[PATCH net v2] net: xilinx: axienet: Free outstanding DMA buffers on dmaengine stop

[Suraj Gupta <suraj.gupta2@amd.com>]

In the dmaengine path the driver pre-submits RX buffers and holds
in-flight TX buffers whose SKBs are DMA-mapped by the driver and freed
only in the completion callbacks. On ndo_stop() the driver calls
dmaengine_terminate_sync(), which aborts these descriptors without
running their callbacks, and then frees only the ring shells with
kfree(). Every SKB still owned by the engine, and its DMA mapping, is
thus leaked on each ifdown. With 128 RX buffers pre-posted per channel,
the mapping leak can eventually exhaust a limited IOMMU aperture.

Clear the slot's skb pointer in the TX and RX callbacks so a non-NULL
skb marks a slot that still owns a live, DMA-mapped buffer, and on stop
unmap and free every such buffer.

axienet_dma_rx_cb() re-arms the RX ring on every completion, so a
completion racing with axienet_stop() could resubmit a buffer after the
terminate that the teardown then frees while the engine still owns it.
Set @stopping before terminating and check it in the callback to fence
resubmission, and release the channels before freeing the rings so a
late completion cannot touch a freed ring.

Fixes: 6a91b846af85 ("net: axienet: Introduce dmaengine support")
Cc: stable@vger.kernel.org
Signed-off-by: Suraj Gupta <suraj.gupta2@amd.com>
---
Changes in v2:
- Fence RX descriptor resubmission in axienet_dma_rx_cb() against a stop
  in progress via the existing @stopping flag, and release the DMA
  channels before freeing the rings (Jakub Kicinski).
- Drop the redundant dmaengine_synchronize() calls that followed
  dmaengine_terminate_sync(), which already ends with a synchronize
  (Jakub Kicinski).
- Expand the commit message to describe the race fix.

v1: https://lore.kernel.org/netdev/20260910141946.3017164-1-suraj.gupta2@amd.com/
---
 drivers/net/ethernet/xilinx/xilinx_axienet.h  |  5 +-
 .../net/ethernet/xilinx/xilinx_axienet_main.c | 50 +++++++++++++++----
 2 files changed, 44 insertions(+), 11 deletions(-)

diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet.h b/drivers/net/ethernet/xilinx/xilinx_axienet.h
index fcd3aaef27fc..7c75e313dd33 100644
--- a/drivers/net/ethernet/xilinx/xilinx_axienet.h
+++ b/drivers/net/ethernet/xilinx/xilinx_axienet.h
@@ -523,8 +523,9 @@ struct skbuf_dma_descriptor {
  * @stats_work: Work for reading the hardware statistics counters often enough
  *              to catch overflows.
  * @dma_err_task: Work structure to process Axi DMA errors
- * @stopping:   Set when @dma_err_task shouldn't do anything because we are
- *              about to stop the device.
+ * @stopping:   Set when we are about to stop the device: makes @dma_err_task
+ *              a no-op (legacy DMA path) and fences RX descriptor
+ *              resubmission in axienet_dma_rx_cb() (dmaengine path).
  * @tx_irq:	Axidma TX IRQ number
  * @rx_irq:	Axidma RX IRQ number
  * @eth_irq:	Ethernet core IRQ number
diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
index 782f903d318f..36a487f791a2 100644
--- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
+++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
@@ -881,6 +881,7 @@ static void axienet_dma_tx_cb(void *data, const struct dmaengine_result *result)
 	u64_stats_update_end(&lp->tx_stat_sync);
 	dma_unmap_sg(lp->dev, skbuf_dma->sgl, skbuf_dma->sg_len, DMA_TO_DEVICE);
 	dev_consume_skb_any(skbuf_dma->skb);
+	skbuf_dma->skb = NULL;
 	netif_txq_completed_wake(txq, 1, len,
 				 CIRC_SPACE(lp->tx_ring_head, lp->tx_ring_tail, TX_BD_NUM_MAX),
 				 2);
@@ -1171,6 +1172,7 @@ static void axienet_dma_rx_cb(void *data, const struct dmaengine_result *result)
 						       &meta_max_len);
 	dma_unmap_single(lp->dev, skbuf_dma->dma_address, lp->max_frm_size,
 			 DMA_FROM_DEVICE);
+	skbuf_dma->skb = NULL;
 
 	if (IS_ERR(app_metadata)) {
 		if (net_ratelimit())
@@ -1193,6 +1195,12 @@ static void axienet_dma_rx_cb(void *data, const struct dmaengine_result *result)
 	u64_stats_update_end(&lp->rx_stat_sync);
 
 rx_submit:
+	/* Do not re-arm the RX ring while a stop is in progress, or the
+	 * teardown could free a buffer still handed to the engine.
+	 */
+	if (READ_ONCE(lp->stopping))
+		return;
+
 	for (i = 0; i < CIRC_SPACE(lp->rx_ring_head, lp->rx_ring_tail,
 				   RX_BUF_NUM_DEFAULT); i++)
 		axienet_rx_submit_desc(lp->ndev);
@@ -1541,6 +1549,7 @@ static int axienet_init_dmaengine(struct net_device *ndev)
 	lp->tx_ring_head = 0;
 	lp->rx_ring_tail = 0;
 	lp->rx_ring_head = 0;
+	lp->stopping = false;
 	lp->tx_skb_ring = kzalloc_objs(*lp->tx_skb_ring, TX_BD_NUM_MAX);
 	if (!lp->tx_skb_ring) {
 		ret = -ENOMEM;
@@ -1752,20 +1761,43 @@ static int axienet_stop(struct net_device *ndev)
 		free_irq(lp->rx_irq, ndev);
 		axienet_dma_bd_release(ndev);
 	} else {
+		struct skbuf_dma_descriptor *skbuf_dma;
+
+		WRITE_ONCE(lp->stopping, true);
 		dmaengine_terminate_sync(lp->tx_chan);
-		dmaengine_synchronize(lp->tx_chan);
 		dmaengine_terminate_sync(lp->rx_chan);
-		dmaengine_synchronize(lp->rx_chan);
-
-		for (i = 0; i < TX_BD_NUM_MAX; i++)
-			kfree(lp->tx_skb_ring[i]);
-		kfree(lp->tx_skb_ring);
-		for (i = 0; i < RX_BUF_NUM_DEFAULT; i++)
-			kfree(lp->rx_skb_ring[i]);
-		kfree(lp->rx_skb_ring);
 
+		/* Release the channels before freeing the rings, so the DMA is
+		 * fully torn down before the memory its descriptors reference is
+		 * freed.
+		 */
 		dma_release_channel(lp->rx_chan);
 		dma_release_channel(lp->tx_chan);
+
+		/* Unmap and free any buffer the terminate did not reclaim, so it
+		 * is not leaked; a non-NULL skb marks such a slot.
+		 */
+		for (i = 0; i < TX_BD_NUM_MAX; i++) {
+			skbuf_dma = lp->tx_skb_ring[i];
+			if (skbuf_dma && skbuf_dma->skb) {
+				dma_unmap_sg(lp->dev, skbuf_dma->sgl,
+					     skbuf_dma->sg_len, DMA_TO_DEVICE);
+				dev_kfree_skb_any(skbuf_dma->skb);
+			}
+			kfree(skbuf_dma);
+		}
+		kfree(lp->tx_skb_ring);
+
+		for (i = 0; i < RX_BUF_NUM_DEFAULT; i++) {
+			skbuf_dma = lp->rx_skb_ring[i];
+			if (skbuf_dma && skbuf_dma->skb) {
+				dma_unmap_single(lp->dev, skbuf_dma->dma_address,
+						 lp->max_frm_size, DMA_FROM_DEVICE);
+				dev_kfree_skb_any(skbuf_dma->skb);
+			}
+			kfree(skbuf_dma);
+		}
+		kfree(lp->rx_skb_ring);
 	}
 
 	netdev_reset_queue(ndev);
-- 
2.25.1