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

["De Lara Guarch, Pablo" <pablo.de.lara.guarch@intel.com>]

Hi Thomas,

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> Sent: Tuesday, March 22, 2016 11:42 AM
> To: De Lara Guarch, Pablo; Gonzalez Monroy, Sergio
> Cc: dev@dpdk.org; Dhana Eadala; Richardson, Bruce; Qiu, Michael
> Subject: Re: [dpdk-dev] [PATCH] hash: fix memcmp function pointer in multi-
> process environment
> 
> Hi,
> 
> Pablo, Sergio, please could you help with this issue?

I agree this is not the best way to fix this. I will try to have a fix without having to use ifdefs.

Thanks,
Pablo
> 
> 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.