Subject: net: fix race in process_backlog
The recent NAPI rework (4fa57c9ea9f36f9ca852f3a88ca5d2f1aebbc960)

introduced a race between netif_rx() and process_backlog() which

resulted in softirq processing to drop dead.
netif_rx()		process_backlog()
			irq_disable();

			skb = __skb_dequeue();

			irq_enable();
irq_disable();

__skb_queue_tail();

napi_schedule();

irq_enable();
			if (!skb)

			  napi_complete();  <-- oops!
we cleared the napi bit, even though there is data to process.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

---

 net/core/dev.c |    4 ++--

 1 file changed, 2 insertions(+), 2 deletions(-)
Index: linux-2.6/net/core/dev.c

===================================================================

--- linux-2.6.orig/net/core/dev.c

+++ linux-2.6/net/core/dev.c

@@ -2095,11 +2095,11 @@ static int process_backlog(struct napi_s
 		local_irq_disable();

 		skb = __skb_dequeue(&queue->input_pkt_queue);

-		local_irq_enable();

 		if (!skb) {

-			napi_complete(napi);

+			__napi_complete(napi);

 			break;

 		}

+		local_irq_enable();
 		dev = skb->dev;
-

From: Peter Zijlstra <a.p.zijlstra@chello.nl>
What re-enables interrupts in the !skb path?

-

On Wed, 03 Oct 2007 14:58:07 -0700 (PDT)
This looks like a better fix. the irq_enable is needed in both cases.
--- a/net/core/dev.c	2007-09-27 07:19:10.000000000 -0700

+++ b/net/core/dev.c	2007-10-03 15:03:54.000000000 -0700

@@ -2077,12 +2077,14 @@ static int process_backlog(struct napi_s
 		local_irq_disable();

 		skb = __skb_dequeue(&queue->input_pkt_queue);

-		local_irq_enable();

 		if (!skb) {

-			napi_complete(napi);

+			__napi_complete(napi);

+			local_irq_enable();

 			break;

 		}
+		local_irq_enable();

+

 		dev = skb->dev;
 		netif_receive_skb(skb);
--

Stephen Hemminger <shemminger@linux-foundation.org>

-

From: Stephen Hemminger <shemminger@linux-foundation.org>
Yep, applied, thanks Peter and Stephen.

-

On Wed, 03 Oct 2007 17:44:53 +0200
Acked-by: Stephen Hemminger <shemminger@linux-foundation.org>
--

Stephen Hemminger <shemminger@linux-foundation.org>

-