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: Joshua Hoke <joshua.hoke@sixnet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 16 insertions, 11 deletions

diff --git a/drivers/net/macb.c b/drivers/net/macb.c
index 4297f6e8c4bc..f69e73e2191e 100644
--- a/drivers/net/macb.c
+++ b/drivers/net/macb.c
@@ -515,14 +515,15 @@ 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);
 
-        /*
-         * We've done what we can to clean the buffers. Make sure we
-         * get notified when new packets arrive.
-         */
-        macb_writel(bp, IER, MACB_RX_INT_FLAGS);
+                /*
+                 * We've done what we can to clean the buffers. Make sure we
+                 * 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) {
+                        /*
+                         * There's no point taking any more interrupts
+                         * 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)) {
-                                /*
-                                 * There's no point taking any more interrupts
-                                 * until we have processed the buffers
-                                 */
-                                macb_writel(bp, IDR, MACB_RX_INT_FLAGS);
                                 dev_dbg(&bp->pdev->dev,
                                         "scheduling RX softirq\n");
                                 __napi_schedule(&bp->napi);