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Commit 9ab9831b authored by Zoltan Kiss's avatar Zoltan Kiss Committed by David S. Miller
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xen-netback: Fix Rx stall due to race condition 



The recent patch to fix receive side flow control
(11b57f90: 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: default avatarZoltan Kiss <zoltan.kiss@citrix.com>
Reviewed-by: default avatarPaul Durrant <paul.durrant@citrix.com>
Acked-by: default avatarWei Liu <wei.liu2@citrix.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent bce3ea81

drivers/net/xen-netback/common.h

+1 −5
Original line number Diff line number Diff line
@@ -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;

drivers/net/xen-netback/interface.c

+0 −1
Original line number Diff line number Diff line
@@ -100,7 +100,6 @@ static irqreturn_t xenvif_rx_interrupt(int irq, void *dev_id) 
{

	struct xenvif *vif = dev_id;



	vif->rx_event = true;

	xenvif_kick_thread(vif);



	return IRQ_HANDLED;

drivers/net/xen-netback/netback.c

+6 −10
Original line number Diff line number Diff line
@@ -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);