xen-netback: Fix Rx stall due to race condition

The recent patch to fix receive side flow control
(11b57f90257c1d6a91cee720151b69e0c2020cf6: xen-netback: stop vif thread
spinning if frontend is unresponsive) solved the spinning thread problem,
however caused an another one. The receive side can stall, if:
- [THREAD] xenvif_rx_action sets rx_queue_stopped to true
- [INTERRUPT] interrupt happens, and sets rx_event to true
- [THREAD] then xenvif_kthread sets rx_event to false
- [THREAD] rx_work_todo doesn't return true anymore

Also, if interrupt sent but there is still no room in the ring, it take quite a
long time until xenvif_rx_action realize it. This patch ditch that two variable,
and rework rx_work_todo. If the thread finds it can't fit more skb's into the
ring, it saves the last slot estimation into rx_last_skb_slots, otherwise it's
kept as 0. Then rx_work_todo will check if:
- there is something to send to the ring (like before)
- there is space for the topmost packet in the queue

I think that's more natural and optimal thing to test than two bool which are
set somewhere else.

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 5 deletions

diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
index 4c76bcb9a879..ae413a2cbee7 100644
--- a/drivers/net/xen-netback/common.h
+++ b/drivers/net/xen-netback/common.h
@@ -143,11 +143,7 @@ struct xenvif {
         char rx_irq_name[IFNAMSIZ+4]; /* DEVNAME-rx */
         struct xen_netif_rx_back_ring rx;
         struct sk_buff_head rx_queue;
-        bool rx_queue_stopped;
-        /* Set when the RX interrupt is triggered by the frontend.
-         * The worker thread may need to wake the queue.
-         */
-        bool rx_event;
+        RING_IDX rx_last_skb_slots;
 
         /* This array is allocated seperately as it is large */
         struct gnttab_copy *grant_copy_op;