<snip>
> Subject: [PATCH] hash: document breakage with multi-writer thread
>
> The code in rte_cuckoo_hash multi-writer support is broken if write
> operations are called from a non-EAL thread.
>
> rte_lcore_id() wil return LCORE_ID_ANY (UINT32_MAX) for non EAL thread
> and that leads to using wrong local cache.
>
> Add error checks and document the restriction.
Having multiple non-EAL writer threads is a valid use case. Should we fix the
issue instead?
>
> Fixes: 9d033dac7d7c ("hash: support no free on delete")
> Fixes: 5915699153d7 ("hash: fix scaling by reducing contention")
> Signed-off-by: Stephen Hemminger <step...@networkplumber.org>
> Cc: honnappa.nagaraha...@arm.com
> Cc: pablo.de.lara.gua...@intel.com
> ---
> doc/guides/prog_guide/hash_lib.rst | 1 + lib/librte_hash/rte_cuckoo_hash.c
> | 9 +++++++++
> 2 files changed, 10 insertions(+)
>
> diff --git a/doc/guides/prog_guide/hash_lib.rst
> b/doc/guides/prog_guide/hash_lib.rst
> index d06c7de2ead1..29b41a425a43 100644
> --- a/doc/guides/prog_guide/hash_lib.rst
> +++ b/doc/guides/prog_guide/hash_lib.rst
> @@ -85,6 +85,7 @@ For concurrent writes, and concurrent reads and writes
> the following flag values
>
> * If the multi-writer flag (RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD) is
> set, multiple threads writing to the table is allowed.
> Key add, delete, and table reset are protected from other writer threads.
> With only this flag set, readers are not protected from ongoing writes.
> + The writer threads must be EAL threads, it is not safe to write to a
> multi-
> writer hash table from an interrupt, control or other threads.
>
> * If the read/write concurrency
> (RTE_HASH_EXTRA_FLAGS_RW_CONCURRENCY) is set, multithread
> read/write operation is safe
> (i.e., application does not need to stop the readers from accessing the
> hash
> table until writers finish their updates. Readers and writers can operate on
> the table concurrently).
> diff --git a/lib/librte_hash/rte_cuckoo_hash.c
> b/lib/librte_hash/rte_cuckoo_hash.c
> index 90cb99b0eef8..79c94107a582 100644
> --- a/lib/librte_hash/rte_cuckoo_hash.c
> +++ b/lib/librte_hash/rte_cuckoo_hash.c
> @@ -979,6 +979,9 @@ __rte_hash_add_key_with_hash(const struct
> rte_hash *h, const void *key,
> /* Did not find a match, so get a new slot for storing the new key */
> if (h->use_local_cache) {
> lcore_id = rte_lcore_id();
> + if (lcore_id == LCORE_ID_ANY)
> + return -EINVAL;
> +
> cached_free_slots = &h->local_free_slots[lcore_id];
> /* Try to get a free slot from the local cache */
> if (cached_free_slots->len == 0) {
> @@ -1382,6 +1385,10 @@ remove_entry(const struct rte_hash *h, struct
> rte_hash_bucket *bkt, unsigned i)
>
> if (h->use_local_cache) {
> lcore_id = rte_lcore_id();
> + ERR_IF_TRUE((lcore_id == LCORE_ID_ANY),
> + "%s: attempt to remove entry from non EAL
> thread\n",
> + __func__);
> +
> cached_free_slots = &h->local_free_slots[lcore_id];
> /* Cache full, need to free it. */
> if (cached_free_slots->len == LCORE_CACHE_SIZE) { @@ -
> 1637,6 +1644,8 @@ rte_hash_free_key_with_position(const struct rte_hash
> *h,
>
> if (h->use_local_cache) {
> lcore_id = rte_lcore_id();
> + RETURN_IF_TRUE((lcore_id == LCORE_ID_ANY), -EINVAL);
> +
> cached_free_slots = &h->local_free_slots[lcore_id];
> /* Cache full, need to free it. */
> if (cached_free_slots->len == LCORE_CACHE_SIZE) {
> --
> 2.26.2
