8.0-RELEASE-p1 Panic "panic: sbdrop"

[Linda Messerschmidt]

This is a new one on me:

panic: sbdrop
cpuid = 3
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2a
panic() at panic+0x182
sbdrop_internal() at sbdrop_internal+0x323
soisdisconnected() at soisdisconnected+0xbe
tcp_close() at tcp_close+0x45
tcp_do_segment() at tcp_do_segment+0x122f
tcp_input() at tcp_input+0xc92
ip_input() at ip_input+0xac
netisr_dispatch_src() at netisr_dispatch_src+0x7e
ether_demux() at ether_demux+0x15d
ether_input() at ether_input+0x17b
em_rxeof() at em_rxeof+0x287
em_handle_rxtx() at em_handle_rxtx+0x2f
taskqueue_run() at taskqueue_run+0x93
taskqueue_thread_loop() at taskqueue_thread_loop+0x46
fork_exit() at fork_exit+0x118
fork_trampoline() at fork_trampoline+0xe
--- trap 0, rip = 0, rsp = 0xffffff8000117d30, rbp = 0 ---

This machine runs squid as a reverse proxy, and this has happened a
couple of times now in the past day.

Unfortunately it's a production machine, so we'll have to go back to
7.2.  I can probably leave it as-is for 24 hours or so if anybody
wants me to check something, but it doesn't have a dump or a debug
kernel and I unfortunately can't put it back in production to provoke
another crash.  :(   But I wanted to at least report this before we
did in case it's useful to anyone.