[PATCH v3] nvmet-tcp: Fix a possible sporadic response drops in weakly ordered arch

[Meir Elisha]

The order in which queue->cmd and rcv_state are updated is crucial.
If these assignments are reordered by the compiler, the worker might not
get queued in nvmet_tcp_queue_response(), hanging the IO. to enforce the
the correct reordering, set rcv_state using smp_store_release().

Fixes: bdaf13279192 ("nvmet-tcp: fix a segmentation fault during io parsing error")

Signed-off-by: Meir Elisha <meir.elisha at volumez.com>
---
Changes from v2:
	- Cosmetic changes to queue_state declaration

Changes from v1:
	- Fix barrier semantics
	- Use rcv_state instead of state variable

This ordering is critical on weakly ordered architectures (such as ARM)
so that any observer which sees the new rcv_state is guaranteed to also
see the updated cmd. Without this guarantee (i.e if the two stores were
reordered), a parallel context might see the new state while queue->cmd
still holds a stale value. This could cause the inline-data check to
return early and ultimately hang the IO.
Additionally, I reviewed the assembly code for ARM and confirmed that
the instructions were reordered(unlike x86), reinforcing the need for
this change.

This scenario was encountered during fio testing, which involved
running 2 min of 4K random writes using an ARM-based machine as the
target. We observed hanging I/O typically after 10-20 iterations.

fio config used:
[global]
ioengine=libaio
max_latency=45s
end_fsync=1
create_serialize=0
size=3200m
directory=/mnt/volumez/vol0
ramp_time=30
lat_percentiles=1
direct=1
filename_format=fiodata.$jobnum
verify_dump=1
numjobs=16
fallocate=native
stonewall=1
group_reporting=1
file_service_type=random
iodepth=16
runtime=5m
time_based=1
[random_0_100_4k]
bs=4k
rw=randwrite

 drivers/nvme/target/tcp.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/nvme/target/tcp.c b/drivers/nvme/target/tcp.c
index 7c51c2a8c109..4f9cac8a5abe 100644
--- a/drivers/nvme/target/tcp.c
+++ b/drivers/nvme/target/tcp.c
@@ -571,10 +571,16 @@ static void nvmet_tcp_queue_response(struct nvmet_req *req)
 	struct nvmet_tcp_cmd *cmd =
 		container_of(req, struct nvmet_tcp_cmd, req);
 	struct nvmet_tcp_queue	*queue = cmd->queue;
+	enum nvmet_tcp_recv_state queue_state;
+	struct nvmet_tcp_cmd *queue_cmd;
 	struct nvme_sgl_desc *sgl;
 	u32 len;
 
-	if (unlikely(cmd == queue->cmd)) {
+	/* Pairs with store_release in nvmet_prepare_receive_pdu() */
+	queue_state = smp_load_acquire(&queue->rcv_state);
+	queue_cmd = READ_ONCE(queue->cmd);
+
+	if (unlikely(cmd == queue_cmd)) {
 		sgl = &cmd->req.cmd->common.dptr.sgl;
 		len = le32_to_cpu(sgl->length);
 
@@ -583,7 +589,7 @@ static void nvmet_tcp_queue_response(struct nvmet_req *req)
 		 * Avoid using helpers, this might happen before
 		 * nvmet_req_init is completed.
 		 */
-		if (queue->rcv_state == NVMET_TCP_RECV_PDU &&
+		if (queue_state == NVMET_TCP_RECV_PDU &&
 		    len && len <= cmd->req.port->inline_data_size &&
 		    nvme_is_write(cmd->req.cmd))
 			return;
@@ -847,8 +853,9 @@ static void nvmet_prepare_receive_pdu(struct nvmet_tcp_queue *queue)
 {
 	queue->offset = 0;
 	queue->left = sizeof(struct nvme_tcp_hdr);
-	queue->cmd = NULL;
-	queue->rcv_state = NVMET_TCP_RECV_PDU;
+	WRITE_ONCE(queue->cmd, NULL);
+	/* Ensure rcv_state is visible only after queue->cmd is set */
+	smp_store_release(&queue->rcv_state, NVMET_TCP_RECV_PDU);
 }
 
 static void nvmet_tcp_free_crypto(struct nvmet_tcp_queue *queue)
-- 
2.34.1