Re: [dpdk-dev] [PATCH v1 2/8] examples/vhost_blk: replace smp with thread fence

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 12/21/20 4:50 PM, Joyce Kong wrote:
> Simply replace the rte_smp_mb barriers with SEQ_CST atomic thread fence,
> if there is no load/store operations.
> 
> Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
>  examples/vhost_blk/vhost_blk.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/examples/vhost_blk/vhost_blk.c b/examples/vhost_blk/vhost_blk.c
> index bb293d492..7ea60863d 100644
> --- a/examples/vhost_blk/vhost_blk.c
> +++ b/examples/vhost_blk/vhost_blk.c
> @@ -86,9 +86,9 @@ enqueue_task(struct vhost_blk_task *task)
>  	 */
>  	used->ring[used->idx & (vq->vring.size - 1)].id = task->req_idx;
>  	used->ring[used->idx & (vq->vring.size - 1)].len = task->data_len;

From here
> -	rte_smp_mb();
> +	rte_atomic_thread_fence(__ATOMIC_SEQ_CST);
>  	used->idx++;
> -	rte_smp_mb();
> +	rte_atomic_thread_fence(__ATOMIC_SEQ_CST);

to here, couldn't it be replaced with:
__atomic_add_fetch(&used->idx, 1, __ATOMIC_RELEASE);

?

>  	rte_vhost_clr_inflight_desc_split(task->ctrlr->vid,
>  		vq->id, used->idx, task->req_idx);
> @@ -112,12 +112,12 @@ enqueue_task_packed(struct vhost_blk_task *task)
>  	desc->id = task->buffer_id;
>  	desc->addr = 0;
>  
> -	rte_smp_mb();
> +	rte_atomic_thread_fence(__ATOMIC_SEQ_CST);
>  	if (vq->used_wrap_counter)
>  		desc->flags |= VIRTQ_DESC_F_AVAIL | VIRTQ_DESC_F_USED;
>  	else
>  		desc->flags &= ~(VIRTQ_DESC_F_AVAIL | VIRTQ_DESC_F_USED);
> -	rte_smp_mb();
> +	rte_atomic_thread_fence(__ATOMIC_SEQ_CST);
>  
>  	rte_vhost_clr_inflight_desc_packed(task->ctrlr->vid, vq->id,
>  					   task->inflight_idx);
>