kern/53982: unpluging a tty/serial can panic

[manuel.bouyer%lip6.fr@localhost]

>Number:         53982
>Category:       kern
>Synopsis:       unpluging a tty/serial can panic
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Feb 14 19:40:00 +0000 2019
>Originator:     Manuel Bouyer
>Release:        NetBSD 8.0_STABLE
>Organization:
>Environment:
System: NetBSD armandeche.soc.lip6.fr 8.0_STABLE NetBSD 8.0_STABLE (GENERIC) #0: Mon Feb 11 13:45:33 UTC 2019 mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:
	when a USB/serial adapter is disconnected while in use (e.g.
	by cu(1)), the system may panic:
ucom3: detached
umodem2: detached
umodem2: at uhub6 port 3 (addr 8) disconnected
fatal page fault in supervisor mode
trap type 6 code 0 rip 0xffffffff80460681 cs 0x8 rflags 0x10202 cr2 0x200000004 
ilevel 0x5 rsp 0xffff8000ae306fb0
curlwp 0xfffffe821f5fa440 pid 0.6 lowest kstack 0xffff8000ae3042c0
panic: trap
cpu0: Begin traceback...
vpanic() at netbsd:vpanic+0x15d
snprintf() at netbsd:snprintf
trap() at netbsd:trap+0xa00
--- trap (number 6) ---
usb_transfer_complete() at netbsd:usb_transfer_complete+0x86
ehci_softintr() at netbsd:ehci_softintr+0x159
usb_soft_intr() at netbsd:usb_soft_intr+0x38
softint_dispatch() at netbsd:softint_dispatch+0x91
DDB lost frame for netbsd:Xsoftintr+0x4f, trying 0xffff8000ae3070f0
Xsoftintr() at netbsd:Xsoftintr+0x4f
--- interrupt ---
0:
cpu0: End traceback...

	unfortunably this happens quite often for me ...
>How-To-Repeat:
	plug in a USB/serial adapter, open the device, unplug it
>Fix:
	please