panic in 8-CURRENT

I got the following panic with the Oct 14 8-CURRENT kernel.  A 2-way
(Opteron) box running GENERIC FreeBSD/i386 kernel with serial console
access, and typing some keys just after "Trying to mount..." line
appears seems to prevent this panic.  Without typing any keys the box
remain stopped after displaying the line, and then typing a key will
cause this panic.  Whether the panic occurs depends on the time
between displaying the line and typing a key.

----(from here)
WARNING: WITNESS option enabled, expect reduced performance.
Trying to mount root from ufs:/dev/ad4s1a
spin lock 0xc0c17a6c (sio) held by 0xc3f0d630 (tid 100004) too long
panic: spin lock held too long
cpuid = 0
KDB: enter: panic
[thread pid 46 tid 100056 ]
Stopped at      kdb_enter+0x32: leave
db> bt
Tracing pid 46 tid 100056 td 0xc40b9a50
kdb_enter(c0a9c1bb,0,c0a9afae,e44f303c,0,...) at kdb_enter+0x32
panic(c0a9afae,c3f0d630,c0b73ea0,c3f0d630,186a4,...) at panic+0x124
_mtx_lock_spin_failed(1,c40b9a50,c0c17a6c,0,e44f3094,...) at _mtx_lock_spin_failed+0x51
_mtx_lock_spin(c0c17a6c,c40b9a50,0,c0ac3e92,9fc,...) at _mtx_lock_spin+0x75
_mtx_lock_spin_flags(c0c17a6c,0,c0ac3e92,9fc,a,...) at _mtx_lock_spin_flags+0x10e
sio_cnputc(c0b73ee0,73,e44f324c,5,73,...) at sio_cnputc+0x8d
cnputc(73,e44f324c,e44f310c,c077ac51,c0a0f1de,...) at cnputc+0x5f
putcons(c0a0f1de,1,1000006,c0c17a6c,c0a9af7f,...) at putcons+0x17
putchar(73,e44f324c,ee9898c0,257,0,...) at putchar+0x61
kvprintf(c0a9af7e,c077abf0,e44f324c,a,e44f3278,...) at kvprintf+0x75
printf(c0a9af7e,c0c17a6c,c0b73ea0,c3f0d630,186a4,...) at printf+0x4e
_mtx_lock_spin_failed(1,c40b9a50,c0c17a6c,0,e44f32d0,...) at _mtx_lock_spin_failed+0x39
_mtx_lock_spin(c0c17a6c,c40b9a50,0,c0ac3e92,9fc,...) at _mtx_lock_spin+0x75
_mtx_lock_spin_flags(c0c17a6c,0,c0ac3e92,9fc,a,...) at _mtx_lock_spin_flags+0x10e
sio_cnputc(c0b73ee0,73,e44f3488,5,73,...) at sio_cnputc+0x8d
cnputc(73,e44f3488,e44f3348,c077ac51,c0a0f1de,...) at cnputc+0x5f
putcons(c0a0f1de,1,1000006,c0c17a6c,c0a9af7f,...) at putcons+0x17
putchar(73,e44f3488,ee9898c0,257,c40b9a50,...) at putchar+0x61
kvprintf(c0a9af7e,c077abf0,e44f3488,a,e44f34b4,...) at kvprintf+0x75
printf(c0a9af7e,c0c17a6c,c0b73ea0,c3f0d630,186a4,...) at printf+0x4e
_mtx_lock_spin_failed(1,c40b9a50,c0c17a6c,0,e44f350c,...) at _mtx_lock_spin_failed+0x39
_mtx_lock_spin(c0c17a6c,c40b9a50,0,c0ac3e92,9fc,...) at _mtx_lock_spin+0x75
_mtx_lock_spin_flags(c0c17a6c,0,c0ac3e92,9fc,2570000,...) at _mtx_lock_spin_flags+0x10e
sio_cnputc(c0b73ee0,73,e44f36c4,5,73,...) at sio_cnputc+0x8d
cnputc(73,e44f36c4,e44f3584,c077ac51,c075c4e8,...) at cnputc+0x5f
putcons(c075c4e8,1,1000006,c0c17a6c,c0a9af7f,...) at putcons+0x17
putchar(73,e44f36c4,ee9898c0,257,c0bb3580,...) at putchar+0x61
kvprintf(c0a9af7e,c077abf0,e44f36c4,a,e44f36f0,...) at kvprintf+0x75
printf(c0a9af7e,c0c17a6c,c0b73ea0,c3f0d630,186a4,...) at printf+0x4e
_mtx_lock_spin_failed(1,c40b9a50,c0c17a6c,0,e44f3748,...) at _mtx_lock_spin_failed+0x39
_mtx_lock_spin(c0c17a6c,c40b9a50,0,c0ac3e92,80d,...) at _mtx_lock_spin+0x75
_mtx_lock_spin_flags(c0c17a6c,0,c0ac3e92,80d,0,...) at _mtx_lock_spin_flags+0x10e
comstart(c40a2000,0,c0ac3e92,76d,3ac3e92,...) at comstart+0x278
comparam(c40a2000,c40a20c4,0,8,c0a976e6,...) at comparam+0x2d1
ttyopen(c40a8700,3,2000,c40b9a50,c0b55080,...) at ttyopen+0x262
giant_open(c40a8700,3,2000,c40b9a50,c4291100,...) at giant_open+0x4f
devfs_open(e44f387c,e44f3950,e44f3950,0,e44f390c,...) at devfs_open+0x230
VOP_OPEN_APV(c0b45500,e44f387c,0,c4235240,c40ca990,...) at VOP_OPEN_APV+0xa5
vn_open_cred(e44f3950,c0bfe954,0,c3f14800,0,...) at vn_open_cred+0x44b
vn_open(e44f3950,c0bfe954,0,0,c0743fcd,...) at vn_open+0x33
cn_devopen(c3f5d800,c40caaa0,e44f39f4,c07200bf,c3f5d800,...) at cn_devopen+0xf9
cnopen(c3f5d800,3,2000,c40b9a50,c0b55460,...) at cnopen+0x3e
giant_open(c3f5d800,3,2000,c40b9a50,c4262d80,...) at giant_open+0x4f
devfs_open(e44f3a8c,c40b9a50,e44f3b80,0,e44f3b1c,...) at devfs_open+0x230
VOP_OPEN_APV(c0b45500,e44f3a8c,e44f3a78,246,c40caaa0,...) at VOP_OPEN_APV+0xa5
vn_open_cred(e44f3b80,e44f3c78,400,c3f14800,c42a6000,...) at vn_open_cred+0x44b
vn_open(e44f3b80,e44f3c78,400,c42a6000,e44f3b90,...) at vn_open+0x33
kern_open(c40b9a50,80c34a5,0,3,80d3600,...) at kern_open+0xe7
open(c40b9a50,e44f3cfc,c,c0acdafe,c0b48698,...) at open+0x30
syscall(e44f3d38) at syscall+0x2b3
Xint0x80_syscall() at Xint0x80_syscall+0x20
--- syscall (5, FreeBSD ELF32, open), eip = 0x8088e3b, esp = 0xbfbfe90c, ebp = 0xbfbfe928 ---
db>
----(to here)

--
| Hiroki SATO