Subject: Re: [Xen-devel] [PATCH net V2] xen-netback: don't move event pointer in TX credit timeout callback

From: David Vrabel <[email-masked]>
Date: Thu, 15 May 2014 14:40:58 +0100

On 15/05/14 12:59, Wei Liu wrote:
> ... otherwise the frontend will try to send TX event all the time, even
> if no progress can be made. The pointer should only be advanced by the
> routine that actually processes the ring (that is, xenvif_poll).

No it does not.  RING_FINAL_CHECK_FOR_REQUESTS() only advances the event
index if the ring is empty.

This will also result in xenvif_up() failing to properly enable the event.

I think Jacek's bug may be that netback fails to call napi_complete()
when credit is exceeded and there still outstanding requests on the
from-guest ring and thus napi repeatedly polls.

David

> 
> Reported-by: Jacek Konieczny <[email-masked]>
> Signed-off-by: Wei Liu <[email-masked]>
> Acked-by: Ian Campbell <[email-masked]>
> Cc: Paul Durrant <[email-masked]>
> ---
>  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 7666540..8e2cbeb 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -658,7 +658,7 @@ void xenvif_check_rx_xenvif(struct xenvif *vif)
>  {
>  	int more_to_do;
>  
> -	RING_FINAL_CHECK_FOR_REQUESTS(&vif->tx, more_to_do);
> +	more_to_do = RING_HAS_UNCONSUMED_REQUESTS(&vif->tx);
>  
>  	if (more_to_do)
>  		napi_schedule(&vif->napi);
>