kern/51129: Disconnecting USB SD card without umount causes file operations to hang

[coypu%sdf.org@localhost]

>Number:         51129
>Category:       kern
>Synopsis:       Disconnecting USB SD card without umount causes file operations to hang
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue May 10 06:35:00 +0000 2016
>Originator:     coypu
>Release:        NetBSD 7.99.29
>Organization:
>Environment:
NetBSD net.Home 7.99.29 NetBSD 7.99.29 (GENERIC) #1: Tue May 10 07:40:15 IDT 2016  fly%net.Home@localhost:/usr/obj/sys/arch/amd64/compile/GENERIC amd64
>Description:
attach SD card reader with rpi.img on it
mount /dev/sd0a /new
detach SD card reader

any operation will hang. ls, umount (even umount -f), etc.
>How-To-Repeat:

>Fix: