RELENG_7 - Witness warnings re USB audio

[Neil Hoggarth]

I'm running RELENG_7 on a dual-core AMD64 system, with a kernel that is 
configured the same as GENERIC but with the WITNESS and INVARIANTS options 
added.

Warnings of the form shown below are getting logged when I play multimedia 
files with mplayer. Is this anything to be concerned about? Should I open 
a PR?

uma_zalloc_arg: zone "4096" with the following non-sleepable locks held:
exclusive sleep mutex pcm0:play:dsp0.p0 (pcm play channel) r = 0 (0xffffff00033017c0) locked @ /usr/src/sys/modules/sound/sound/../../../dev/sound/pcm/vchan.c:448
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2a
witness_warn() at witness_warn+0x248
uma_zalloc_arg() at uma_zalloc_arg+0x33a
malloc() at malloc+0x8a
usbd_alloc_buffer() at usbd_alloc_buffer+0x8a
uaudio_chan_alloc_buffers() at uaudio_chan_alloc_buffers+0x4d
uaudio_trigger_output() at uaudio_trigger_output+0x3b
ua_chan_trigger() at ua_chan_trigger+0x7a
chn_trigger() at chn_trigger+0x117
chn_notify() at chn_notify+0xd5
vchan_trigger() at vchan_trigger+0x95
chn_trigger() at chn_trigger+0x117
chn_write() at chn_write+0x11b
dsp_write() at dsp_write+0x2ef
devfs_write_f() at devfs_write_f+0x9c
dofilewrite() at dofilewrite+0x85
kern_writev() at kern_writev+0x4c
write() at write+0x54
syscall() at syscall+0x1ce
Xfast_syscall() at Xfast_syscall+0xab
--- syscall (4, FreeBSD ELF64, write), rip = 0x80311fdcc, rsp = 0x7fffffffd6b8, rbp = 0x9485471f ---