MFC6_LOCKPTR
#define MFC6_LOCK() mtx_lock(MFC6_LOCKPTR())
#define MFC6_UNLOCK() mtx_unlock(MFC6_LOCKPTR())
#define MFC6_LOCK_ASSERT() mtx_assert(MFC6_LOCKPTR(), MA_OWNED)
#define MFC6_LOCK_INIT() mtx_init(MFC6_LOCKPTR(), \
#define MFC6_LOCK_DESTROY() mtx_destroy(MFC6_LOCKPTR())
callout_init_mtx(&V_expire_upcalls_ch, MFC6_LOCKPTR(), 0);