socket Rx

tasklet_init(&ap->tsk, ppp_async_process, (unsigned long) ap);
ppp_async_process -> ppp_input -> ppp_do_recv -> ppp_receive_frame
->ppp_receive_nonmp_frame ->netif_rx ->enqueue_to_backlog ->____napi_schedule -> __raise_softirq_irqoff(NET_RX_SOFTIRQ);

net_rx_action -> n->poll(n, weight)[process_backlog] -> __netif_receive_skb -> deliver_skb
-> pt_prev->func[ip_rcv] -> ip_rcv_finish -> ip_local_deliver -> ip_local_deliver_finish -> udp_rcv
-> __udp4_lib_rcv

[   66.927588:0] Backtrace:
[   66.930335:0] [<c041d9a4>] (tcp_recvmsg+0x0/0x4c) from [<c043e1f4>] (inet_recvmsg+0xa8/0xc0)
[   66.938793:0]  r4:dc3d0aa8 r3:00002000
[   66.942682:0] [<c043e14c>] (inet_recvmsg+0x0/0xc0) from [<c03c0684>] (sock_recvmsg+0xac/0xc8)
[   66.951225:0]  r8:00000000 r7:00000000 r6:00000000 r5:00002000 r4:dcc71de8
[   66.958300:0] [<c03c05d8>] (sock_recvmsg+0x0/0xc8) from [<c03c0d7c>] (sys_recvfrom+0x90/0xe8)
[   66.966844:0]  r9:dcc70000 r8:00000000 r6:4229a438 r5:e1592e78 r4:00002000
[   66.973932:0] [<c03c0cec>] (sys_recvfrom+0x0/0xe8) from [<c000e040>] (ret_fast_syscall+0x0/0x30)