BUILD_OK (no warnings) -- guest cc:
$ cc -O2 -pthread -o /tmp/dsp_race /tmp/dsp_race.c
BUILD_OK

Guest env:
DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul  2 06:02:54 UTC 2026  root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC  x86_64
crw-rw-rw-  1 root  wheel  33, 0x180000ff Sep 19 11:03 /dev/dsp
crw-rw-rw- 1 root  wheel  33, 0x18000000 Sep 19 11:03 /dev/dsp0
crw-rw-rw- 1 root  wheel  33, 0x18000001 Sep 19 11:03 /dev/dsp1
hw.snd.default_unit: 0
hw.snd.maxautovchans: 16
pcm0: <AudioPCI ES1370> port 0xc000-0xc0ff irq 11 at device 4.0 on pci0
pcm0: <Playback: DAC1,DAC2 / Record: ADC>
hdac0: <Intel 82801F HDA Controller> mem 0xfebd0000-0xfebd3fff irq 10 at device 5.0 on pci0
hdacc0: <Generic (0x1af40022) HDA CODEC> at cad 0 on hdac0
hdaa0: <Generic (0x1af40022) Audio Function Group> at nid 1 on hdacc0
pcm1: <Generic (0x1af40022) (Analog)> at nid 3 and 5 on hdaa0