On Wed, Apr 10, 2019 at 12:58 PM Xiaolong Ye <xiaolong...@intel.com> wrote:
> Since tx callback only picks mbufs that can be copied to the xdp desc, it
> should call xsk_ring_prod__submit with valid count other than nb_pkts.
>
> Fixes: f1debd77efaf ("net/af_xdp: introduce AF_XDP PMD")
>
> Reported-by: David Marchand <david.march...@redhat.com>
> Signed-off-by: Xiaolong Ye <xiaolong...@intel.com>
> ---
> drivers/net/af_xdp/rte_eth_af_xdp.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/af_xdp/rte_eth_af_xdp.c
> b/drivers/net/af_xdp/rte_eth_af_xdp.c
> index 5cc643ce2..8c2ba5740 100644
> --- a/drivers/net/af_xdp/rte_eth_af_xdp.c
> +++ b/drivers/net/af_xdp/rte_eth_af_xdp.c
> @@ -300,7 +300,7 @@ eth_af_xdp_tx(void *queue, struct rte_mbuf **bufs,
> uint16_t nb_pkts)
> rte_pktmbuf_free(mbuf);
> }
>
> - xsk_ring_prod__submit(&txq->tx, nb_pkts);
> + xsk_ring_prod__submit(&txq->tx, valid);
>
Err, well, I think we will have an issue here.
Taking from the 5.1.0-rc4 sources I have:
static inline __u32 xsk_prod_nb_free(struct xsk_ring_prod *r, __u32 nb)
{
__u32 free_entries = r->cached_cons - r->cached_prod;
if (free_entries >= nb)
return free_entries;
/* Refresh the local tail pointer.
* cached_cons is r->size bigger than the real consumer pointer so
* that this addition can be avoided in the more frequently
* executed code that computs free_entries in the beginning of
* this function. Without this optimization it whould have been
* free_entries = r->cached_prod - r->cached_cons + r->size.
*/
r->cached_cons = *r->consumer + r->size;
return r->cached_cons - r->cached_prod;
}
static inline size_t xsk_ring_prod__reserve(struct xsk_ring_prod *prod,
size_t nb, __u32 *idx)
{
if (unlikely(xsk_prod_nb_free(prod, nb) < nb))
return 0;
*idx = prod->cached_prod;
prod->cached_prod += nb;
return nb;
}
static inline void xsk_ring_prod__submit(struct xsk_ring_prod *prod, size_t
nb)
{
/* Make sure everything has been written to the ring before
signalling
* this to the kernel.
*/
smp_wmb();
*prod->producer += nb;
}
If we reserve N slots, but only submit n slots, we end up with an incorrect
opinion of the number of available slots later.
Either xsk_ring_prod__submit should also update cached_prod (but I am not
sure it was the intent of this api), or we must ensure that both reserve
and submit are consistent.
Did I miss some subtility ?
--
David Marchand
