rds_ib_recv_refill() is a function that refills an IB receive
queue. It can be called from both the CQE handler (tasklet) and a
worker thread.

Just after the call to ib_post_recv(), a debug message is printed with
rdsdebug():

            ret = ib_post_recv(ic->i_cm_id->qp, &recv->r_wr, &failed_wr);
            rdsdebug("recv %p ibinc %p page %p addr %lu ret %d\n", recv,
                     recv->r_ibinc, sg_page(&recv->r_frag->f_sg),
                     (long) ib_sg_dma_address(
                            ic->i_cm_id->device,
                            &recv->r_frag->f_sg),
                    ret);

Now consider an invocation of rds_ib_recv_refill() from the worker
thread, which is preemptible. Further, assume that the worker thread
is preempted between the ib_post_recv() and rdsdebug() statements.

Then, if the preemption is due to a receive CQE event, the
rds_ib_recv_cqe_handler() will be invoked. This function processes
receive completions, inc...