Re: [dpdk-dev] [PATCH] kni: fix kni rx fifo producer synchronization

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 8/9/2018 11:23 AM, Kiran Kumar wrote:
> With existing code in kni_fifo_put, rx_q values are not being updated
> before updating fifo_write. While reading rx_q in kni_net_rx_normal,
> This is causing the sync issue on other core. So adding a write
> barrier to make sure the values being synced before updating fifo_write.
> 
> Fixes: 3fc5ca2f6352 ("kni: initial import")
> 
> Signed-off-by: Kiran Kumar <kkokkilagadda@caviumnetworks.com>
> ---
>  lib/librte_kni/rte_kni_fifo.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/lib/librte_kni/rte_kni_fifo.h b/lib/librte_kni/rte_kni_fifo.h
> index ac26a8c..4d6b33e 100644
> --- a/lib/librte_kni/rte_kni_fifo.h
> +++ b/lib/librte_kni/rte_kni_fifo.h
> @@ -39,6 +39,7 @@ kni_fifo_put(struct rte_kni_fifo *fifo, void **data, unsigned num)
>  		fifo->buffer[fifo_write] = data[i];
>  		fifo_write = new_write;
>  	}
> +	rte_smp_wmb();
>  	fifo->write = fifo_write;
>  	return i;

For Intel this is just a compiler barrier so no issue but not sure if a memory
barrier is required here,

Related code block is:

|-          for (i = 0; i < num; i++) {
||                  new_write = (new_write + 1) & (fifo->len - 1);
||
||                  if (new_write == fifo_read)
||                          break;
||                  fifo->buffer[fifo_write] = data[i];
||                  fifo_write = new_write;
||          }
|           fifo->write = fifo_write;

"fifo_write" is updated in the loop, so there is a dependency to it for
"fifo->write". Can memory writes be reordered when there is a dependency?

Cc'ed a few more people for comment.