panic: sbflush_locked

I just got the following panic for a second time in the last three days
doing a "make -jN buildworld".  This is a with a recent copy of HEAD.
If anyone wants more detail, let me know.

panic: sbflush_locked: cc 4 || mb 0xffffff0052afa400 || mbcnt 0
cpuid = 1
KDB: enter: panic
[thread pid 12163 tid 100188 ]
Stopped at      kdb_enter+0x2f: nop     
db> trace
Tracing pid 12163 tid 100188 td 0xffffff008d169500
kdb_enter() at kdb_enter+0x2f
panic() at panic+0x291
sbflush_locked() at sbflush_locked+0x64
sbrelease_locked() at sbrelease_locked+0x1c
sbrelease() at sbrelease+0x48
sorflush() at sorflush+0x15c
sofree() at sofree+0x204
soclose() at soclose+0x3af
fifo_cleanup() at fifo_cleanup+0x38
fifo_close() at fifo_close+0x79
ufsfifo_close() at ufsfifo_close+0x7d
vn_close() at vn_close+0x8e
vn_closefile() at vn_closefile+0x65
fdrop_locked() at fdrop_locked+0xc0
closef() at closef+0x39
close() at close+0x1a5
syscall() at syscall+0x51e
Xfast_syscall() at Xfast_syscall+0xa8
--- syscall (6, FreeBSD ELF64, close), rip = 0x41e2c0, rsp = 0x7fffffffded8, rbp = 0x57a540 ---