USB stack getting confused

[Rozhuk Ivan]

On Sat, 9 Mar 2019 18:26:40 +0200
Konstantin Belousov <kostikbel at gmail.com> wrote:

> In fact I saw something similar with apcupsd and either usb/com
> adapters or native usb control card for APC UPSes.  For reasons I do
> not understand, these devices are often disconnected.  For older
> versions of apcupsd, it required restart for newly reattached device
> to be recreated in /dev. Sometimes it hangs whole usb stack.
> 
> Newer apcupsd seems to open /dev/ugen only for the duration of the
> query, which makes the erratic behaviour is much less likely, but
> could still cause breakage when device disappear while apcupsd has it
> opened.
> 

Same problem with usb sound cards.
I try to fix it, but fail with dsp, only mixer can be fixed with small code change.
https://reviews.freebsd.org/D11140