(tbl, params.hashfn(key, key_len,
 						   tbl->hash_rnd));
 }
 

@@ -3133,13 +3133,12 @@ static inline u32 netlink_hash(const void *data, u32 seed)
 	struct netlink_compare_arg arg;
 
 	netlink_compare_arg_init(&arg, sock_net(&nlk->sk), nlk->portid);
-	return jhash(&arg, netlink_compare_arg_len, seed);
+	return jhash2((u32 *)&arg, netlink_compare_arg_len / sizeof(u32), seed);
 }
 
 static const struct rhashtable_params netlink_rhashtable_params = {
 	.head_offset = offsetof(struct netlink_sock, node),
 	.key_len = netlink_compare_arg_len,
-	.hashfn = jhash,
 	.obj_hashfn = netlink_hash,
 	.obj_cmpfn = netlink_compare,
 	.max_size = 65536,

@@ -35,7 +35,6 @@
  */
 
 #include <linux/rhashtable.h>
-#include <linux/jhash.h>
 #include "core.h"
 #include "name_table.h"
 #include "node.h"

@@ -2294,7 +2293,6 @@ static const struct rhashtable_params tsk_rht_params = {
 	.head_offset = offsetof(struct tipc_sock, node),
 	.key_offset = offsetof(struct tipc_sock, portid),
 	.key_len = sizeof(u32), /* portid */
-	.hashfn = jhash,
 	.max_size = 1048576,
 	.min_size = 256,
 };

@@ -13,7 +13,6 @@
 #include <linux/module.h>
 #include <linux/list.h>
 #include <linux/log2.h>
-#include <linux/jhash.h>
 #include <linux/netlink.h>
 #include <linux/rhashtable.h>
 #include <linux/netfilter.h>

@@ -169,7 +168,6 @@ static unsigned int nft_hash_privsize(const struct nlattr * const nla[])
 static const struct rhashtable_params nft_hash_params = {
 	.head_offset = offsetof(struct nft_hash_elem, node),
 	.key_offset = offsetof(struct nft_hash_elem, key),
-	.hashfn = jhash,
 };
 
 static int nft_hash_init(const struct nft_set *set,

@@ -252,19 +252,6 @@ static inline bool rht_grow_above_75(const struct rhashtable *ht,
 	       (!ht->p.max_size || tbl->size < ht->p.max_size);
 }
 
-/**
- * rht_shrink_below_30 - returns true if nelems < 0.3 * table-size
- * @ht:		hash table
- * @tbl:	current table
- */
-static inline bool rht_shrink_below_30(const struct rhashtable *ht,
-				       const struct bucket_table *tbl)
-{
-	/* Shrink table beneath 30% load */
-	return atomic_read(&ht->nelems) < (tbl->size * 3 / 10) &&
-	       tbl->size > ht->p.min_size;
-}
-
 /* The bucket lock is selected based on the hash and protects mutations
  * on a group of hash buckets.
  *

@@ -745,8 +732,6 @@ static inline int rhashtable_remove_fast(
 		goto out;
 
 	atomic_dec(&ht->nelems);
-	if (rht_shrink_below_30(ht, tbl))
-		schedule_work(&ht->run_work);
 
 out:
 	rcu_read_unlock();

@@ -261,30 +261,48 @@ EXPORT_SYMBOL_GPL(rhashtable_expand);
  * rhashtable_shrink - Shrink hash table while allowing concurrent lookups
  * @ht:		the hash table to shrink
  *
- * This function may only be called in a context where it is safe to call
- * synchronize_rcu(), e.g. not within a rcu_read_lock() section.
- *
- * The caller must ensure that no concurrent resizing occurs by holding
- * ht->mutex.
- *
- * The caller must ensure that no concurrent table mutations take place.
- * It is however valid to have concurrent lookups if they are RCU protected.
+ * This function shrinks the hash table to fit, i.e., the smallest
+ * size would not cause it to expand right away automatically.
  *
  * It is valid to have concurrent insertions and deletions protected by per
  * bucket locks or concurrent RCU protected lookups and traversals.
  */
 int rhashtable_shrink(struct rhashtable *ht)
 {
-	struct bucket_table *new_tbl, *old_tbl = rht_dereference(ht->tbl, ht);
+	unsigned size = roundup_pow_of_two(atomic_read(&ht->nelems) * 4 / 3);
+	struct bucket_table *new_tbl;
+	struct bucket_table *tbl;
+	int err;
 
-	ASSERT_RHT_MUTEX(ht);
+	if (size < ht->p.min_size)
+		size = ht->p.min_size;
 
-	new_tbl = bucket_table_alloc(ht, old_tbl->size / 2);
+	new_tbl = bucket_table_alloc(ht, size);
 	if (new_tbl == NULL)
 		return -ENOMEM;
 
+	err = -EEXIST;
+
+	mutex_lock(&ht->mutex);
+	tbl = rht_dereference(ht->tbl, ht);
+	if (rht_dereference(tbl->future_tbl, ht))
+		goto out_free_tbl;
+
+	err = 0;
+	if (tbl->size <= size)
+		goto out_free_tbl;
+
 	rhashtable_rehash(ht, new_tbl);
-	return 0;
+
+	mutex_unlock(&ht->mutex);
+
+out:
+	return err;
+
+out_free_tbl:
+	mutex_unlock(&ht->mutex);
+	bucket_table_free(new_tbl);
+	goto out;
 }
 EXPORT_SYMBOL_GPL(rhashtable_shrink);
 

@@ -302,8 +320,6 @@ static void rht_deferred_worker(struct work_struct *work)
 
 	if (rht_grow_above_75(ht, tbl))
 		rhashtable_expand(ht);
-	else if (rht_shrink_below_30(ht, tbl))
-		rhashtable_shrink(ht);
 unlock:
 	mutex_unlock(&ht->mutex);
 }

@@ -167,17 +167,13 @@ static int __init test_rhashtable(struct rhashtable *ht)
 		rcu_read_unlock();
 	}
 
-	for (i = 0; i < TEST_NEXPANDS; i++) {
-		pr_info("  Table shrinkage iteration %u...\n", i);
-		mutex_lock(&ht->mutex);
-		rhashtable_shrink(ht);
-		mutex_unlock(&ht->mutex);
+	pr_info("  Table shrinkage...\n");
+	rhashtable_shrink(ht);
 
-		rcu_read_lock();
-		pr_info("  Verifying lookups...\n");
-		test_rht_lookup(ht);
-		rcu_read_unlock();
-	}
+	rcu_read_lock();
+	pr_info("  Verifying lookups...\n");
+	test_rht_lookup(ht);
+	rcu_read_unlock();
 
 	rcu_read_lock();
 	test_bucket_stats(ht, true);

@@ -58,7 +58,8 @@ EXPORT_SYMBOL_GPL(lockdep_rht_bucket_is_held);
 #endif
 
 
-static int alloc_bucket_locks(struct rhashtable *ht, struct bucket_table *tbl)
+static int alloc_bucket_locks(struct rhashtable *ht, struct bucket_table *tbl,
+			      gfp_t gfp)
 {
 	unsigned int i, size;
 #if defined(CONFIG_PROVE_LOCKING)

@@ -75,12 +76,13 @@ static int alloc_bucket_locks(struct rhashtable *ht, struct bucket_table *tbl)
 
 	if (sizeof(spinlock_t) != 0) {
 #ifdef CONFIG_NUMA
-		if (size * sizeof(spinlock_t) > PAGE_SIZE)
+		if (size * sizeof(spinlock_t) > PAGE_SIZE &&
+		    gfp == GFP_KERNEL)
 			tbl->locks = vmalloc(size * sizeof(spinlock_t));
 		else
 #endif
 		tbl->locks = kmalloc_array(size, sizeof(spinlock_t),
-					   GFP_KERNEL);
+					   gfp);
 		if (!tbl->locks)
 			return -ENOMEM;
 		for (i = 0; i < size; i++)

@@ -105,23 +107,25 @@ static void bucket_table_free_rcu(struct rcu_head *head)
 }
 
 static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
-					       size_t nbuckets)
+					       size_t nbuckets,
+					       gfp_t gfp)
 {
 	struct bucket_table *tbl = NULL;
 	size_t size;
 	int i;
 
 	size = sizeof(*tbl) + nbuckets * sizeof(tbl->buckets[0]);
-	if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
-		tbl = kzalloc(size, GFP_KERNEL | __GFP_NOWARN | __GFP_NORETRY);
-	if (tbl == NULL)
+	if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER) ||
+	    gfp != GFP_KERNEL)
+		tbl = kzalloc(size, gfp | __GFP_NOWARN | __GFP_NORETRY);
+	if (tbl == NULL && gfp == GFP_KERNEL)
 		tbl = vzalloc(size);
 	if (tbl == NULL)
 		return NULL;
 
 	tbl->size = nbuckets;
 
-	if (alloc_bucket_locks(ht, tbl) < 0) {
+	if (alloc_bucket_locks(ht, tbl, gfp) < 0) {
 		bucket_table_free(tbl);
 		return NULL;
 	}

@@ -273,7 +277,7 @@ static int rhashtable_expand(struct rhashtable *ht)
 
 	old_tbl = rhashtable_last_table(ht, old_tbl);
 
-	new_tbl = bucket_table_alloc(ht, old_tbl->size * 2);
+	new_tbl = bucket_table_alloc(ht, old_tbl->size * 2, GFP_KERNEL);
 	if (new_tbl == NULL)
 		return -ENOMEM;
 

@@ -304,7 +308,7 @@ int rhashtable_shrink(struct rhashtable *ht)
 	if (size < ht->p.min_size)
 		size = ht->p.min_size;
 
-	new_tbl = bucket_table_alloc(ht, size);
+	new_tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
 	if (new_tbl == NULL)
 		return -ENOMEM;
 

@@ -678,7 +682,7 @@ int rhashtable_init(struct rhashtable *ht,
 		}
 	}
 
-	tbl = bucket_table_alloc(ht, size);
+	tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
 	if (tbl == NULL)
 		return -ENOMEM;
 

@@ -294,7 +294,6 @@ int rhashtable_insert_slow(struct rhashtable *ht, const void *key,
 			   struct rhash_head *obj,
 			   struct bucket_table *old_tbl);
 
-int rhashtable_expand(struct rhashtable *ht);
 int rhashtable_shrink(struct rhashtable *ht);
 
 int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter);

@@ -527,17 +526,22 @@ static inline int __rhashtable_insert_fast(
 	rcu_read_lock();
 
 	tbl = rht_dereference_rcu(ht->tbl, ht);
-	hash = rht_head_hashfn(ht, tbl, obj, params);
-	lock = rht_bucket_lock(tbl, hash);
-
-	spin_lock_bh(lock);
 
-	/* Because we have already taken the bucket lock in tbl,
-	 * if we find that future_tbl is not yet visible then
-	 * that guarantees all other insertions of the same entry
-	 * will also grab the bucket lock in tbl because until
-	 * the rehash completes ht->tbl won't be changed.
+	/* All insertions must grab the oldest table containing
+	 * the hashed bucket that is yet to be rehashed.
 	 */
+	for (;;) {
+		hash = rht_head_hashfn(ht, tbl, obj, params);
+		lock = rht_bucket_lock(tbl, hash);
+		spin_lock_bh(lock);
+
+		if (tbl->rehash <= hash)
+			break;
+
+		spin_unlock_bh(lock);
+		tbl = rht_dereference_rcu(tbl->future_tbl, ht);
+	}
+
 	new_tbl = rht_dereference_rcu(tbl->future_tbl, ht);
 	if (unlikely(new_tbl)) {
 		err = rhashtable_insert_slow(ht, key, obj, new_tbl);

@@ -136,11 +136,24 @@ static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
 	return tbl;
 }
 
+static struct bucket_table *rhashtable_last_table(struct rhashtable *ht,
+						  struct bucket_table *tbl)
+{
+	struct bucket_table *new_tbl;
+
+	do {
+		new_tbl = tbl;
+		tbl = rht_dereference_rcu(tbl->future_tbl, ht);
+	} while (tbl);
+
+	return new_tbl;
+}
+
 static int rhashtable_rehash_one(struct rhashtable *ht, unsigned old_hash)
 {
 	struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
-	struct bucket_table *new_tbl =
-		rht_dereference(old_tbl->future_tbl, ht) ?: old_tbl;
+	struct bucket_table *new_tbl = rhashtable_last_table(ht,
+		rht_dereference_rcu(old_tbl->future_tbl, ht));
 	struct rhash_head __rcu **pprev = &old_tbl->buckets[old_hash];
 	int err = -ENOENT;
 	struct rhash_head *head, *next, *entry;

@@ -196,12 +209,18 @@ static void rhashtable_rehash_chain(struct rhashtable *ht, unsigned old_hash)
 	spin_unlock_bh(old_bucket_lock);
 }
 
-static void rhashtable_rehash(struct rhashtable *ht,
-			      struct bucket_table *new_tbl)
+static int rhashtable_rehash_attach(struct rhashtable *ht,
+				    struct bucket_table *old_tbl,
+				    struct bucket_table *new_tbl)
 {
-	struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
-	struct rhashtable_walker *walker;
-	unsigned old_hash;
+	/* Protect future_tbl using the first bucket lock. */
+	spin_lock_bh(old_tbl->locks);
+
+	/* Did somebody beat us to it? */
+	if (rcu_access_pointer(old_tbl->future_tbl)) {
+		spin_unlock_bh(old_tbl->locks);
+		return -EEXIST;
+	}
 
 	/* Make insertions go into the new, empty table right away. Deletions
 	 * and lookups will be attempted in both tables until we synchronize.

@@ -211,6 +230,22 @@ static void rhashtable_rehash(struct rhashtable *ht,
 	/* Ensure the new table is visible to readers. */
 	smp_wmb();
 
+	spin_unlock_bh(old_tbl->locks);
+
+	return 0;
+}
+
+static int rhashtable_rehash_table(struct rhashtable *ht)
+{
+	struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
+	struct bucket_table *new_tbl;
+	struct rhashtable_walker *walker;
+	unsigned old_hash;
+
+	new_tbl = rht_dereference(old_tbl->future_tbl, ht);
+	if (!new_tbl)
+		return 0;
+
 	for (old_hash = 0; old_hash < old_tbl->size; old_hash++)
 		rhashtable_rehash_chain(ht, old_hash);
 

@@ -225,37 +260,29 @@ static void rhashtable_rehash(struct rhashtable *ht,
 	 * remain.
 	 */
 	call_rcu(&old_tbl->rcu, bucket_table_free_rcu);
+
+	return rht_dereference(new_tbl->future_tbl, ht) ? -EAGAIN : 0;
 }
 
-/**
- * rhashtable_expand - Expand hash table while allowing concurrent lookups
- * @ht:		the hash table to expand
- *
- * A secondary bucket array is allocated and the hash entries are migrated.
- *
- * This function may only be called in a context where it is safe to call
- * synchronize_rcu(), e.g. not within a rcu_read_lock() section.
- *
- * The caller must ensure that no concurrent resizing occurs by holding
- * ht->mutex.
- *
- * It is valid to have concurrent insertions and deletions protected by per
- * bucket locks or concurrent RCU protected lookups and traversals.
- */
-int rhashtable_expand(struct rhashtable *ht)
+static int rhashtable_expand(struct rhashtable *ht)
 {
 	struct bucket_table *new_tbl, *old_tbl = rht_dereference(ht->tbl, ht);
+	int err;
 
 	ASSERT_RHT_MUTEX(ht);
 
+	old_tbl = rhashtable_last_table(ht, old_tbl);
+
 	new_tbl = bucket_table_alloc(ht, old_tbl->size * 2);
 	if (new_tbl == NULL)
 		return -ENOMEM;
 
-	rhashtable_rehash(ht, new_tbl);
-	return 0;
+	err = rhashtable_rehash_attach(ht, old_tbl, new_tbl);
+	if (err)
+		bucket_table_free(new_tbl);
+
+	return err;
 }
-EXPORT_SYMBOL_GPL(rhashtable_expand);
 
 /**
  * rhashtable_shrink - Shrink hash table while allowing concurrent lookups

@@ -281,18 +308,19 @@ int rhashtable_shrink(struct rhashtable *ht)
 	if (new_tbl == NULL)
 		return -ENOMEM;
 
-	err = -EEXIST;
-
 	mutex_lock(&ht->mutex);
 	tbl = rht_dereference(ht->tbl, ht);
-	if (rht_dereference(tbl->future_tbl, ht))
-		goto out_free_tbl;
+	tbl = rhashtable_last_table(ht, tbl);
 
 	err = 0;
 	if (tbl->size <= size)
 		goto out_free_tbl;
 
-	rhashtable_rehash(ht, new_tbl);
+	err = rhashtable_rehash_attach(ht, tbl, new_tbl);
+	if (err)
+		goto out_free_tbl;
+
+	schedule_work(&ht->run_work);
 
 	mutex_unlock(&ht->mutex);
 

@@ -310,6 +338,7 @@ static void rht_deferred_worker(struct work_struct *work)
 {
 	struct rhashtable *ht;
 	struct bucket_table *tbl;
+	int err = 0;
 
 	ht = container_of(work, struct rhashtable, run_work);
 	mutex_lock(&ht->mutex);

@@ -317,11 +346,18 @@ static void rht_deferred_worker(struct work_struct *work)
 		goto unlock;
 
 	tbl = rht_dereference(ht->tbl, ht);
+	tbl = rhashtable_last_table(ht, tbl);
 
 	if (rht_grow_above_75(ht, tbl))
 		rhashtable_expand(ht);
+
+	err = rhashtable_rehash_table(ht);
+
 unlock:
 	mutex_unlock(&ht->mutex);
+
+	if (err)
+		schedule_work(&ht->run_work);
 }
 
 int rhashtable_insert_slow(struct rhashtable *ht, const void *key,

@@ -332,6 +368,7 @@ int rhashtable_insert_slow(struct rhashtable *ht, const void *key,
 	unsigned hash;
 	int err = -EEXIST;
 
+	tbl = rhashtable_last_table(ht, tbl);
 	hash = head_hashfn(ht, tbl, obj);
 	spin_lock_nested(rht_bucket_lock(tbl, hash), SINGLE_DEPTH_NESTING);
 

@@ -151,26 +151,6 @@ static int __init test_rhashtable(struct rhashtable *ht)
 	}
 
 	rcu_read_lock();
-	test_bucket_stats(ht, true);
-	test_rht_lookup(ht);
-	rcu_read_unlock();
-
-	for (i = 0; i < TEST_NEXPANDS; i++) {
-		pr_info("  Table expansion iteration %u...\n", i);
-		mutex_lock(&ht->mutex);
-		rhashtable_expand(ht);
-		mutex_unlock(&ht->mutex);
-
-		rcu_read_lock();
-		pr_info("  Verifying lookups...\n");
-		test_rht_lookup(ht);
-		rcu_read_unlock();
-	}
-
-	pr_info("  Table shrinkage...\n");
-	rhashtable_shrink(ht);
-
-	rcu_read_lock();
 	pr_info("  Verifying lookups...\n");
 	test_rht_lookup(ht);
 	rcu_read_unlock();

@@ -190,6 +170,9 @@ static int __init test_rhashtable(struct rhashtable *ht)
 		kfree(obj);
 	}
 
+	pr_info("  Table shrinkage...\n");
+	rhashtable_shrink(ht);
+
 	return 0;
 
 error:

@@ -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;

@@ -252,6 +256,17 @@ static inline bool rht_grow_above_75(const struct rhashtable *ht,
 	       (!ht->p.max_size || tbl->size < ht->p.max_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.
  *

@@ -517,11 +532,12 @@ 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;
 
 	rcu_read_lock();
 

@@ -543,22 +559,24 @@ static inline int __rhashtable_insert_fast(
 	}
 
 	new_tbl = rht_dereference_rcu(tbl->future_tbl, ht);
-	if (unlikely(new_tbl)) {
+	if (unlikely(new_tbl || rht_grow_above_100(ht, tbl))) {
+slow_path:
 		err = rhashtable_insert_slow(ht, key, obj, new_tbl);
 		goto out;
 	}
 
-	if (!key)
-		goto skip_lookup;
-
+	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;
 	}
 
-skip_lookup:
 	err = 0;
 
 	head = rht_dereference_bucket(tbl->buckets[hash], tbl, hash);

@@ -364,21 +364,80 @@ unlock:
 		schedule_work(&ht->run_work);
 }
 
+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_expand_or_rehash(struct rhashtable *ht,
+				struct bucket_table *tbl)
+{
+	struct bucket_table *old_tbl;
+	struct bucket_table *new_tbl;
+	unsigned int size;
+	int err;
+
+	old_tbl = rht_dereference_rcu(ht->tbl, ht);
+	if (!tbl)
+		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) {
+		bucket_table_free(new_tbl);
+		if (err == -EEXIST)
+			err = 0;
+	} else
+		schedule_work(&ht->run_work);
+
+	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;
+
+	if (!tbl)
+		goto rehash;
 
+restart:
 	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);

@@ -391,6 +450,13 @@ int rhashtable_insert_slow(struct rhashtable *ht, const void *key,
 
 exit:
 	spin_unlock(rht_bucket_lock(tbl, hash));
+ 
+	if (err == -EAGAIN) {
+rehash:
+		err = rhashtable_expand_or_rehash(ht, tbl);
+		if (!err)
+			goto restart;
+	}
 
 	return err;
 }

@@ -667,6 +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);
 	else