macb: Don't re-enable interrupts while in polling mode

On a busy network, the macb driver could get stuck in the interrupt
handler, quickly triggering the watchdog, due to a confluence of
factors:

 1. macb_poll re-enables interrupts unconditionally, even when it will
    be called again because it exhausted its rx budget

 2. macb_interrupt only disables interrupts after scheduling
    macb_poll, but scheduling fails when macb_poll is already scheduled
    because it didn't call napi_complete

 3. macb_interrupt loops until the interrupt status register is clear,
    which will never happen in this case if the driver doesn't disable
    the RX interrupt

Since macb_interrupt runs in interrupt context, this effectively locks
up the machine, triggering the hardware watchdog.

This issue was readily reproducible on a flooded network with a
modified 2.6.27.48 kernel. The same problem appears to still be in the
2.6.36-rc8 driver code, so I am submitting this patch against that
version. I have not tested this version of the patch except to make
sure the kernel compiles.
Signed-off-by: NJoshua Hoke <joshua.hoke@sixnet.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/macb.c

@@ -515,7 +515,7 @@ static int macb_poll(struct napi_struct *napi, int budget)
 		(unsigned long)status, budget);
 
 	work_done = macb_rx(bp, budget);
-	if (work_done < budget)
+	if (work_done < budget) {
 		napi_complete(napi);
 
 		/*
@@ -523,6 +523,7 @@ static int macb_poll(struct napi_struct *napi, int budget)
 		 * get notified when new packets arrive.
 		 */
 		macb_writel(bp, IER, MACB_RX_INT_FLAGS);
+	}
 
 	/* TODO: Handle errors */
 
@@ -550,12 +551,16 @@ static irqreturn_t macb_interrupt(int irq, void *dev_id)
 		}
 
 		if (status & MACB_RX_INT_FLAGS) {
-			if (napi_schedule_prep(&bp->napi)) {
 			/*
 			 * There's no point taking any more interrupts
-				 * until we have processed the buffers
+			 * until we have processed the buffers. The
+			 * scheduling call may fail if the poll routine
+			 * is already scheduled, so disable interrupts
+			 * now.
 			 */
 			macb_writel(bp, IDR, MACB_RX_INT_FLAGS);
+
+			if (napi_schedule_prep(&bp->napi)) {
 				dev_dbg(&bp->pdev->dev,
 					"scheduling RX softirq\n");
 				__napi_schedule(&bp->napi);