SLP_BUSY
while ((~slp->ns_flags & (SLP_BUSY|SLP_VALID)) == 0) {
KASSERT((slp->ns_flags & SLP_BUSY) == 0);
slp->ns_flags |= SLP_BUSY;
KASSERT((slp->ns_flags & SLP_BUSY) != 0);
slp->ns_flags &= ~SLP_BUSY;
while ((slp->ns_flags & SLP_BUSY) != 0) {
KASSERT((slp->ns_flags & SLP_BUSY) != 0);