Re: [PATCH net v3 2/2] xen-netback: napi: don't prematurely request a tx event

[Malcolm Crossley <malcolm.crossley@citrix.com>]

On 10/12/13 13:39, Paul Durrant wrote:
> This patch changes the RING_FINAL_CHECK_FOR_REQUESTS in
> xenvif_build_tx_gops to a check for RING_HAS_UNCONSUMED_REQUESTS as the
> former call has the side effect of advancing the ring event pointer and
> therefore inviting another interrupt from the frontend before the napi
> poll has actually finished, thereby defeating the point of napi.
Reported-by: Malcolm Crossley <malcolm.crossley@citrix.com>
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> Cc: Wei Liu <wei.liu2@citrix.com>
> Cc: Ian Campbell <ian.campbell@citrix.com>
> Cc: David Vrabel <david.vrabel@citrix.com>
> ---
>   drivers/net/xen-netback/netback.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index 83b4e5b..fb512a9 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1381,7 +1381,7 @@ static unsigned xenvif_tx_build_gops(struct xenvif *vif, int budget)
>   			continue;
>   		}
>   
> -		RING_FINAL_CHECK_FOR_REQUESTS(&vif->tx, work_to_do);
> +		work_to_do = RING_HAS_UNCONSUMED_REQUESTS(&vif->tx);
>   		if (!work_to_do)
>   			break;
>