Date: Wed, 1 Nov 2023 12:18:58 -0700
From: <alexey.pakhunov@...cex.com>
To: <siva.kallam@...adcom.com>
CC: <vincent.wong2@...cex.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <prashant@...adcom.com>,
        <mchan@...adcom.com>, Alex Pakhunov <alexey.pakhunov@...cex.com>
Subject: [PATCH 2/2] tg3: Fix the TX ring stall

From: Alex Pakhunov <alexey.pakhunov@...cex.com>

The TX ring maintained by the tg3 driver can end up in a state, when it
has packets queued for sending but the NIC hardware is not informed, so no
progress is made. This leads to a multi-second interruption in network
traffic followed by dev_watchdog() firing and resetting the queue.

The specific sequence of steps is:

1. tg3_start_xmit() is called at least once and queues packet(s) without
   updating tnapi->prodmbox (netdev_xmit_more() returns true)
2. tg3_start_xmit() is called with an SKB which causes tg3_tso_bug() to be
   called.
3. tg3_tso_bug() determines that the SKB is too large, ...

        if (unlikely(tg3_tx_avail(tnapi) <= frag_cnt_est)) {

   ... stops the queue, and returns NETDEV_TX_BUSY:

        netif_tx_stop_queue(txq);
        ...
        if (tg3_tx_avail(tnapi) <= frag_cnt_est)
                return NETDEV_TX_BUSY;

4. Since all tg3_tso_bug() call sites directly return, the code updating
   tnapi->prodmbox is skipped.

5. The queue is stuck now. tg3_start_xmit() is not called while the queue
   is stopped. The NIC is not processing new packets because
   tnapi->prodmbox wasn't updated. tg3_tx() is not called by
   tg3_poll_work() because the all TX descriptions that could be freed has
   been freed:

        /* run TX completion thread */
        if (tnapi->hw_status->idx[0].tx_consumer != tnapi->tx_cons) {
                tg3_tx(tnapi);

6. Eventually, dev_watchdog() fires triggering a reset of the queue.

This fix makes sure that the tnapi->prodmbox update happens regardless of
the reason tg3_start_xmit() returned.

Signed-off-by: Alex Pakhunov <alexey.pakhunov@...cex.com>
Signed-off-by: Vincent Wong <vincent.wong2@...cex.com>
---
 drivers/net/ethernet/broadcom/tg3.c | 46 ++++++++++++++++++++++-------
 1 file changed, 35 insertions(+), 11 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/tg3.c b/drivers/net/ethernet/broadcom/tg3.c
index 99638e6c9e16..c3512409434e 100644
--- a/drivers/net/ethernet/broadcom/tg3.c
+++ b/drivers/net/ethernet/broadcom/tg3.c
@@ -6603,9 +6603,9 @@ static void tg3_tx(struct tg3_napi *tnapi)
 
 	tnapi->tx_cons = sw_idx;
 
-	/* Need to make the tx_cons update visible to tg3_start_xmit()
+	/* Need to make the tx_cons update visible to __tg3_start_xmit()
 	 * before checking for netif_queue_stopped().  Without the
-	 * memory barrier, there is a small possibility that tg3_start_xmit()
+	 * memory barrier, there is a small possibility that __tg3_start_xmit()
 	 * will miss it and cause the queue to be stopped forever.
 	 */
 	smp_mb();
@@ -7845,7 +7845,7 @@ static bool tg3_tso_bug_gso_check(struct tg3_napi *tnapi, struct sk_buff *skb)
 	return skb_shinfo(skb)->gso_segs < tnapi->tx_pending / 3;
 }
 
-static netdev_tx_t tg3_start_xmit(struct sk_buff *, struct net_device *);
+static netdev_tx_t __tg3_start_xmit(struct sk_buff *, struct net_device *);
 
 /* Use GSO to workaround all TSO packets that meet HW bug conditions
  * indicated in tg3_tx_frag_set()
@@ -7881,7 +7881,7 @@ static int tg3_tso_bug(struct tg3 *tp, struct tg3_napi *tnapi,
 
 	skb_list_walk_safe(segs, seg, next) {
 		skb_mark_not_on_list(seg);
-		tg3_start_xmit(seg, tp->dev);
+		__tg3_start_xmit(seg, tp->dev);
 	}
 
 tg3_tso_bug_end:
@@ -7891,7 +7891,7 @@ static int tg3_tso_bug(struct tg3 *tp, struct tg3_napi *tnapi,
 }
 
 /* hard_start_xmit for all devices */
-static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t __tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct tg3 *tp = netdev_priv(dev);
 	u32 len, entry, base_flags, mss, vlan = 0;
@@ -8135,11 +8135,6 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 			netif_tx_wake_queue(txq);
 	}
 
-	if (!netdev_xmit_more() || netif_xmit_stopped(txq)) {
-		/* Packets are ready, update Tx producer idx on card. */
-		tw32_tx_mbox(tnapi->prodmbox, entry);
-	}
-
 	return NETDEV_TX_OK;
 
 dma_error:
@@ -8152,6 +8147,35 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	return NETDEV_TX_OK;
 }
 
+static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
+{
+	u16 skb_queue_mapping = skb_get_queue_mapping(skb);
+	struct netdev_queue *txq = netdev_get_tx_queue(dev, skb_queue_mapping);
+
+	netdev_tx_t ret = __tg3_start_xmit(skb, dev);
+
+	/* Notify the hardware that packets are ready by updating the TX ring
+	 * tail pointer. We respect netdev_xmit_more() thus avoiding poking
+	 * the hardware for every packet. To guarantee forward progress the TX
+	 * ring must be drained when it is full as indicated by
+	 * netif_xmit_stopped(). This needs to happen even when the current
+	 * skb was dropped or rejected with NETDEV_TX_BUSY. Otherwise packets
+	 * queued by previous __tg3_start_xmit() calls might get stuck in
+	 * the queue forever.
+	 */
+	if (!netdev_xmit_more() || netif_xmit_stopped(txq)) {
+		struct tg3 *tp = netdev_priv(dev);
+		struct tg3_napi *tnapi = &tp->napi[skb_queue_mapping];
+
+		if (tg3_flag(tp, ENABLE_TSS))
+			tnapi++;
+
+		tw32_tx_mbox(tnapi->prodmbox, tnapi->tx_prod);
+	}
+
+	return ret;
+}
+
 static void tg3_mac_loopback(struct tg3 *tp, bool enable)
 {
 	if (enable) {
@@ -17682,7 +17706,7 @@ static int tg3_init_one(struct pci_dev *pdev,
 	 * device behind the EPB cannot support DMA addresses > 40-bit.
 	 * On 64-bit systems with IOMMU, use 40-bit dma_mask.
 	 * On 64-bit systems without IOMMU, use 64-bit dma_mask and
-	 * do DMA address check in tg3_start_xmit().
+	 * do DMA address check in __tg3_start_xmit().
 	 */
 	if (tg3_flag(tp, IS_5788))
 		persist_dma_mask = dma_mask = DMA_BIT_MASK(32);
-- 
2.39.3

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