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

[Jonathan Nicklin]

Thanks, Sagi. I have tested the changes to eliminate queue->more_requests and
confirm that it addresses the bug (1000 runs of the repro scenario executed
without issue).

Tested-by: Jonathan Nicklin <jnicklin at blockbridge.com>

On Mon, Sep 5, 2022 at 11:07 AM Sagi Grimberg <sagi at grimberg.me> wrote:
>
> 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
>