nvme-tcp: fix io_work priority inversion

Dispatching requests inline with the .queue_rq() call may block while
holding the send_mutex. If the tcp io_work also happens to schedule, it
may see the req_list is non-empty, leaving "pending" true and remaining
in TASK_RUNNING. Since io_work is of higher scheduling priority, the
.queue_rq task may not get a chance to run, blocking forward progress
and leading to io timeouts.

Instead of checking for pending requests within io_work, let the queueing
restart io_work outside the send_mutex lock if there is more work to be
done.

Fixes: a0fdd141 ("nvme-tcp: rerun io_work if req_list is not empty")
Reported-by: Samuel Jones <sjones@kalrayinc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/tcp.c

@@ -274,6 +274,12 @@ static inline void nvme_tcp_send_all(struct nvme_tcp_queue *queue)
 	} while (ret > 0);
 }
 
+static inline bool nvme_tcp_queue_more(struct nvme_tcp_queue *queue)
+{
+	return !list_empty(&queue->send_list) ||
+		!llist_empty(&queue->req_list) || queue->more_requests;
+}
+
 static inline void nvme_tcp_queue_request(struct nvme_tcp_request *req,
 		bool sync, bool last)
 {
@@ -294,9 +300,10 @@ static inline void nvme_tcp_queue_request(struct nvme_tcp_request *req,
 		nvme_tcp_send_all(queue);
 		queue->more_requests = false;
 		mutex_unlock(&queue->send_mutex);
-	} else if (last) {
-		queue_work_on(queue->io_cpu, nvme_tcp_wq, &queue->io_work);
 	}
+
+	if (last && nvme_tcp_queue_more(queue))
+		queue_work_on(queue->io_cpu, nvme_tcp_wq, &queue->io_work);
 }
 
 static void nvme_tcp_process_req_list(struct nvme_tcp_queue *queue)
@@ -906,12 +913,6 @@ static void nvme_tcp_state_change(struct sock *sk)
 	read_unlock_bh(&sk->sk_callback_lock);
 }
 
-static inline bool nvme_tcp_queue_more(struct nvme_tcp_queue *queue)
-{
-	return !list_empty(&queue->send_list) ||
-		!llist_empty(&queue->req_list) || queue->more_requests;
-}
-
 static inline void nvme_tcp_done_send_req(struct nvme_tcp_queue *queue)
 {
 	queue->request = NULL;
@@ -1145,8 +1146,7 @@ static void nvme_tcp_io_work(struct work_struct *w)
 				pending = true;
 			else if (unlikely(result < 0))
 				break;
-		} else
-			pending = !llist_empty(&queue->req_list);
+		}
 
 		result = nvme_tcp_try_recv(queue);
 		if (result > 0)