Patch "amd-xgbe: Use wmb before updating current descriptor count" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

amd-xgbe: Use wmb before updating current descriptor count

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
amd-xgbe-use-wmb-before-updating-current-descriptor-count.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Lendacky, Thomas" <[email protected]>
Date: Wed, 21 Oct 2015 15:37:05 -0500
Subject: amd-xgbe: Use wmb before updating current descriptor count

From: "Lendacky, Thomas" <[email protected]>

[ Upstream commit 20a41fba679d665cdae2808e2b9cae97c073351f ]

The code currently uses the lightweight dma_wmb barrier before updating the current descriptor count. Under heavy load, the Tx cleanup routine was seeing the updated current descriptor count before the updated descriptor information. As a result, the Tx descriptor was being cleaned up before it was used because it was not "owned" by the hardware yet, resulting in a Tx queue hang.

Using the wmb barrier insures that the descriptor is updated before the descriptor counter preventing the Tx queue hang. For extra insurance, the Tx cleanup routine is changed to grab the current decriptor count on entry and uses that initial value in the processing loop rather than trying to chase the current value.

---
 drivers/net/ethernet/amd/xgbe/xgbe-dev.c |    2 +-
 drivers/net/ethernet/amd/xgbe/xgbe-drv.c |    4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

@@ -1595,7 +1595,7 @@ static void xgbe_dev_xmit(struct xgbe_ch
 				  packet->rdesc_count, 1);
 
 	/* Make sure ownership is written to the descriptor */
-	dma_wmb();
+	wmb();
 
 	ring->cur = cur_index + 1;
 	if (!packet->skb->xmit_more ||

@@ -1807,6 +1807,7 @@ static int xgbe_tx_poll(struct xgbe_chan
 	struct netdev_queue *txq;
 	int processed = 0;
 	unsigned int tx_packets = 0, tx_bytes = 0;
+	unsigned int cur;
 
 	DBGPR("-->xgbe_tx_poll\n");
 

@@ -1814,10 +1815,11 @@ static int xgbe_tx_poll(struct xgbe_chan
 	if (!ring)
 		return 0;
 
+	cur = ring->cur;
 	txq = netdev_get_tx_queue(netdev, channel->queue_index);
 
 	while ((processed < XGBE_TX_DESC_MAX_PROC) &&
-	       (ring->dirty != ring->cur)) {
+	       (ring->dirty != cur)) {
 		rdata = XGBE_GET_DESC_DATA(ring, ring->dirty);
 		rdesc = rdata->rdesc;
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/amd-xgbe-fix-race-between-access-of-desc-and-desc-index.patch queue-4.2/amd-xgbe-use-wmb-before-updating-current-descriptor-count.patch