Staging: octeon-ethernet: Fix race freeing transmit buffers.

The existing code had the following race:

Thread-1                       Thread-2

inc/read in_use
                               inc/read in_use
inc tx_free_list[qos].len
                               inc tx_free_list[qos].len

The actual in_use value was incremented twice, but thread-1 is going
to free memory based on its stale value, and will free one too many
times.  The result is that memory is freed back to the kernel while
its packet is still in the transmit buffer.  If the memory is
overwritten before it is transmitted, the hardware will put a valid
checksum on it and send it out (just like it does with good packets).
If by chance the TCP flags are clobbered but not the addresses or
ports, the result can be a broken TCP stream.

The fix is to track the number of freed packets in a single location
(a Fetch-and-Add Unit register).  That way it can never get out of sync
with itself.

We try to free up to MAX_SKB_TO_FREE (currently 10) buffers at a time.
If fewer are available we adjust the free count with the difference.
The action of claiming buffers to free is atomic so two threads cannot
claim the same buffers.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 25 insertions, 0 deletions

diff --git a/drivers/staging/octeon/ethernet-tx.h b/drivers/staging/octeon/ethernet-tx.h
index 5106236fe981..c0bebf750bc0 100644
--- a/drivers/staging/octeon/ethernet-tx.h
+++ b/drivers/staging/octeon/ethernet-tx.h
@@ -30,3 +30,28 @@ int cvm_oct_xmit_pow(struct sk_buff *skb, struct net_device *dev);
 int cvm_oct_transmit_qos(struct net_device *dev, void *work_queue_entry,
 			 int do_free, int qos);
 void cvm_oct_tx_shutdown(struct net_device *dev);
+
+/**
+ * Free dead transmit skbs.
+ *
+ * @priv:		The driver data
+ * @skb_to_free:	The number of SKBs to free (free none if negative).
+ * @qos:		The queue to free from.
+ * @take_lock:		If true, acquire the skb list lock.
+ */
+static inline void cvm_oct_free_tx_skbs(struct octeon_ethernet *priv,
+					int skb_to_free,
+					int qos, int take_lock)
+{
+	/* Free skbuffs not in use by the hardware.  */
+	if (skb_to_free > 0) {
+		if (take_lock)
+			spin_lock(&priv->tx_free_list[qos].lock);
+		while (skb_to_free > 0) {
+			dev_kfree_skb(__skb_dequeue(&priv->tx_free_list[qos]));
+			skb_to_free--;
+		}
+		if (take_lock)
+			spin_unlock(&priv->tx_free_list[qos].lock);
+	}
+}