Re: [dpdk-dev] [PATCH 3/4] hash: fix rw concurrency while moving keys

["Wang, Yipeng1" <yipeng1.wang@intel.com>]

Reply inlined:

>-----Original Message-----
>From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Honnappa Nagarahalli
>Sent: Thursday, September 6, 2018 10:12 AM
>To: Richardson, Bruce <bruce.richardson@intel.com>; De Lara Guarch, Pablo <pablo.de.lara.guarch@intel.com>
>Cc: dev@dpdk.org; honnappa.nagarahalli@dpdk.org; gavin.hu@arm.com; steve.capper@arm.com; ola.liljedahl@arm.com;
>nd@arm.com; Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
>Subject: [dpdk-dev] [PATCH 3/4] hash: fix rw concurrency while moving keys
>
>Reader-writer concurrency issue, caused by moving the keys
>to their alternative locations during key insert, is solved
>by introducing a global counter(tbl_chng_cnt) indicating a
>change in table.
>
>@@ -662,6 +679,20 @@ rte_hash_cuckoo_move_insert_mw(const struct rte_hash *h,
> 		curr_bkt = curr_node->bkt;
> 	}
>
>+	/* Inform the previous move. The current move need
>+	 * not be informed now as the current bucket entry
>+	 * is present in both primary and secondary.
>+	 * Since there is one writer, load acquires on
>+	 * tbl_chng_cnt are not required.
>+	 */
>+	__atomic_store_n(&h->tbl_chng_cnt,
>+			 h->tbl_chng_cnt + 1,
>+			 __ATOMIC_RELEASE);
>+	/* The stores to sig_alt and sig_current should not
>+	 * move above the store to tbl_chng_cnt.
>+	 */
>+	__atomic_thread_fence(__ATOMIC_RELEASE);
>+
[Wang, Yipeng] I believe for X86 this fence should not be compiled to any code, otherwise
we need macros for the compile time check.

>@@ -926,30 +957,56 @@ __rte_hash_lookup_with_hash(const struct rte_hash *h, const void *key,
> 	uint32_t bucket_idx;
> 	hash_sig_t alt_hash;
> 	struct rte_hash_bucket *bkt;
>+	uint32_t cnt_b, cnt_a;
> 	int ret;
>
>-	bucket_idx = sig & h->bucket_bitmask;
>-	bkt = &h->buckets[bucket_idx];
>-
> 	__hash_rw_reader_lock(h);
>
>-	/* Check if key is in primary location */
>-	ret = search_one_bucket(h, key, sig, data, bkt);
>-	if (ret != -1) {
>-		__hash_rw_reader_unlock(h);
>-		return ret;
>-	}
>-	/* Calculate secondary hash */
>-	alt_hash = rte_hash_secondary_hash(sig);
>-	bucket_idx = alt_hash & h->bucket_bitmask;
>-	bkt = &h->buckets[bucket_idx];
>+	do {
[Wang, Yipeng] As far as I know, the MemC3 paper "MemC3: Compact and Concurrent
MemCache with Dumber Caching and Smarter Hashing"
as well as OvS cmap uses similar version counter to implement read-write concurrency for hash table,
but one difference is reader checks even/odd of the version counter to make sure there is no
concurrent writer. Could you just double check and confirm that this is not needed for your implementation?

>--- a/lib/librte_hash/rte_hash.h
>+++ b/lib/librte_hash/rte_hash.h
>@@ -156,7 +156,7 @@ rte_hash_count(const struct rte_hash *h);
>  *   - -ENOSPC if there is no space in the hash for this key.
>  */
> int
>-rte_hash_add_key_data(const struct rte_hash *h, const void *key, void *data);
>+rte_hash_add_key_data(struct rte_hash *h, const void *key, void *data);
>
> /**
>  * Add a key-value pair with a pre-computed hash value
>@@ -180,7 +180,7 @@ rte_hash_add_key_data(const struct rte_hash *h, const void *key, void *data);
>  *   - -ENOSPC if there is no space in the hash for this key.
>  */
> int32_t
>-rte_hash_add_key_with_hash_data(const struct rte_hash *h, const void *key,
>+rte_hash_add_key_with_hash_data(struct rte_hash *h, const void *key,
> 						hash_sig_t sig, void *data);
>
> /**
>@@ -200,7 +200,7 @@ rte_hash_add_key_with_hash_data(const struct rte_hash *h, const void *key,
>  *     array of user data. This value is unique for this key.
>  */
> int32_t
>-rte_hash_add_key(const struct rte_hash *h, const void *key);
>+rte_hash_add_key(struct rte_hash *h, const void *key);
>
> /**
>  * Add a key to an existing hash table.
>@@ -222,7 +222,7 @@ rte_hash_add_key(const struct rte_hash *h, const void *key);
>  *     array of user data. This value is unique for this key.
>  */
> int32_t
>-rte_hash_add_key_with_hash(const struct rte_hash *h, const void *key, hash_sig_t sig);
>+rte_hash_add_key_with_hash(struct rte_hash *h, const void *key, hash_sig_t sig);
>
> /

I think the above changes will break ABI by changing the parameter type? Other people may know better on this.