vtnet + gif (IPv4 in IPv4) + iperf3 leads to crash on ALPHA6

 I'm preparing some benchmarking setup, which includes gif tunnel from
FreeBSD to FreeBSD, where one end is 12.0-ALPHA6/r338707 installed as
guest in VirtualBox with virtual NIC (vtnet).

 This tunnel works for simple pings, but when I run iperf3 on this
"link" ALPHA4 system crashes. It is 100% reproducible for me.

panic: Assertion !in_epoch(net_epoch_preempt) &&
!mtx_owned(&(&(tcbinfo))->ipi_lock) failed at
/data/src/sys/netinet/tcp_input.c:803
cpuid = 0
time = 1537187018
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame
0xfffffe000044a310
vpanic() at vpanic+0x1a3/frame 0xfffffe000044a370
panic() at panic+0x43/frame 0xfffffe000044a3d0
tcp_input() at tcp_input+0x16a9/frame 0xfffffe000044a520
ip_input() at ip_input+0x126/frame 0xfffffe000044a5a0
netisr_dispatch_src() at netisr_dispatch_src+0x83/frame 0xfffffe000044a600
gif_input() at gif_input+0x2db/frame 0xfffffe000044a640
in_gif_input() at in_gif_input+0x73/frame 0xfffffe000044a680
encap_input() at encap_input+0x1cf/frame 0xfffffe000044a6f0
encap4_input() at encap4_input+0x28/frame 0xfffffe000044a720
ip_input() at ip_input+0x126/frame 0xfffffe000044a7a0
netisr_dispatch_src() at netisr_dispatch_src+0x83/frame 0xfffffe000044a800
ether_demux() at ether_demux+0x15e/frame 0xfffffe000044a830
ether_nh_input() at ether_nh_input+0x373/frame 0xfffffe000044a880
netisr_dispatch_src() at netisr_dispatch_src+0x83/frame 0xfffffe000044a8e0
ether_input() at ether_input+0x42/frame 0xfffffe000044a900
vtnet_rxq_eof() at vtnet_rxq_eof+0x736/frame 0xfffffe000044a9a0
vtnet_rx_vq_intr() at vtnet_rx_vq_intr+0x58/frame 0xfffffe000044a9d0
vtpci_legacy_intr() at vtpci_legacy_intr+0xb0/frame 0xfffffe000044aa10
ithread_loop() at ithread_loop+0x140/frame 0xfffffe000044aa70
fork_exit() at fork_exit+0x84/frame 0xfffffe000044aab0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe000044aab0

-- 
// Lev Serebryakov