Occasional wedging entering DDB via serial break

I'm seeing occasional wedging attempting to enter DDB via a serial break
on a dual-Xeon box (4 logical processors).  The symptoms usually look
something like this:

hippy# ./tmp.csh
~KDB: enter: Line break on console
[thread pid 560 tid 100202 ]
Stopped at      kdb_enter+0x2c: leave
db> show alllocks
db> cont
load: 0.04  cmd: super-smack 619 [runnable] 0.02u 0.51s 2% 2216k
~KDB: enter: Line break on console
[thread pid 560 tid 100201 ]
Stopped at      kdb_enter+0x2c: leave
db> show alllocks
Process 560 (mysqld)
Thread 0x18769
exclusive sleep mutex so_rcv r = 0 (0xc2a25bcc) locked _at_
kern/uipc_usrreq.c:464
exclusive sleep mutex unp r = 0 (0xc091a740) locked _at_
kern/uipc_usrreq.c:392
db> cont
load: 0.92  cmd: super-smack 619 [runnable] 0.05u 1.36s 6% 2216k
~KDB: enter: Line break on console
[thread pid 560 tid 100199 ]
Stopped at      kdb_enter+0x2c: leave
db> show alllocks
db> cont
load: 2.12  cmd: super-smack 616 [running] 0.08u 2.90s 10% 2216k
~KDB: enter: Line break on console
<wedge>

After that point, a serial break will no longer drop to DDB, respond to
pings, etc.  The box is running 6.x-CURRENT from this morning.  Is anyone
else seeing this, or does anyone else have ideas about what might cause
this?

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert_at_fledge.watson.org      Principal Research Scientist, McAfee Research