panic during shutdown in lockmgr()

Hi,

with a -CURRENT kernel built today I am consistently getting a panic
during shutdown. Unfortunately a debug kernel refuses to do a core
dump, so I only have a naked stack trace:

panic: lockmgr: thread 0xc1918720, not exclusive lock holder 0xc06b5660 unlocking
#0  0xc04fc40a in doadump ()
#1  0xc04fcaac in boot ()
#2  0xc04fce03 in poweroff_wait ()
#3  0xc04eff3c in lockmgr ()
#4  0xc054b2b2 in vop_stdunlock ()
#5  0xc054b15a in vop_defaultop ()
#6  0xc05e2f73 in ufs_vnoperate ()
#7  0xc0555cd8 in vput ()
#8  0xc05d3a86 in ffs_sync ()
#9  0xc0558a5d in sync ()
#10 0xc04fc5a4 in boot ()
#11 0xc04fc1bf in reboot ()
#12 0xc0632372 in syscall ()
#13 0xc0622fdd in Xint0x80_syscall ()

This panic occurs when I shut the machine down with all filesystems
mounted. I can avoid it when I go to single user mode and umount all
filesystems first.

Please let me know if I can provide further information.

Regards
        -Thorsten

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