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

[Meir Elisha]

Just making sure it wasn't forgotten since there was a lot of traffic. Thanks for your time!

On 26/02/2025 9:28, Meir Elisha wrote:
> 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)