Deterministic panic due to non-sleepable lock with if_alc when reconfiguring interfaces

[Garrett Cooper]

    When loading if_alc as a module on my netbook and running
/etc/rc.d/netif restart, I can deterministically panic my netbook with
the following message:

) at _bus_dmamap_sync+0x51
alc_stop(c3dbb000,0,c0c51844,93a,80206910,...) at alc_stop+0x24e
alc_ioctl(c3d07400,80206910,c40423c0,c06a7935,c0914e3c,...) at alc_ioctl+0x22e
ifioctl(c45029c0,80206910,c40423c0,c40505c0,c4528c00,...) at ifioctl+0xc98
soo_ioctl(c4574e00,80206910,c40423c0,c413e680,c40505c0,...) at soo_ioctl+0x401
kern_ioctl(c40505c0,3,80206910,c40423c0,c40423c0,...) at kern_ioctl+0x1d7
ioctl(c40505c0,e6ca3cec,e6ca3d28,c08e929d,0,...) at ioctl+0x118
syscallenter(c40505c0,e6ca3ce4,e6ca3ce4,0,0,...) at syscallenter+0x23f
syscall(e6ca3d28) at syscall+0x2e
Xint0x80_syscall() at Xint0x80_syscall+0x21
--- syscall (54kernel trap 12 with interrupts disabled
Kernel page fault with the following non-sleepable locks held:
exclusive sleep mutex alc0 (network driver) r = 0 (0xc3dbc608) locked
@ /usr/src/sys/modules/alc/../../dev/alc/if_alc.c:2362
KDB: stack backtrace:
db_trace_self_wrapper(c08e727a,80,6e726500,74206c65,20706172,...) at
db_trace_self_wrapper+0x26
kdb_backtrace(93a,0,ffffffff,c0ad6114,e6ca323c,...) at kdb_backtrace+0x2a
_witness_debugger(c08e9f67,e6ca3250,4,1,0,...) at _witness_debugger+0x1e
witness_warn(5,0,c0924fe1,c097df50,c3e42b00,...) at witness_warn+0x1f1
trap(e6ca32dc) at trap+0x15a
calltrap() at calltrap+0x6

    I tried to track down what the exact issue was, but I got lost
(the locking sort of looks ok to me, but I'm still not an expert with
mutex(9)).
    I still have the vmcore and can provide more helpful details when requested.
Thanks,
-Garrett