[Bug 226870] [panic] cdstrategy violates GEOM I/O contract

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=226870

            Bug ID: 226870
           Summary: [panic] cdstrategy violates GEOM I/O contract
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: avg at FreeBSD.org

anic: sleepq_add: td 0xfffff80008e1c000 to sleep on wchan 0xfffff801e58b8048
with sleeping prohibited
cpuid = 0
curthread: 0xfffff80008e1c000
stack: 0xfffffe07e1883000 - 0xfffffe07e1887000
stack pointer: 0xfffffe07e1886820
time = 1521652565
KDB: stack backtrace:
db_trace_self_wrapper() at 0xffffffff804670bb =
db_trace_self_wrapper+0x2b/frame 0xfffffe07e1886750
kdb_backtrace() at 0xffffffff806d4c79 = kdb_backtrace+0x39/frame
0xfffffe07e1886800
vpanic() at 0xffffffff80699da6 = vpanic+0x166/frame 0xfffffe07e1886840
kassert_panic() at 0xffffffff80699adb = kassert_panic+0x16b/frame
0xfffffe07e18868b0
sleepq_add() at 0xffffffff806e1902 = sleepq_add+0x342/frame 0xfffffe07e1886910
_sleep() at 0xffffffff806a41f7 = _sleep+0x2b7/frame 0xfffffe07e18869b0
cam_periph_ccbwait() at 0xffffffff802b53ad = cam_periph_ccbwait+0x5d/frame
0xfffffe07e18869e0
cam_periph_runccb() at 0xffffffff802b51d8 = cam_periph_runccb+0xb8/frame
0xfffffe07e1886a40
cdrunccb() at 0xffffffff802d52fc = cdrunccb+0x3c/frame 0xfffffe07e1886a60
cdprevent() at 0xffffffff802d4beb = cdprevent+0x7b/frame 0xfffffe07e1886aa0
cdcheckmedia() at 0xffffffff802d48b2 = cdcheckmedia+0x22/frame
0xfffffe07e1886af0
cdstrategy() at 0xffffffff802d2a36 = cdstrategy+0x56/frame 0xfffffe07e1886b20
g_disk_start() at 0xffffffff80608330 = g_disk_start+0x170/frame
0xfffffe07e1886b70
g_io_schedule_down() at 0xffffffff8060c1db = g_io_schedule_down+0x1eb/frame
0xfffffe07e1886b90
g_down_procbody() at 0xffffffff8060cbdd = g_down_procbody+0x6d/frame
0xfffffe07e1886ba0
fork_exit() at 0xffffffff8065e410 = fork_exit+0xd0/frame 0xfffffe07e1886bf0
fork_trampoline() at 0xffffffff808e4aee = fork_trampoline+0xe/frame
0xfffffe07e1886bf0

The cause is obvious.
There must no sleeping in the GEOM I/O threads (up and down).
But cdstrategy may violate this contract if it detects that a disk has been
removed.
In this case it calls cdcheckmedia() that closes the CD-ROM tray, locks it and
tastes the disk (if present).
These are all blocking operations and can potentially take a long time since
mechanical actions are involved.

I am not sure how to fix this.
A naive approach is to just fail the request when the CD-ROM is without media
(including when its tray is open).

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