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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

Hi,

Pablo, Sergio, please could you help with this issue?

2016-03-13 22:16, Dhana Eadala:
> 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 function call 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);
> 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.

Introducing #ifdef RTE_LIB_MP_NO_FUNC_PTR is not recommended.