ata panic: "mtx_lock of destroyed mutex"

[Bruce Cran]

I've been setting up an amd64 VirtualBox machine with the latest
9-CURRENT and got the following panic when booting it (another machine
updated and booting at the same time didn't panic):

ata1: WARNING - READ_TOC read data overrun 18>12
panic: mtx_lock of destroyed mutex @ /usr/src/sys/kern/kern_sema.c:79
cpuid = 0

with the following stack trace:

kdb_enter
panic
_mtx_lock_flags
_sema_post
ata_completed
taskqueue_run
intr_event_execute_handlers
ithread_loop
fork_exit
fork_trampoline

-- 
Bruce Cran