B_BARRIER
if ((bp->b_flags & B_BARRIER) != 0) {
bp->b_flags &= ~B_BARRIER;
if ((bp->b_flags & B_BARRIER) != 0)
KASSERT((bp->b_flags & B_BARRIER) == 0,
bp->b_flags |= B_ASYNC | B_BARRIER;
bp->b_flags |= B_BARRIER;
bp->b_flags |= (tbp->b_flags & B_BARRIER);
tbp->b_flags &= ~(B_DONE | B_BARRIER);
bp->b_flags &= ~B_BARRIER;
bp->b_flags &= ~B_BARRIER;