Panic after USB deadlock followed by kldunload umass.ko

A few days ago a couple of usbconfig processed did not exit:

fk_at_r500 ~ $sudo procstat -kk $(pgrep usbconfig)
  PID    TID COMM             TDNAME           KSTACK                       
 1624 100781 usbconfig        -                mi_switch+0xe1 sleepq_wait+0x3a _sx_xlock_hard+0x522 _sx_xlock+0x5d usbd_enum_lock+0x3a usb_ref_device+0x157 usb_open+0xbf devfs_open+0x122 VOP_OPEN_APV+0xa1 vn_open_vnode+0x234 vn_open_cred+0x351 kern_openat+0x26f amd64_syscall+0x3fb Xfast_syscall+0xfb 
 1617 100779 usbconfig        -                mi_switch+0xe1 sleepq_wait+0x3a _sx_xlock_hard+0x522 _sx_xlock+0x5d usbd_enum_lock+0x3a usb_ref_device+0x157 usb_open+0xbf devfs_open+0x122 VOP_OPEN_APV+0xa1 vn_open_vnode+0x234 vn_open_cred+0x351 kern_openat+0x26f amd64_syscall+0x3fb Xfast_syscall+0xfb 
 1615 100777 usbconfig        -                mi_switch+0xe1 sleepq_wait+0x3a _sx_xlock_hard+0x522 _sx_xlock+0x5d usbd_enum_lock+0x3a usb_ref_device+0x157 usb_open+0xbf devfs_open+0x122 VOP_OPEN_APV+0xa1 vn_open_vnode+0x234 vn_open_cred+0x351 kern_openat+0x26f amd64_syscall+0x3fb Xfast_syscall+0xfb 
 1601 100774 usbconfig        -                mi_switch+0xe1 sleepq_timedwait+0x3a _sleep+0x294 pause_sbt+0xd0 usb_pause_mtx+0x85 usb_ioctl+0x3e7 devfs_ioctl_f+0x13b kern_ioctl+0x3cd sys_ioctl+0x13c amd64_syscall+0x3fb Xfast_syscall+0xfb 

kldunload umass.ko lead to a panic, dumping didn't work.

Screenshots are available at:
http://www.fabiankeil.de/bilder/freebsd/kernel-panic-r273434-usb/

I've seen locked-up usbconfig processes in the past,
usually after executing a shell function that does:

| usbconfig_output="$(sudo usbconfig -d ${device} add_quirk UQ_MSC_NO_INQUIRY)"
| [... error handling snipped ]
| usbconfig_output="$(sudo usbconfig -d ${device} reset)"

Sometimes the second command seems to mess up the USB system.

Fabian