[dpdk-dev] [PATCH] hash: document breakage with multi-writer thread

[Honnappa Nagarahalli]

<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 <stephen at networkplumber.org>
> Cc: honnappa.nagarahalli at arm.com
> Cc: pablo.de.lara.guarch at 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