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

["Qiu, Michael" <michael.qiu@intel.com>]

On 3/2/2016 2:57 AM, Dhananjaya Reddy Eadala wrote:
> 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.

Could you send the patch in the mail?

Learn how to send a patch:

http://www.dpdk.org/dev

Thanks,
Michael
> 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
>