Re: [PATCH 2/2] ethdev: fix race condition in fast-path ops setup

[fengchengwen <fengchengwen@huawei.com>]

On 2023/2/20 14:08, Ashok Kaladi wrote:
> If ethdev enqueue or dequeue function is called during
> eth_dev_fp_ops_setup(), it may get pre-empted after setting
> the function pointers, but before setting the pointer to port data.
> In this case the newly registered enqueue/dequeue function will use
> dummy port data and end up in seg fault.
> 
> This patch moves the updation of each data pointers before updating
> corresponding function pointers.
> 
> Fixes: c87d435a4d79 ("ethdev: copy fast-path API into separate structure")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Ashok Kaladi <ashok.k.kaladi@intel.com>
> 
> diff --git a/lib/ethdev/ethdev_private.c b/lib/ethdev/ethdev_private.c
> index 48090c879a..a0232c669f 100644
> --- a/lib/ethdev/ethdev_private.c
> +++ b/lib/ethdev/ethdev_private.c
> @@ -270,17 +270,17 @@ void
>  eth_dev_fp_ops_setup(struct rte_eth_fp_ops *fpo,
>  		const struct rte_eth_dev *dev)
>  {
> +	fpo->rxq.data = dev->data->rx_queues;
>  	fpo->rx_pkt_burst = dev->rx_pkt_burst;
> +	fpo->txq.data = dev->data->tx_queues;
>  	fpo->tx_pkt_burst = dev->tx_pkt_burst;
>  	fpo->tx_pkt_prepare = dev->tx_pkt_prepare;
>  	fpo->rx_queue_count = dev->rx_queue_count;
>  	fpo->rx_descriptor_status = dev->rx_descriptor_status;
>  	fpo->tx_descriptor_status = dev->tx_descriptor_status;
>  
> -	fpo->rxq.data = dev->data->rx_queues;
>  	fpo->rxq.clbk = (void **)(uintptr_t)dev->post_rx_burst_cbs;
>  
> -	fpo->txq.data = dev->data->tx_queues;
>  	fpo->txq.clbk = (void **)(uintptr_t)dev->pre_tx_burst_cbs;

Hi Ashok,

The modification is OK for the x86 platform (which has strong memory order, and will keep write-after-write order in here, and read-after-read in rte_eth_rx/tx_burst),
but for other weak memory order (like ARM platform) will fail.

For the weak memory order, suggest add write-mb in here, and read-mb in rte_eth_rx/tx_burst.
But the read-mb in rte_eth_rx/tx_burst will affect performance, especially the variable will changes only once when start.

So I suggest use write-mb + delay in here:
   fpo->rxq.data = dev->data->rx_queues;
   fpo->txq.data = dev->data->tx_queues;
   mdelay(5); // delay e.g. 5ms
   fpo->rx_pkt_burst = dev->rx_pkt_burst;
   fpo->tx_pkt_burst = dev->tx_pkt_burst;

And also cc ARMv8 maintainer.

>  }
>  
>