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, 6 insertions, 10 deletions

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 6b62c3eb8e18..e5284bca2d90 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -476,7 +476,6 @@ static void xenvif_rx_action(struct xenvif *vif)
         unsigned long offset;
         struct skb_cb_overlay *sco;
         bool need_to_notify = false;
-        bool ring_full = false;
 
         struct netrx_pending_operations npo = {
                 .copy  = vif->grant_copy_op,
@@ -486,7 +485,7 @@ static void xenvif_rx_action(struct xenvif *vif)
         skb_queue_head_init(&rxq);
 
         while ((skb = skb_dequeue(&vif->rx_queue)) != NULL) {
-                int max_slots_needed;
+                RING_IDX max_slots_needed;
                 int i;
 
                 /* We need a cheap worse case estimate for the number of
@@ -509,9 +508,10 @@ static void xenvif_rx_action(struct xenvif *vif)
                 if (!xenvif_rx_ring_slots_available(vif, max_slots_needed)) {
                         skb_queue_head(&vif->rx_queue, skb);
                         need_to_notify = true;
-                        ring_full = true;
+                        vif->rx_last_skb_slots = max_slots_needed;
                         break;
-                }
+                } else
+                        vif->rx_last_skb_slots = 0;
 
                 sco = (struct skb_cb_overlay *)skb->cb;
                 sco->meta_slots_used = xenvif_gop_skb(skb, &npo);
@@ -522,8 +522,6 @@ static void xenvif_rx_action(struct xenvif *vif)
 
         BUG_ON(npo.meta_prod > ARRAY_SIZE(vif->meta));
 
-        vif->rx_queue_stopped = !npo.copy_prod && ring_full;
-
         if (!npo.copy_prod)
                 goto done;
 
@@ -1473,8 +1471,8 @@ static struct xen_netif_rx_response *make_rx_response(struct xenvif *vif,
 
 static inline int rx_work_todo(struct xenvif *vif)
 {
-        return (!skb_queue_empty(&vif->rx_queue) && !vif->rx_queue_stopped) ||
-                vif->rx_event;
+        return !skb_queue_empty(&vif->rx_queue) &&
+               xenvif_rx_ring_slots_available(vif, vif->rx_last_skb_slots);
 }
 
 static inline int tx_work_todo(struct xenvif *vif)
@@ -1560,8 +1558,6 @@ int xenvif_kthread(void *data)
                 if (!skb_queue_empty(&vif->rx_queue))
                         xenvif_rx_action(vif);
 
-                vif->rx_event = false;
-
                 if (skb_queue_empty(&vif->rx_queue) &&
                     netif_queue_stopped(vif->dev))
                         xenvif_start_queue(vif);