hash: load value after full key compare

hash: load value after full key compare

When a hash entry is added, there are 2 sets of stores.

1) The application writes its data to memory (whose address
is provided in rte_hash_add_key_with_hash_data API (or NULL))
2) The rte_hash library writes to its own internal data structures;
key store entry and the hash table.

The only ordering requirement between these 2 is that - store
to the application data must complete before the store to key_index.
There are no ordering requirements between the stores to
key/signature and store to application data. The synchronization
point for application data can be any point between the 'store to
application data' and 'store to the key_index'. So, 'pdata' should not
be a guard variable for the data in hash table. It should be a guard
variable only for the application data written to the memory location
pointed by 'pdata'. Hence, in the lookup functions, 'pdata' can be
loaded after full key comparison succeeds.

The synchronization point for the application data (store-release
to 'pdata' in key store) is changed to be consistent with the order
of loads in lookup function. However, this change is cosmetic and
does not affect the functionality.

Fixes: e605a1d36 ("hash: add lock-free r/w concurrency")
Cc: stable@dpdk.org
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Tested-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Yipeng Wang <yipeng1.wang@intel.com>