Sleeping on "isp_mboxwaiting" with the following non-sleepablelocks held:

This happened to me just now on alpha:

(da0:isp0:0:0:0): Retrying Command (per Sense Data)
(da0:isp0:0:0:0): READ(06). CDB: 8 5 f 30 10 0
(da0:isp0:0:0:0): CAM Status: SCSI Status Error
(da0:isp0:0:0:0): SCSI Status: Check Condition
(da0:isp0:0:0:0): MEDIUM ERROR info:50f30 asc:11,0
(da0:isp0:0:0:0): Unrecovered read error field replaceable unit: e4 actual retry count: 257
(da0:isp0:0:0:0): Retrying Command (per Sense Data)
swap_pager: indefinite wait buffer: device: da0b, blkno: 331568, size: 8192
(da0:isp0:0:0:0): READ(06). CDB: 8 5 f 30 10 0
(da0:isp0:0:0:0): CAM Status: SCSI Status Error
(da0:isp0:0:0:0): SCSI Status: Check Condition
(da0:isp0:0:0:0): MEDIUM ERROR info:50f30 asc:11,0
(da0:isp0:0:0:0): Unrecovered read error field replaceable unit: e4 actual retry count: 257
(da0:isp0:0:0:0): Retries Exhausted
swap_pager: I/O error - pagein failed; blkno 331568,size 8192, error 5
vm_fault: pager read error, pid 90537 (aspell)
Sleeping on "isp_mboxwaiting" with the following non-sleepablelocks held:
exclusive sleep mutex g_xdown r = 0 (0xfffffe0006bfdc78) locked _at_ /a/asami/portbuild/alpha/src-client/sys/geom/geom_io.c:345
witness_warn
Stopped at      Debugger+0x38:  zapnot  v0,#0xf,v0      <v0=0x0>
db> trace
Debugger() at Debugger+0x38
witness_warn() at witness_warn+0x284
msleep() at msleep+0xa8
isp_mbox_wait_complete() at isp_mbox_wait_complete+0x94
isp_mboxcmd() at isp_mboxcmd+0x258
isp_update_bus() at isp_update_bus+0x2f0
isp_update() at isp_update+0x54
isp_start() at isp_start+0x208
isp_action() at isp_action+0x1bc
xpt_run_dev_sendq() at xpt_run_dev_sendq+0x23c
xpt_action() at xpt_action+0x2a0
dastart() at dastart+0x220
xpt_run_dev_allocq() at xpt_run_dev_allocq+0xf0
xpt_schedule() at xpt_schedule+0xc0
dastrategy() at dastrategy+0x70
g_disk_start() at g_disk_start+0x1ec
g_io_schedule_down() at g_io_schedule_down+0x234
g_down_procbody() at g_down_procbody+0x5c
fork_exit() at fork_exit+0x100
exception_return() at exception_return
--- root of call graph ---

Kris