[NET] back: Fix wrap to zero in transmit credit scheduler.

This could happen when credit_bytes == ~0UL (i.e., scheduling is 'disabled').
Signed-off-by: Kirk Allan <kallan@novell.com>

diff --git a/linux-2.6-xen-sparse/drivers/xen/netback/netback.c b/linux-2.6-xen-sparse/drivers/xen/netback/netback.c
index 5706322a8f03c6ec12ca0be4472425b0689c4da3..c4fb3e438359210fc9535d694b2e3af1fb8ed2d2 100644 (file)
--- a/linux-2.6-xen-sparse/drivers/xen/netback/netback.c
+++ b/linux-2.6-xen-sparse/drivers/xen/netback/netback.c
@@ -814,7 +814,7 @@ void netif_deschedule_work(netif_t *netif)
 
 static void tx_add_credit(netif_t *netif)
 {
-       unsigned long max_burst;
+       unsigned long max_burst, max_credit;
 
        /*
         * Allow a burst big enough to transmit a jumbo packet of up to 128kB.
@@ -824,9 +824,10 @@ static void tx_add_credit(netif_t *netif)
        max_burst = min(max_burst, 131072UL);
        max_burst = max(max_burst, netif->credit_bytes);
 
-       netif->remaining_credit = min(netif->remaining_credit +
-                                     netif->credit_bytes,
-                                     max_burst);
+       /* Take care that adding a new chunk of credit doesn't wrap to zero. */
+       max_credit = max(netif->remaining_credit + netif->credit_bytes, ~0UL);
+
+       netif->remaining_credit = min(max_credit, max_burst);
 }
 
 static void tx_credit_callback(unsigned long data)