[PATCH] nvme-tcp: Fix regression that causes sporadic requests to time out

[Sagi Grimberg]

When we queue requests, we strive to batch as much as possible and also
signal the network stack that more data is about to be sent over a socket
with MSG_SENDPAGE_NOTLAST. This flag looks at the pending requests queued
as well as queue->more_requests that is derived from the block layer
last-in-batch indication.

We set more_request=true when we flush the request directly from .queue_rq
submission context (in nvme_tcp_send_all), however this is wrongly
assuming that no other requests may be queued during the execution of
nvme_tcp_send_all.

Due to this, a race condition may happen where:
1. request X is queued as !last-in-batch
2. request X submission context calls nvme_tcp_send_all directly
3. nvme_tcp_send_all is preempted and schedules to a different cpu
4. request Y is queued as last-in-batch
5. nvme_tcp_send_all context sends request X+Y, however signals for
both MSG_SENDPAGE_NOTLAST because queue->more_requests=true.
==> none of the requests is pushed down to the wire as the network
stack is waiting for more data, both requests timeout.

To fix this, we eliminate queue->more_requests and only rely on
the queue req_list and send_list to be not-empty.

Fixes: 122e5b9f3d37 ("nvme-tcp: optimize network stack with setting msg
flags according to batch size")
Reported-by: Jonathan Nicklin <jnicklin at blockbridge.com>
Signed-off-by: Sagi Grimberg <sagi at grimberg.me>
---
 drivers/nvme/host/tcp.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/nvme/host/tcp.c b/drivers/nvme/host/tcp.c
index 54b4e8a7fe42..d5871fd6f769 100644
--- a/drivers/nvme/host/tcp.c
+++ b/drivers/nvme/host/tcp.c
@@ -121,7 +121,6 @@ struct nvme_tcp_queue {
 	struct mutex		send_mutex;
 	struct llist_head	req_list;
 	struct list_head	send_list;
-	bool			more_requests;
 
 	/* recv state */
 	void			*pdu;
@@ -320,7 +319,7 @@ static inline void nvme_tcp_send_all(struct nvme_tcp_queue *queue)
 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;
+		!llist_empty(&queue->req_list);
 }
 
 static inline void nvme_tcp_queue_request(struct nvme_tcp_request *req,
@@ -339,9 +338,7 @@ static inline void nvme_tcp_queue_request(struct nvme_tcp_request *req,
 	 */
 	if (queue->io_cpu == raw_smp_processor_id() &&
 	    sync && empty && mutex_trylock(&queue->send_mutex)) {
-		queue->more_requests = !last;
 		nvme_tcp_send_all(queue);
-		queue->more_requests = false;
 		mutex_unlock(&queue->send_mutex);
 	}
 
-- 
2.34.1