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)
This is a known issue. The solution is not just straight forward, due to mmap() use. See also. https://reviews.freebsd.org/D11140 --HPS
A commit in branch main references this bug: URL: https://cgit.FreeBSD.org/src/commit/?id=9eff58c6d52b66eb8abe7f724dabcd804a566df4 commit 9eff58c6d52b66eb8abe7f724dabcd804a566df4 Author: Christos Margiolis <christos@FreeBSD.org> AuthorDate: 2024-03-31 14:13:43 +0000 Commit: Christos Margiolis <christos@FreeBSD.org> CommitDate: 2024-03-31 14:13:43 +0000 sound: Implement asynchronous device detach Hot-unplugging a sound device, such as a USB sound card, whilst being consumed by an application, results in an infinite loop until either the application closes the device's file descriptor, or the channel automatically times out after hw.snd.timeout seconds. In the case of a detach however, the timeout approach is still not ideal, since we want all resources to be released immediatelly, without waiting for N seconds until we can use the bus again. The timeout mechanism works by calling chn_sleep() in chn_read() and chn_write() (see pcm/channel.c) in order to send the thread to sleep, using cv_timedwait_sig(). Since chn_sleep() sets the CHN_F_SLEEPING flag while waiting for cv_timedwait_sig() to return, we can test this flag in pcm_unregister() (called during detach) and wakeup the sleeping thread(s) to immediately kill the channel(s) being consumed. Sponsored by: The FreeBSD Foundation MFC after: 2 months PR: 194727, 278055, 202275, 220949, 272286 Reviewed by: dev_submerge.ch, markj Differential Revision: https://reviews.freebsd.org/D43545 share/man/man4/snd_uaudio.4 | 11 +---------- sys/dev/sound/pcm/dsp.c | 2 +- sys/dev/sound/pcm/mixer.c | 11 ----------- sys/dev/sound/pcm/sound.c | 24 ++++++++++-------------- sys/dev/sound/usb/uaudio.c | 13 +++---------- 5 files changed, 15 insertions(+), 46 deletions(-)
Fixed as of 44e128fe9d92c1a544b801cb56e907a66ef34691
*** This bug has been marked as a duplicate of bug 194727 ***