nvme-tcp: have queue prod/cons send list become a llist

The queue processing will splice to a queue local list, this should
alleviate some contention on the send_list lock, but also prepares
us to the next patch where we look on these lists for network stack
flag optimization.

Remove queue lock as its not used anymore.
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Tested-by: Mark Wunderlich <mark.wunderlich@intel.com>
[hch: simplified a loop]
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/tcp.c

@@ -46,6 +46,7 @@ struct nvme_tcp_request {
 	u32			pdu_sent;
 	u16			ttag;
 	struct list_head	entry;
+	struct llist_node	lentry;
 	__le32			ddgst;
 
 	struct bio		*curr_bio;
@@ -75,8 +76,8 @@ struct nvme_tcp_queue {
 	struct work_struct	io_work;
 	int			io_cpu;
 
-	spinlock_t		lock;
 	struct mutex		send_mutex;
+	struct llist_head	req_list;
 	struct list_head	send_list;
 
 	/* recv state */
@@ -266,10 +267,8 @@ static inline void nvme_tcp_queue_request(struct nvme_tcp_request *req,
 	struct nvme_tcp_queue *queue = req->queue;
 	bool empty;
 
-	spin_lock(&queue->lock);
-	empty = list_empty(&queue->send_list) && !queue->request;
-	list_add_tail(&req->entry, &queue->send_list);
-	spin_unlock(&queue->lock);
+	empty = llist_add(&req->lentry, &queue->req_list) &&
+		list_empty(&queue->send_list) && !queue->request;
 
 	/*
 	 * if we're the first on the send_list and we can try to send
@@ -285,18 +284,33 @@ static inline void nvme_tcp_queue_request(struct nvme_tcp_request *req,
 	}
 }
 
+static void nvme_tcp_process_req_list(struct nvme_tcp_queue *queue)
+{
+	struct nvme_tcp_request *req;
+	struct llist_node *node;
+
+	for (node = llist_del_all(&queue->req_list); node; node = node->next) {
+		req = llist_entry(node, struct nvme_tcp_request, lentry);
+		list_add(&req->entry, &queue->send_list);
+	}
+}
+
 static inline struct nvme_tcp_request *
 nvme_tcp_fetch_request(struct nvme_tcp_queue *queue)
 {
 	struct nvme_tcp_request *req;
 
-	spin_lock(&queue->lock);
 	req = list_first_entry_or_null(&queue->send_list,
 			struct nvme_tcp_request, entry);
-	if (req)
-		list_del(&req->entry);
-	spin_unlock(&queue->lock);
+	if (!req) {
+		nvme_tcp_process_req_list(queue);
+		req = list_first_entry_or_null(&queue->send_list,
+				struct nvme_tcp_request, entry);
+		if (unlikely(!req))
+			return NULL;
+	}
 
+	list_del(&req->entry);
 	return req;
 }
 
@@ -1344,8 +1358,8 @@ static int nvme_tcp_alloc_queue(struct nvme_ctrl *nctrl,
 	int ret, rcv_pdu_size;
 
 	queue->ctrl = ctrl;
+	init_llist_head(&queue->req_list);
 	INIT_LIST_HEAD(&queue->send_list);
-	spin_lock_init(&queue->lock);
 	mutex_init(&queue->send_mutex);
 	INIT_WORK(&queue->io_work, nvme_tcp_io_work);
 	queue->queue_size = queue_size;