[Bug 220949] When the only USB soundcard is disconnected, defunct /dev/dsp0 is left, kernel warnings, programs hang

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=220949

            Bug ID: 220949
           Summary: When the only USB soundcard is disconnected, defunct
                    /dev/dsp0 is left, kernel warnings, programs hang
           Product: Base System
           Version: 11.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: usb
          Assignee: freebsd-usb at FreeBSD.org
          Reporter: yuri at rawbw.com

I have a USB webcam that is also a soundcard with only a microphone, and no
speaker. When there are no other audio devices, disconnecting this USB webcam
causes system dysfunction.

There are a lot of repeating messages in log:
> Jul 23 09:14:21 yv kernel: pcm0: unregister: mixer busy
> Jul 23 09:14:21 yv kernel: pcm0: Waiting for sound application to exit!
> Jul 23 09:14:23 yv kernel: pcm0: unregister: mixer busy
> Jul 23 09:14:23 yv kernel: pcm0: Waiting for sound application to exit!

Programs that have this device open hanged and became unkillable with kill -9.

'kldunload snd_uaudio.ko' also hangs in unkillable state.

USB is hot-pluggable. Device is supposed to disappear and reappear without any
long-term effects.

IMO, /dev/dspN should disappear when device is removed. Programs that have it
opened should behave gracefully. They should only get errors from it and not
hang.

USB webcam: HD Pro Webcam C920 (vendor 0x046d, product 0x6860)

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