[Bug 239730] System fails to remove the audio device because some app keeps it opened, kernel enters the endless cycle: unregister: mixer busy Waiting for sound application to exit!

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239730

            Bug ID: 239730
           Summary: System fails to remove the audio device because some
                    app keeps it opened, kernel enters the endless cycle:
                    unregister: mixer busy Waiting for sound application
                    to exit!
           Product: Base System
           Version: 12.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: usb
          Assignee: usb at FreeBSD.org
          Reporter: yuri at freebsd.org

I connected the USB audio device through a (possibly unstable) USB hub.
When the USB hub has disconnected the system began to print messages like this:
> Aug  8 15:57:12 sw kernel: pcm3: unregister: mixer busy
> Aug  8 15:57:12 sw kernel: pcm3: Waiting for sound application to exit!

IMHO, this is an entirely wrong behavior.

Once the device disappears, the device file should be just gone. If some
application has it opened, the file handle should remain and all subsequent API
functions that this application invokes with this file handle should fail. The
file handle should disappear when the application would close it. In the next
second the same device can reappear, and then any applications should be able
to just use it again, regardless of some application that is potentially
holding the old instance of it open.

Unregistering the sound device should not print any such messages as it
currently prints. Unregistering shouldn't fail just because some application
has it open, because this can happen any time.


----system log----
Aug  8 15:53:55 sw kernel: uaudio0: <vendor 0x046d HD Pro Webcam C920, class
239/2, rev 2.00/0.11, addr 2> on usbus0
Aug  8 15:53:55 sw kernel: uaudio0: No playback.
Aug  8 15:53:55 sw kernel: uaudio0: Record: 32000 Hz, 2 ch, 16-bit S-LE PCM
format, 2x8ms buffer.
Aug  8 15:53:55 sw kernel: uaudio0: Record: 24000 Hz, 2 ch, 16-bit S-LE PCM
format, 2x8ms buffer.
Aug  8 15:53:55 sw kernel: uaudio0: Record: 16000 Hz, 2 ch, 16-bit S-LE PCM
format, 2x8ms buffer.
Aug  8 15:53:55 sw kernel: uaudio0: No MIDI sequencer.
Aug  8 15:53:55 sw kernel: pcm3: <USB audio> on uaudio0
Aug  8 15:53:55 sw kernel: uaudio0: No HID volume keys found.
...
Aug  8 15:56:38 sw kernel: ugen0.2: <vendor 0x214b USB2.0 HUB> at usbus0
(disconnected)
Aug  8 15:56:38 sw kernel: uhub2: at uhub0, port 5, addr 1 (disconnected)
Aug  8 15:56:38 sw kernel: ugen0.3: <vendor 0x046d HD Pro Webcam C920> at
usbus0 (disconnected)
Aug  8 15:56:38 sw kernel: uaudio0: at uhub2, port 1, addr 2 (disconnected)
Aug  8 15:56:38 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:56:38 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:56:40 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:56:40 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:56:42 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:56:42 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:56:44 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:56:44 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:56:46 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:56:46 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:56:48 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:56:48 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:56:50 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:56:50 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:56:52 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:56:58 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:56:58 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:00 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:00 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:02 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:02 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:04 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:04 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:06 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:06 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:08 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:08 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:10 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:10 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:12 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:12 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:14 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:14 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:16 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:16 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:18 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:18 sw kernel: pcm3: Waiting for sound application to exit!
Aug  8 15:57:20 sw kernel: pcm3: unregister: mixer busy
Aug  8 15:57:20 sw kernel: pcm3: Waiting for sound application to exit!

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