[dpdk-dev] [PATCH] hash: fix memcmp function pointer in multi-process

[Dhananjaya Reddy Eadala <edreddy@gmail.com>]

Hi

We found a problem in dpdk-2.2 using under multi-process environment.
Here is the brief description how we are using the dpdk:

We have two processes proc1, proc2 using dpdk. These proc1 and proc2 are
two different compiled binaries.
proc1 is started as primary process and proc2 as secondary process.

proc1:
Calls srcHash = rte_hash_create("src_hash_name") to create rte_hash
structure.
As part of this, this api initalized the rte_hash structure and set the
srcHash->rte_hash_cmp_eq to the address of memcmp() from proc1 address
space.

proc2:
calls srcHash =  rte_hash_find_existing("src_hash_name"). This returns the
rte_hash created by proc1.
This srcHash->rte_hash_cmp_eq still points to the address of memcmp() from
proc1 address space.
Later proc2  calls rte_hash_lookup_with_hash(srcHash, (const void*) &key,
key.sig);
Under the hood, rte_hash_lookup_with_hash() invokes
__rte_hash_lookup_with_hash(), which in turn calls h->rte_hash_cmp_eq(key,
k->key, h->key_len).
This leads to a crash as h->rte_hash_cmp_eq is an address from proc1
address space and is invalid address in proc2 address space.

We found, from dpdk documentation, that
"
 The use of function pointers between multiple processes running based of
different compiled
 binaries is not supported, since the location of a given function in one
process may be different to
 its location in a second. This prevents the librte_hash library from
behaving properly as in a  multi-
 threaded instance, since it uses a pointer to the hash function internally.


 To work around this issue, it is recommended that multi-process
applications perform the hash
 calculations by directly calling the hashing function from the code and
then using the
 rte_hash_add_with_hash()/rte_hash_lookup_with_hash() functions instead of
the functions which do
 the hashing internally, such as rte_hash_add()/rte_hash_lookup().
"

We did follow the recommended steps by invoking rte_hash_lookup_with_hash().
It was no issue up to and including dpdk-2.0. In later releases started
crashing because rte_hash_cmp_eq is introduced in dpdk-2.1

We fixed it with the following patch and would like to submit the patch to
dpdk.org.
Patch is created such that, if anyone wanted to use dpdk in multi-process
environment with function pointers not shared, they need to
define RTE_LIB_MP_NO_FUNC_PTR in their Makefile. Without defining this flag
in Makefile, it works as it is now.


Please find here attached is the patch file.

Thanks
Dhana