[PATCH 2/2] net/atlantic: fix Tx cleanup descriptor read race

[mike]

atl_xmit_cleanup() scans hw_ring descriptors for a set dd bit to
know how many are safe to reclaim, reading dd straight from
DMA-coherent memory the NIC concurrently writes, with no barrier.

A torn or stale read here can make the scan believe nothing is
done when descriptors have genuinely completed, so to_clean comes
back 0 and cleanup is skipped. Since atl_xmit_pkts() only refuses
to send when tx_free is too low, and only cleanup ever grows
tx_free back, a missed cleanup pass can leave tx_free permanently
short: every later send call sees insufficient free descriptors
and transmits nothing, indefinitely, with no way to recover.

Add a read barrier once the scan's dd checks are complete and
before acting on to_clean.

Fixes: 2b1472d7150c ("net/atlantic: implement Tx path")
Cc: pavel.belous at aquantia.com

Signed-off-by: Mike Murphy <mike at sugarmillgang.com>
---
 drivers/net/atlantic/atl_rxtx.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/atlantic/atl_rxtx.c b/drivers/net/atlantic/atl_rxtx.c
index 2d551a341f..34f53cd6cc 100644
--- a/drivers/net/atlantic/atl_rxtx.c
+++ b/drivers/net/atlantic/atl_rxtx.c
@@ -1162,6 +1162,12 @@ atl_xmit_cleanup(struct atl_tx_queue *txq)
 				break;
 		}
 
+		/*
+		 * Same dd/DMA race as the Rx path; barrier before
+		 * acting on what was just read.
+		 */
+		rte_rmb();
+
 		if (to_clean == 0)
 			return;
 
-- 
2.34.1