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Subject[PATCH 4.12 119/196] nvme-rdma: remove race conditions from IB signalling
Date
4.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Marta Rybczynska <mrybczyn@kalray.eu>

commit 5e599d73c1c1816af07f94ddba879499aa39b43c upstream.

This patch improves the way the RDMA IB signalling is done by using atomic
operations for the signalling variable. This avoids race conditions on
sig_count.

The signalling interval changes slightly and is now the largest power of
two not larger than queue depth / 2.

ilog() usage idea by Bart Van Assche.

Signed-off-by: Marta Rybczynska <marta.rybczynska@kalray.eu>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/nvme/host/rdma.c | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)

--- a/drivers/nvme/host/rdma.c
+++ b/drivers/nvme/host/rdma.c
@@ -88,7 +88,7 @@ enum nvme_rdma_queue_flags {

struct nvme_rdma_queue {
struct nvme_rdma_qe *rsp_ring;
- u8 sig_count;
+ atomic_t sig_count;
int queue_size;
size_t cmnd_capsule_len;
struct nvme_rdma_ctrl *ctrl;
@@ -553,6 +553,7 @@ static int nvme_rdma_init_queue(struct n
queue->cmnd_capsule_len = sizeof(struct nvme_command);

queue->queue_size = queue_size;
+ atomic_set(&queue->sig_count, 0);

queue->cm_id = rdma_create_id(&init_net, nvme_rdma_cm_handler, queue,
RDMA_PS_TCP, IB_QPT_RC);
@@ -1040,17 +1041,16 @@ static void nvme_rdma_send_done(struct i
nvme_rdma_wr_error(cq, wc, "SEND");
}

-static inline int nvme_rdma_queue_sig_limit(struct nvme_rdma_queue *queue)
+/*
+ * We want to signal completion at least every queue depth/2. This returns the
+ * largest power of two that is not above half of (queue size + 1) to optimize
+ * (avoid divisions).
+ */
+static inline bool nvme_rdma_queue_sig_limit(struct nvme_rdma_queue *queue)
{
- int sig_limit;
+ int limit = 1 << ilog2((queue->queue_size + 1) / 2);

- /*
- * We signal completion every queue depth/2 and also handle the
- * degenerated case of a 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;
+ return (atomic_inc_return(&queue->sig_count) & (limit - 1)) == 0;
}

static int nvme_rdma_post_send(struct nvme_rdma_queue *queue,

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