Here's a bit more detail for what you described. The fact that flushfiles is flushing out so much on a quiescent filesystem is rather suspect (as a counter point, run lsof and look for writeable files).
The next line is a tty interrogation with ^T
load: 0.22 cmd: mount 2123 [biowr] 5.82r 0.02u 0.08s 7% 1044k
This is the alternate ddb entry, "~^b"
~KDB: enter: Break sequence on console
[thread pid 10 tid 64003 ]
Stopped at kdb_enter+0x3a: movl $0,kdb_why
# Just what is the hung mount doing...
db> bt 2123
Tracing pid 2123 tid 64074 td 0xc25612e0
sched_switch(c25612e0,0,104,191,10e78686,...) at sched_switch+0x399
mi_switch(104,0,c0c8be83,1eb,4c,...) at mi_switch+0x1da
sleepq_switch(c25612e0,0,c0c8be83,260,0,...) at sleepq_switch+0x15f
sleepq_wait(cf240fd0,4c,c0c92fe4,0,0,...) at sleepq_wait+0x63
_sleep(cf240fd0,c212c524,4c,c0c92fe4,0,...) at _sleep+0x372
bwait(cf240fd0,4c,c0c92fe4,cf240fd0,d412a69c,...) at bwait+0x6f
bufwait(cf240fd0,cf240fd0,df,cf240fd0,c22fb5e4,...) at bufwait+0x48
bufwrite(cf240fd0,0,c0cb0aac,772,0,...) at bufwrite+0x165
ffs_bufwrite(cf240fd0,c22fd600,80,1000,0,...) at ffs_bufwrite+0x27b
ffs_update(c238610c,1,c0cb10b8,162,d412a798,...) at ffs_update+0x2ac
ffs_syncvnode(c238610c,1,c25612e0,c2320110,c23861b4,...) at ffs_syncvnode+0x48f
ffs_fsync(d412a858,c238610c,0,c238610c,d412a880,...) at ffs_fsync+0x27
VOP_FSYNC_APV(c11dcc20,d412a858,c0c955dc,9cb,0,...) at VOP_FSYNC_APV+0xa5
vflush(c22f2c94,0,6,c25612e0,c22e0200,...) at vflush+0x2b4
ffs_flushfiles(c22f2c94,6,c25612e0,e6,c095ca37,...) at ffs_flushfiles+0x7b
ffs_mount(c22f2c94,0,c0c94deb,3ed,0,...) at ffs_mount+0x452
vfs_donmount(c25612e0,10090003,c286aa00,c286aa00,bfbfdca9,...) at vfs_donmount+0x1012
nmount(c25612e0,d412acec,c25612e0,c1214c80,207,...) at nmount+0x64
syscall(d412ad28) at syscall+0x24e
--More--^M ^MXint0x80_syscall() at Xint0x80_syscall+0x21
--- syscall (378, FreeBSD ELF32, nmount), eip = 0x280e746b, esp = 0xbfbfdc7c, ebp = 0xbfbfe1d8 ---
db> show mount
0xc22f2c94 /dev/ufs/pfsense1 on / (ufs)
0xc22f3000 devfs on /dev (devfs)
0xc22f2a10 /dev/md0 on /tmp (ufs)
0xc22f278c /dev/md1 on /var (ufs)
0xc22f2508 /dev/ufs/cf on /cf (ufs)
0xc22f2000 devfs on /var/dhcpd/dev (devfs)