net/mlx5e: Sync txq2sq updates with mlx5e_xmit for HTB queues

This commit makes necessary changes to guarantee that txq2sq remains
stable while mlx5e_xmit is running. Proper synchronization is added for
HTB TX queues.

All updates to txq2sq are performed while the corresponding queue is
disabled (i.e. mlx5e_xmit doesn't run on that queue). smp_wmb after each
change guarantees that mlx5e_xmit can see the updated value after the
queue is enabled. Comments explaining this mechanism are added to
mlx5e_xmit.

When an HTB SQ can be deleted (after deleting an HTB node), synchronize
with RCU to wait for mlx5e_select_queue to finish and stop selecting
that queue, before we re-enable it to avoid TX timeout watchdog alarms.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/qos.c b/drivers/net/ethernet/mellanox/mlx5/core/en/qos.c
index c1e0749..ff45840 100644 (file)
--- a/drivers/net/ethernet/mellanox/mlx5/core/en/qos.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en/qos.c
@@ -50,7 +50,6 @@ static int mlx5e_find_unused_qos_qid(struct mlx5e_priv *priv)
 
 struct mlx5e_qos_node {
        struct hlist_node hnode;
-       struct rcu_head rcu;
        struct mlx5e_qos_node *parent;
        u64 rate;
        u32 bw_share;
@@ -132,7 +131,11 @@ static void mlx5e_sw_node_delete(struct mlx5e_priv *priv, struct mlx5e_qos_node
                __clear_bit(node->qid, priv->htb.qos_used_qids);
                mlx5e_update_tx_netdev_queues(priv);
        }
-       kfree_rcu(node, rcu);
+       /* Make sure this qid is no longer selected by mlx5e_select_queue, so
+        * that mlx5e_reactivate_qos_sq can safely restart the netdev TX queue.
+        */
+       synchronize_net();
+       kfree(node);
 }
 
 /* TX datapath API */
@@ -273,10 +276,18 @@ err_free_sq:
 static void mlx5e_activate_qos_sq(struct mlx5e_priv *priv, struct mlx5e_qos_node *node)
 {
        struct mlx5e_txqsq *sq;
+       u16 qid;
 
        sq = mlx5e_get_qos_sq(priv, node->qid);
 
-       WRITE_ONCE(priv->txq2sq[mlx5e_qid_from_qos(&priv->channels, node->qid)], sq);
+       qid = mlx5e_qid_from_qos(&priv->channels, node->qid);
+
+       /* If it's a new queue, it will be marked as started at this point.
+        * Stop it before updating txq2sq.
+        */
+       mlx5e_tx_disable_queue(netdev_get_tx_queue(priv->netdev, qid));
+
+       priv->txq2sq[qid] = sq;
 
        /* Make the change to txq2sq visible before the queue is started.
         * As mlx5e_xmit runs under a spinlock, there is an implicit ACQUIRE,
@@ -299,8 +310,13 @@ static void mlx5e_deactivate_qos_sq(struct mlx5e_priv *priv, u16 qid)
        qos_dbg(priv->mdev, "Deactivate QoS SQ qid %u\n", qid);
        mlx5e_deactivate_txqsq(sq);
 
-       /* The queue is disabled, no synchronization with datapath is needed. */
        priv->txq2sq[mlx5e_qid_from_qos(&priv->channels, qid)] = NULL;
+
+       /* Make the change to txq2sq visible before the queue is started again.
+        * As mlx5e_xmit runs under a spinlock, there is an implicit ACQUIRE,
+        * which pairs with this barrier.
+        */
+       smp_wmb();
 }
 
 static void mlx5e_close_qos_sq(struct mlx5e_priv *priv, u16 qid)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
index 9c91ef0..7266617 100644 (file)
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
@@ -691,8 +691,21 @@ netdev_tx_t mlx5e_xmit(struct sk_buff *skb, struct net_device *dev)
        struct mlx5e_txqsq *sq;
        u16 pi;
 
+       /* All changes to txq2sq are performed in sync with mlx5e_xmit, when the
+        * queue being changed is disabled, and smp_wmb guarantees that the
+        * changes are visible before mlx5e_xmit tries to read from txq2sq. It
+        * guarantees that the value of txq2sq[qid] doesn't change while
+        * mlx5e_xmit is running on queue number qid. smb_wmb is paired with
+        * HARD_TX_LOCK around ndo_start_xmit, which serves as an ACQUIRE.
+        */
        sq = priv->txq2sq[skb_get_queue_mapping(skb)];
        if (unlikely(!sq)) {
+               /* Two cases when sq can be NULL:
+                * 1. The HTB node is registered, and mlx5e_select_queue
+                * selected its queue ID, but the SQ itself is not yet created.
+                * 2. HTB SQ creation failed. Similar to the previous case, but
+                * the SQ won't be created.
+                */
                dev_kfree_skb_any(skb);
                return NETDEV_TX_OK;
        }