linux-kernel - [PATCH 4.2 008/124] amd-xgbe: Use wmb before updating current descriptor count

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Date:	Mon,  7 Dec 2015 09:54:58 -0500
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Tom Lendacky <thomas.lendacky@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: [PATCH 4.2 008/124] amd-xgbe: Use wmb before updating current descriptor count

4.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Lendacky, Thomas" <Thomas.Lendacky@....com>

[ 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.

Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
Tested-by: Christoffer Dall <christoffer.dall@...aro.org>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 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(-)

--- a/drivers/net/ethernet/amd/xgbe/xgbe-dev.c
+++ b/drivers/net/ethernet/amd/xgbe/xgbe-dev.c
@@ -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 ||
--- a/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
+++ b/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
@@ -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;
 


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