Merge branch 'net-use-read_once-write_once-for-ring-index-accesses'

Jakub Kicinski says:

====================
net: use READ_ONCE/WRITE_ONCE for ring index accesses

Small follow up to the lockless ring stop/start macros.
Update the doc and the drivers suggested by Eric:
https://lore.kernel.org/all/CANn89iJrBGSybMX1FqrhCEMWT3Nnz2=2+aStsbbwpWzKHjk51g@mail.gmail.com/

====================

Link: https://lore.kernel.org/r/20230412015038.674023-1-kuba@kernel.orgSigned-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/driver.rst

@@ -47,30 +47,43 @@ for a driver implementing scatter-gather this means:
 
 .. code-block:: c
 
+	static u32 drv_tx_avail(struct drv_ring *dr)
+	{
+		u32 used = READ_ONCE(dr->prod) - READ_ONCE(dr->cons);
+
+		return dr->tx_ring_size - (used & bp->tx_ring_mask);
+	}
+
 	static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,
 					       struct net_device *dev)
 	{
 		struct drv *dp = netdev_priv(dev);
+		struct netdev_queue *txq;
+		struct drv_ring *dr;
+		int idx;
+
+		idx = skb_get_queue_mapping(skb);
+		dr = dp->tx_rings[idx];
+		txq = netdev_get_tx_queue(dev, idx);
 
-		lock_tx(dp);
 		//...
-		/* This is a hard error log it. */
-		if (TX_BUFFS_AVAIL(dp) <= (skb_shinfo(skb)->nr_frags + 1)) {
+		/* This should be a very rare race - log it. */
+		if (drv_tx_avail(dr) <= skb_shinfo(skb)->nr_frags + 1) {
 			netif_stop_queue(dev);
-			unlock_tx(dp);
-			printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
-			       dev->name);
+			netdev_warn(dev, "Tx Ring full when queue awake!\n");
 			return NETDEV_TX_BUSY;
 		}
 
 		//... queue packet to card ...
-		//... update tx consumer index ...
 
-		if (TX_BUFFS_AVAIL(dp) <= (MAX_SKB_FRAGS + 1))
-			netif_stop_queue(dev);
+		netdev_tx_sent_queue(txq, skb->len);
+
+		//... update tx producer index using WRITE_ONCE() ...
+
+		if (!netif_txq_maybe_stop(txq, drv_tx_avail(dr),
+					  MAX_SKB_FRAGS + 1, 2 * MAX_SKB_FRAGS))
+			dr->stats.stopped++;
 
-		//...
-		unlock_tx(dp);
 		//...
 		return NETDEV_TX_OK;
 	}
@@ -79,30 +92,10 @@ And then at the end of your TX reclamation event handling:
 
 .. code-block:: c
 
-	if (netif_queue_stopped(dp->dev) &&
-	    TX_BUFFS_AVAIL(dp) > (MAX_SKB_FRAGS + 1))
-		netif_wake_queue(dp->dev);
-
-For a non-scatter-gather supporting card, the three tests simply become:
-
-.. code-block:: c
-
-		/* This is a hard error log it. */
-		if (TX_BUFFS_AVAIL(dp) <= 0)
-
-and:
-
-.. code-block:: c
-
-		if (TX_BUFFS_AVAIL(dp) == 0)
-
-and:
-
-.. code-block:: c
+	//... update tx consumer index using WRITE_ONCE() ...
 
-	if (netif_queue_stopped(dp->dev) &&
-	    TX_BUFFS_AVAIL(dp) > 0)
-		netif_wake_queue(dp->dev);
+	netif_txq_completed_wake(txq, cmpl_pkts, cmpl_bytes,
+				 drv_tx_avail(dr), 2 * MAX_SKB_FRAGS);
 
 Lockless queue stop / wake helper macros
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -472,7 +472,7 @@ static netdev_tx_t bnxt_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		prod = NEXT_TX(prod);
 		tx_push->doorbell =
 			cpu_to_le32(DB_KEY_TX_PUSH | DB_LONG_TX_PUSH | prod);
-		txr->tx_prod = prod;
+		WRITE_ONCE(txr->tx_prod, prod);
 
 		tx_buf->is_push = 1;
 		netdev_tx_sent_queue(txq, skb->len);
@@ -583,7 +583,7 @@ static netdev_tx_t bnxt_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	wmb();
 
 	prod = NEXT_TX(prod);
-	txr->tx_prod = prod;
+	WRITE_ONCE(txr->tx_prod, prod);
 
 	if (!netdev_xmit_more() || netif_xmit_stopped(txq))
 		bnxt_txr_db_kick(bp, txr, prod);
@@ -688,7 +688,7 @@ static void bnxt_tx_int(struct bnxt *bp, struct bnxt_napi *bnapi, int nr_pkts)
 		dev_kfree_skb_any(skb);
 	}
 
-	txr->tx_cons = cons;
+	WRITE_ONCE(txr->tx_cons, cons);
 
 	__netif_txq_completed_wake(txq, nr_pkts, tx_bytes,
 				   bnxt_tx_avail(bp, txr), bp->tx_wake_thresh,

drivers/net/ethernet/broadcom/bnxt/bnxt.h

@@ -2231,13 +2231,12 @@ struct bnxt {
 #define SFF_MODULE_ID_QSFP28			0x11
 #define BNXT_MAX_PHY_I2C_RESP_SIZE		64
 
-static inline u32 bnxt_tx_avail(struct bnxt *bp, struct bnxt_tx_ring_info *txr)
+static inline u32 bnxt_tx_avail(struct bnxt *bp,
+				const struct bnxt_tx_ring_info *txr)
 {
-	/* Tell compiler to fetch tx indices from memory. */
-	barrier();
+	u32 used = READ_ONCE(txr->tx_prod) - READ_ONCE(txr->tx_cons);
 
-	return bp->tx_ring_size -
-		((txr->tx_prod - txr->tx_cons) & bp->tx_ring_mask);
+	return bp->tx_ring_size - (used & bp->tx_ring_mask);
 }
 
 static inline void bnxt_writeq(struct bnxt *bp, u64 val,

drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c

@@ -64,7 +64,7 @@ struct bnxt_sw_tx_bd *bnxt_xmit_bd(struct bnxt *bp,
 		int frag_len;
 
 		prod = NEXT_TX(prod);
-		txr->tx_prod = prod;
+		WRITE_ONCE(txr->tx_prod, prod);
 
 		/* first fill up the first buffer */
 		frag_tx_buf = &txr->tx_buf_ring[prod];
@@ -94,7 +94,7 @@ struct bnxt_sw_tx_bd *bnxt_xmit_bd(struct bnxt *bp,
 	/* Sync TX BD */
 	wmb();
 	prod = NEXT_TX(prod);
-	txr->tx_prod = prod;
+	WRITE_ONCE(txr->tx_prod, prod);
 
 	return tx_buf;
 }
@@ -161,7 +161,7 @@ void bnxt_tx_int_xdp(struct bnxt *bp, struct bnxt_napi *bnapi, int nr_pkts)
 		}
 		tx_cons = NEXT_TX(tx_cons);
 	}
-	txr->tx_cons = tx_cons;
+	WRITE_ONCE(txr->tx_cons, tx_cons);
 	if (rx_doorbell_needed) {
 		tx_buf = &txr->tx_buf_ring[last_tx_cons];
 		bnxt_db_write(bp, &rxr->rx_db, tx_buf->rx_prod);

drivers/net/ethernet/mellanox/mlx4/en_tx.c

@@ -228,7 +228,9 @@ void mlx4_en_deactivate_tx_ring(struct mlx4_en_priv *priv,
 
 static inline bool mlx4_en_is_tx_ring_full(struct mlx4_en_tx_ring *ring)
 {
-	return ring->prod - ring->cons > ring->full_size;
+	u32 used = READ_ONCE(ring->prod) - READ_ONCE(ring->cons);
+
+	return used > ring->full_size;
 }
 
 static void mlx4_en_stamp_wqe(struct mlx4_en_priv *priv,
@@ -1083,7 +1085,7 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
 			op_own |= cpu_to_be32(MLX4_WQE_CTRL_IIP);
 	}
 
-	ring->prod += nr_txbb;
+	WRITE_ONCE(ring->prod, ring->prod + nr_txbb);
 
 	/* If we used a bounce buffer then copy descriptor back into place */
 	if (unlikely(bounce))
@@ -1214,7 +1216,7 @@ netdev_tx_t mlx4_en_xmit_frame(struct mlx4_en_rx_ring *rx_ring,
 
 	rx_ring->xdp_tx++;
 
-	ring->prod += MLX4_EN_XDP_TX_NRTXBB;
+	WRITE_ONCE(ring->prod, ring->prod + MLX4_EN_XDP_TX_NRTXBB);
 
 	/* Ensure new descriptor hits memory
 	 * before setting ownership of this descriptor to HW