SUNRPC: Partial revert of commit 6f9f1728

The premise of commit 6f9f1728 ("SUNRPC: Mitigate cond_resched() in
xprt_transmit()") was that cond_resched() is expensive and unnecessary
when there has been just a single send.
The point of cond_resched() is to ensure that tasks that should pre-empt
this one get a chance to do so when it is safe to do so. The code prior
to commit 6f9f1728 failed to take into account that it was keeping a
rpc_task pinned for longer than it needed to, and so rather than doing a
full revert, let's just move the cond_resched.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xprt.c

@@ -1603,15 +1603,14 @@ xprt_transmit(struct rpc_task *task)
 {
 	struct rpc_rqst *next, *req = task->tk_rqstp;
 	struct rpc_xprt	*xprt = req->rq_xprt;
-	int counter, status;
+	int status;
 
 	spin_lock(&xprt->queue_lock);
-	counter = 0;
-	while (!list_empty(&xprt->xmit_queue)) {
-		if (++counter == 20)
+	for (;;) {
+		next = list_first_entry_or_null(&xprt->xmit_queue,
+						struct rpc_rqst, rq_xmit);
+		if (!next)
 			break;
-		next = list_first_entry(&xprt->xmit_queue,
-				struct rpc_rqst, rq_xmit);
 		xprt_pin_rqst(next);
 		spin_unlock(&xprt->queue_lock);
 		status = xprt_request_transmit(next, task);
@@ -1619,13 +1618,16 @@ xprt_transmit(struct rpc_task *task)
 			status = 0;
 		spin_lock(&xprt->queue_lock);
 		xprt_unpin_rqst(next);
-		if (status == 0) {
-			if (!xprt_request_data_received(task) ||
-			    test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
-				continue;
-		} else if (test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
-			task->tk_status = status;
-		break;
+		if (status < 0) {
+			if (test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
+				task->tk_status = status;
+			break;
+		}
+		/* Was @task transmitted, and has it received a reply? */
+		if (xprt_request_data_received(task) &&
+		    !test_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
+			break;
+		cond_resched_lock(&xprt->queue_lock);
 	}
 	spin_unlock(&xprt->queue_lock);
 }