panic unmounting filesystems

After upgrading to current as of about 24 hours ago, I now consistently
get panics when I try to unmount filesystems with pending I/O.  If I
just mount and unmount a filesystem, it works but if I try to unmount
a filesystem that has been active, I get:

handle_workitem_remove: vget: got error 16 while accessing filesystem
softdep_waitidle: Failed to flush worklist for 0xc2c6429c
panic: vfs_allocate_syncvnode: insmntque failed

I have a crashdump and the backtrace looks like:
#8  0xc055c545 in panic (fmt=0xc07309f1 "vfs_allocate_syncvnode: insmntque failed")
    at /usr/src/sys/kern/kern_shutdown.c:547
#9  0xc05d6b9a in vfs_allocate_syncvnode (mp=0xc2c6429c) at /usr/src/sys/kern/vfs_subr.c:3111
#10 0xc05d1222 in dounmount (mp=0xc2c6429c, flags=0x8000000, td=0xc2d9c600) at /usr/src/sys/kern/vfs_mount.c:1289
#11 0xc05d16ff in unmount (td=0xc2d9c600, uap=0xd6218cfc) at /usr/src/sys/kern/vfs_mount.c:1170
#12 0xc06de9ea in syscall (frame=0xd6218d38) at /usr/src/sys/i386/i386/trap.c:1008
#13 0xc06cc5c0 in Xint0x80_syscall () at /usr/src/sys/i386/i386/exception.s:196

According to kgdb, insmntque() returned both EXDEV and EBUSY.  The former
is impossible so I suspect kgdb is confused and the latter is correct.

The mountpoint shows as in the process of being unmounted and has
mnt_nvnodelistsize == 0 so insmntque() makes sense at the micro level.
Having a system panic as a result does not make sense.

The softdep_waitidle() error looks suspicious - at a quick glance, it
appears to only wait for 10 ticks (10msec) for the dependency chain to
empty.  This seems unreasonably short for an operation that probably
includes physical I/O.  Is my reasoning correct?

Does anyone have any suggestions on where to look next?

-- 
Peter Jeremy