lockrange
struct rangelock *lockrange(vinum_off_t stripe, struct buf *bp, struct plex *plex);
rqg->lock = lockrange(rqg->lockbase, rqg->rq->bio->bio_buf, &PLEX[rqg->plexno]);
lock = lockrange(plexblkno << DEV_BSHIFT, bp, plex); /* lock it */
*lockp = lockrange(pstripe * plex->stripesize * (plex->subdisks - 1),