xprtrdma: Re-write rpcrdma_flush_cqs()

Currently rpcrdma_flush_cqs() attempts to avoid code duplication,
and simply invokes rpcrdma_recvcq_upcall and rpcrdma_sendcq_upcall.

1. rpcrdma_flush_cqs() can run concurrently with provider upcalls.
   Both flush_cqs() and the upcalls were invoking ib_poll_cq() in
   different threads using the same wc buffers (ep->rep_recv_wcs
   and ep->rep_send_wcs), added by commit 1c00dd0 ("xprtrmda:
   Reduce calls to ib_poll_cq() in completion handlers").

   During transport disconnect processing, this sometimes resulted
   in the same reply getting added to the rpcrdma_tasklets_g list
   more than once, which corrupted the list.

2. The upcall functions drain only a limited number of CQEs,
   thanks to the poll budget added by commit 8301a2c
   ("xprtrdma: Limit work done by completion handler").

Fixes: a7bc211 ("xprtrdma: On disconnect, don't ignore ... ")
BugLink: https://bugzilla.linux-nfs.org/show_bug.cgi?id=276
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/verbs.c

@@ -317,8 +317,15 @@ rpcrdma_recvcq_upcall(struct ib_cq *cq, void *cq_context)
 static void
 rpcrdma_flush_cqs(struct rpcrdma_ep *ep)
 {
-	rpcrdma_recvcq_upcall(ep->rep_attr.recv_cq, ep);
-	rpcrdma_sendcq_upcall(ep->rep_attr.send_cq, ep);
+	struct ib_wc wc;
+	LIST_HEAD(sched_list);
+
+	while (ib_poll_cq(ep->rep_attr.recv_cq, 1, &wc) > 0)
+		rpcrdma_recvcq_process_wc(&wc, &sched_list);
+	if (!list_empty(&sched_list))
+		rpcrdma_schedule_tasklet(&sched_list);
+	while (ib_poll_cq(ep->rep_attr.send_cq, 1, &wc) > 0)
+		rpcrdma_sendcq_process_wc(&wc);
 }
 
 #ifdef RPC_DEBUG