Q_CB
#define Q_MAX_XFER Q_CB
gcq_merge_tail(&t->q[Q_CB], &t->q[Q_NEXT_CB]);
head = Q_CB;
head = Q_CB;
enter_q(t, spipe, Q_CB);
gcq_merge_tail(&t->q[Q_CALLBACKS], &t->q[Q_CB]);
GCQ_GOT_FIRST_TYPED(tvar, &(t)->q[Q_CB], struct slhci_pipe, xq)