Re: [dpdk-dev] [PATCH 4/4] hash: enable lock-free reader-writer concurrency

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> >
> >Add the flag to enable reader-writer concurrency during run time. The
> >rte_hash_del_xxx APIs do not free the keystore element when this flag
> >is enabled. Hence a new API, rte_hash_free_key_with_position, to free
> >the key store element is added.
> >
> >+/** Flag to support lock free reader writer concurrency */ #define
> >+RTE_HASH_EXTRA_FLAGS_RW_CONCURRENCY_LF 0x08
> [Wang, Yipeng] It would be good to indicate that the lockless implementation
> works for single writer multiple readers.
Multi-writers are supported by using the rw-lock or transactional memory. Essentially, we still have single writer. This patch works fine with multi-writer as defined by ' MULTI_WRITER_ADD' flag. I have tested it as well. I will enable this test case in V2.

> Also, if people use a mix of the flags for example set both multiwriter and LF
> flags, then I guess either we need to return an error or maybe multiwriter
> should have higher priority. Currently the RW_CONCURRENCY will assume
> MULTI_WRITER_ADD I think.
As mentioned above, multi-writer and LF combination is supported. Yes, RW_CONCURRENCY currently assumes MULTI_WRITER_ADD. I think we should separate them.

> >+
> > /** Signature of key that is stored internally. */ typedef uint32_t
> > hash_sig_t;
> >
> >@@ -143,6 +148,11 @@ rte_hash_count(const struct rte_hash *h);
> >  * and should only be called from one thread by default.
> >  * Thread safety can be enabled by setting flag during
> >  * table creation.
> >+ * When lock free reader writer concurrency is enabled,
> >+ * if this API is called to update an existing entry,
> >+ * the application should free any memory allocated for
> >+ * previous 'data' only after all the readers have stopped
> >+ * using previous 'data'.
> [Wang, Yipeng] Could you be more specific on this description?
> When add_key API is called, the users do not know if it will update an existing
> entry or inserting a new one, do they?
I think, it will depend on the application. The applications I have worked on so far, added a hash entry as a result of receiving an event and updated it on receiving another event. I can change the comments to indicate that the applications need to be aware of add/update operations.

> 
> >  *
> >  * @param h
> >  *   Hash table to add the key to.
> >@@ -165,6 +175,11 @@ rte_hash_add_key_data(struct rte_hash *h, const
> >void *key, void *data);
> >  * and should only be called from one thread by default.
> >  * Thread safety can be enabled by setting flag during
> >  * table creation.
> >+ * When lock free reader writer concurrency is enabled,
> >+ * if this API is called to update an existing entry,
> >+ * the application should free any memory allocated for
> >+ * previous 'data' only after all the readers have stopped
> >+ * using previous 'data'.
> >  *
> >  * @param h
> >  *   Hash table to add the key to.
> >@@ -230,6 +245,12 @@ rte_hash_add_key_with_hash(struct rte_hash *h,
> >const void *key, hash_sig_t sig);
> >  * and should only be called from one thread by default.
> >  * Thread safety can be enabled by setting flag during
> >  * table creation.
> >+ * If lock free reader writer concurrency is enabled,
> >+ * the hash library's internal memory for the deleted
> >+ * key is not freed. It should be freed by calling
> >+ * rte_hash_free_key_with_position API after all
> >+ * the readers have stopped using the hash entry
> >+ * corresponding to this key.
> >  *
> >  * @param h
> >  *   Hash table to remove the key from.
> >@@ -241,6 +262,8 @@ rte_hash_add_key_with_hash(struct rte_hash *h,
> const void *key, hash_sig_t sig);
> >  *   - A positive value that can be used by the caller as an offset into an
> >  *     array of user data. This value is unique for this key, and is the same
> >  *     value that was returned when the key was added.
> >+ *     When lock free concurrency is enabled, this value should be used
> >+ *     while calling the rte_hash_free_key_with_position API.
> >  */
> > int32_t
> > rte_hash_del_key(const struct rte_hash *h, const void *key); @@ -251,6
> >+274,12 @@ rte_hash_del_key(const struct rte_hash *h, const void *key);
> >  * and should only be called from one thread by default.
> >  * Thread safety can be enabled by setting flag during
> >  * table creation.
> >+ * If lock free reader writer concurrency is enabled,
> >+ * the hash library's internal memory for the deleted
> >+ * key is not freed. It should be freed by calling
> >+ * rte_hash_free_key_with_position API after all
> >+ * the readers have stopped using the hash entry
> >+ * corresponding to this key.
> >  *
> >  * @param h
> >  *   Hash table to remove the key from.
> >@@ -264,6 +293,8 @@ rte_hash_del_key(const struct rte_hash *h, const
> void *key);
> >  *   - A positive value that can be used by the caller as an offset into an
> >  *     array of user data. This value is unique for this key, and is the same
> >  *     value that was returned when the key was added.
> >+ *     When lock free concurrency is enabled, this value should be used
> >+ *     while calling the rte_hash_free_key_with_position API.
> >  */
> > int32_t
> > rte_hash_del_key_with_hash(const struct rte_hash *h, const void *key,
> >hash_sig_t sig); @@ -290,6 +321,30 @@
> rte_hash_get_key_with_position(const struct rte_hash *h, const int32_t
> position,
> > 			       void **key);
> >
> [Wang, Yipeng] If possible, how about having a new delete function instead of
> modifying the current one?
> I think it does not need to be tied with the lockless implementation, it is
> orthogonal to multi-threading implementation.
> people using locks may still want this new deletion behavior.
> If people want old behavior, they can call current API, otherwise they can call
> the new deletion function, followed by Rte_hash_free_key_with_position later.
I like the terms 'delete' and 'free'. I am finding it hard to come up with a good name for the API. It will be on the lines of 'rte_hash_del_key_with_hash_no_free' - I do not like the name much.
Instead, we could have a configuration flag for the hash table, 'RTE_HASH_EXTRA_FLAGS_FREE_MEM_ON_DEL'. If this is enabled, 'rte_hash_del_...' APIs will free the key store index and any internal memory. Enabling lock-free RW concurrency will enable this flag. User can enable this flag explicitly while not using lock-free RW concurrency as well.