DPDK Hash failing to lookup data from secondary process - hash

Question

Failing to use existing rte Hash from secondary process:
h = rte_hash_find_existing("some_hash");
if (h) {
// this will work, in case we re-create
//rte_hash_free(h);
}
else {
h = rte_hash_create (&params);
}
// using the hash will crash the process with:
// Program received signal SIGSEGV, Segmentation fault.
ret = rte_hash_lookup_data (h,name,&data);
DPDK Version: dpdk-19.02
Build Mode Static: CONFIG_RTE_BUILD_SHARED_LIB=n
The Primary and secondary processes are different binaries but linked to the same DPDK library
The Key is added in primary as follows
struct cdev_key {
uint64_t len;
};
struct cdev_key key = { 0 };
if (rte_hash_add_key_data (testptr, &key,(void *) &test) < 0) {
fprintf (stderr,"add failed errno: %s\n", rte_strerror(rte_errno));
}
and used in secondary as follows:
printf("Looking for data\n");
struct cdev_key key = { 0 };
int ret = rte_hash_lookup_data (h,&key,&data);

Answer 1

with DPDK version 19.02, I am able to run 2 separate binaries without issues.
[EDIT-1] based on the update in the ticket, I am able to lookup hash entry added from primary in the secondary process.
Priamry log:
rte_hash_count 1 ret:val 0x0:0x0
Secondary log:
0x17fd61380 rte_hash_count 1
rte_hash_count 1 key:val 0:0
note: if using rte_hash_lookup please remember to disable Linux ASLR via echo 0 | tee /proc/sys/kernel/randomize_va_space.
Binary 1: modified example/skeleton to create hash test
CMD-1: ./build/basicfwd -l 5 -w 0000:08:00.1 --vdev=net_tap0 --socket-limit=2048,1 --file-prefix=test
Binary 2: modified helloworld to lookup for hash test, else assert
CMD-2: for i in {1..20000}; do du -kh /var/run/dpdk/; ./build/helloworld -l 6 --proc-type=secondary --log-level=3 --file-prefix=test; done
Changing or removing the file-prefix results in assert logic to be hit.
note: DPDK 19.02 has the inherent bug which does not cleanup the /var/run/dpdk/; hence recommends to use 19.11.2 LTS
Code-1:
struct rte_hash_parameters test = {0};
test.name = "test";
test.entries = 32;
test.key_len = sizeof(uint64_t);
test.hash_func = rte_jhash;
test.hash_func_init_val = 0;
test.socket_id = 0;
struct rte_hash *testptr = rte_hash_create(&test);
if (testptr == NULL) {
rte_panic("Failed to create test hash, errno = %d\n", rte_errno);
}
Code-2:
assert(rte_hash_find_existing("test"));
printf("hello from core %u::%p\n", lcore_id, rte_hash_find_existing("test"));
printf("hello from core %u::%p\n", lcore_id, rte_hash_find_existing("test1"));

Answer 2

As mentioned in DPDK Programmers Guide, using multiprocessor functionalities come with some restrictions. One of them is that the pointer to a function can not be shared between processes. As a result the hashing function is not available on the secondary process. The suggested work around is to the hashing part in the primary process and the secondary process accessing the hash table using the hash value instead of the key.
From DPDK Guide:
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().
Please refer to the guide for more information [36.3. Multi-process Limitations]
link: https://doc.dpdk.org/guides/prog_guide/multi_proc_support.html
In the time of writing this answer the guide is for DPDK 20.08.