net/hns3: fix crash when Tx multiple buffer packets

net/hns3: fix crash when Tx multiple buffer packets

Currently, there is a possibility that segment faults occur when sending
packets whose payloads are stored in multiple buffers based on hns3
network engine. The related core dump information as follows:

Program terminated with signal 11, Segmentation fault.
0  hns3_reassemble_tx_pkts
2512                            temp = temp->next;
Missing separate debuginfos, use:
(gdb) bt
0  hns3_reassemble_tx_pkts
1  0x0000000000969c60 in hns3_check_non_tso_pkt
2  0x000000000096adbc in hns3_xmit_pkts
3  0x000000000050d4d0 in rte_eth_tx_burst
4  0x000000000050fca4 in pkt_burst_transmit
5  0x00000000004ca6b8 in run_pkt_fwd_on_lcore
6  0x00000000004ca7fc in start_pkt_forward_on_core
7  0x00000000006975a4 in eal_thread_loop
8  0x0000ffffa6f7fc48 in start_thread
9  0x0000ffffa6ed1600 in thread_start

The root cause is that hns3 PMD driver invokes the rte_pktmbuf_free_seg
API function to release the same rte_mbuf multiple times. The rte_mbuf
pointer is not set to NULL in the internal function
hns3_rx_queue_release_mbufs which is invoked during queue setup, stop
and close. As a result the rte_mbuf in Rx queues will be repeatedly
released when the user application setup queues or stop/start the dev
for multiple times.
Probably for performance reasons, DPDK mempool lib does not check for
the repeated rte_mbuf releases. The Address of released rte_mbuf are
directly stored into the per lcore cache of the mempool. This makes the
rte_mbufs obtained from mempool by calling rte_mempool_get_bulk API
function repetitively.  ultimately, it causes to access to a NULL
pointer in PMD driver.

This patch fixes this problem by setting released mbuf pointer to NULL
in the internal function named hns3_rx_queue_release_mbuf. And the other
internal function named hns3_reassemble_tx_pkts is optimized to avoid a
similar problem.

Fixes: bba636698316 ("net/hns3: support Rx/Tx and related operations")
Cc: stable@dpdk.org
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>