This patch fix use case: user unplug USB sound card before all apps close /dev/mixer and /dev/dsp and then USB driver hang/wait until all apps closed, no old devices can be removed, no new devices can be added.
dmesg show for mixer:
pcm4: unregister: mixer busy
pcm4: Waiting for sound application to exit!
And almost same for dsp, but also prints app pid.