[PATCH v2] nvme-rdma: support devices with queue size < 32

[Marta Rybczynska]

In the case of small NVMe-oF queue size (<32) we may enter
a deadlock caused by the fact that the IB completions aren't sent
waiting for 32 and the send queue will fill up.

The error is seen as (using mlx5):
[ 2048.693355] mlx5_0:mlx5_ib_post_send:3765:(pid 7273):
[ 2048.693360] nvme nvme1: nvme_rdma_post_send failed with error code -12

This patch changes the way the signaling is done so
that it depends on the queue depth now. The magic define has
been removed completely.

Signed-off-by: Marta Rybczynska <marta.rybczynska at kalray.eu>
Signed-off-by: Samuel Jones <sjones at kalray.eu>
---
Changes from v1:
* signal by queue size/2, remove hardcoded 32
* support queue depth of 1

 drivers/nvme/host/rdma.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
index 47a479f..4de1b92 100644
--- a/drivers/nvme/host/rdma.c
+++ b/drivers/nvme/host/rdma.c
@@ -1029,6 +1029,18 @@ static void nvme_rdma_send_done(struct ib_cq *cq, struct ib_wc *wc)
                nvme_rdma_wr_error(cq, wc, "SEND");
 }
 
+static inline nvme_rdma_queue_sig_limit(struct nvme_rdma_queue *queue)
+{
+       int sig_limit;
+
+       /* We signal completion every queue depth/2 and also
+        * handle the case of possible device with queue_depth=1,
+        * where we would need to signal every message.
+        */
+       sig_limit = max(queue->queue_size / 2, 1);
+       return (++queue->sig_count % sig_limit) == 0;
+}
+
 static int nvme_rdma_post_send(struct nvme_rdma_queue *queue,
                struct nvme_rdma_qe *qe, struct ib_sge *sge, u32 num_sge,
                struct ib_send_wr *first, bool flush)
@@ -1056,9 +1068,6 @@ static int nvme_rdma_post_send(struct nvme_rdma_queue *queue,
         * Would have been way to obvious to handle this in hardware or
         * at least the RDMA stack..
         *
-        * This messy and racy code sniplet is copy and pasted from the iSER
-        * initiator, and the magic '32' comes from there as well.
-        *
         * Always signal the flushes. The magic request used for the flush
         * sequencer is not allocated in our driver's tagset and it's
         * triggered to be freed by blk_cleanup_queue(). So we need to
@@ -1066,7 +1075,7 @@ static int nvme_rdma_post_send(struct nvme_rdma_queue *queue,
         * embedded in request's payload, is not freed when __ib_process_cq()
         * calls wr_cqe->done().
         */
-       if ((++queue->sig_count % 32) == 0 || flush)
+       if (nvme_rdma_queue_sig_limit(queue) || flush)
                wr.send_flags |= IB_SEND_SIGNALED;
 
        if (first)
-- 
1.8.3.1