netdev - Re: [PATCH 06/11] net: calxedaxgmac: fix race with tx queue stop/wake

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Date:	Tue, 27 Aug 2013 19:31:57 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Rob Herring <robherring2@...il.com>
CC:	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Andreas Herrmann <andreas.herrmann@...xeda.com>,
	Lennert Buytenhek <buytenh@...tstofly.org>,
	Rob Herring <rob.herring@...xeda.com>
Subject: Re: [PATCH 06/11] net: calxedaxgmac: fix race with tx queue
 stop/wake

On Mon, 2013-08-26 at 17:45 -0500, Rob Herring wrote:
> From: Rob Herring <rob.herring@...xeda.com>
> 
> Since the xgmac transmit start and completion work locklessly, it is
> possible for xgmac_xmit to stop the tx queue after the xgmac_tx_complete
> has run resulting in the tx queue never being woken up. Fix this by
> ensuring that ring buffer index updates are visible and serialize the
> queue wake with netif_tx_lock.
>
> The implementation used here was copied from
> drivers/net/ethernet/broadcom/tg3.c.
> 
> Signed-off-by: Rob Herring <rob.herring@...xeda.com>
> ---
>  drivers/net/ethernet/calxeda/xgmac.c | 25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/calxeda/xgmac.c b/drivers/net/ethernet/calxeda/xgmac.c
> index f630855..cd5872c 100644
> --- a/drivers/net/ethernet/calxeda/xgmac.c
> +++ b/drivers/net/ethernet/calxeda/xgmac.c
> @@ -410,6 +410,9 @@ struct xgmac_priv {
>  #define dma_ring_space(h, t, s)	CIRC_SPACE(h, t, s)
>  #define dma_ring_cnt(h, t, s)	CIRC_CNT(h, t, s)
>  
> +#define tx_dma_ring_space(p) \
> +	dma_ring_space((p)->tx_head, (p)->tx_tail, DMA_TX_RING_SZ)
> +
>  /* XGMAC Descriptor Access Helpers */
>  static inline void desc_set_buf_len(struct xgmac_dma_desc *p, u32 buf_sz)
>  {
> @@ -886,9 +889,14 @@ static void xgmac_tx_complete(struct xgmac_priv *priv)
>  		priv->tx_tail = dma_ring_incr(entry, DMA_TX_RING_SZ);
>  	}
>  
> -	if (dma_ring_space(priv->tx_head, priv->tx_tail, DMA_TX_RING_SZ) >
> -	    MAX_SKB_FRAGS)
> -		netif_wake_queue(priv->dev);
> +	/* Ensure tx_tail is visible to xgmac_xmit */
> +	smp_mb();
> +	if (unlikely(netif_queue_stopped(priv->dev))) {
> +		netif_tx_lock(priv->dev);
> +		if (tx_dma_ring_space(priv) > MAX_SKB_FRAGS)
> +			netif_wake_queue(priv->dev);
> +		netif_tx_unlock(priv->dev);
> +	}
>  }

You don't need to take the TX lock for this.  The memory barriers
provide sufficient synchronisation.

>  static void xgmac_tx_timeout_work(struct work_struct *work)
> @@ -1125,10 +1133,15 @@ static netdev_tx_t xgmac_xmit(struct sk_buff *skb, struct net_device *dev)
>  
>  	priv->tx_head = dma_ring_incr(entry, DMA_TX_RING_SZ);
>  
> -	if (dma_ring_space(priv->tx_head, priv->tx_tail, DMA_TX_RING_SZ) <
> -	    MAX_SKB_FRAGS)
> +	/* Ensure tx_head update is visible to tx completion */
> +	smp_mb();
> +	if (unlikely(tx_dma_ring_space(priv) < MAX_SKB_FRAGS)) {
>  		netif_stop_queue(dev);
> -
> +		/* Ensure netif_stop_queue is visible to tx completion */
> +		smp_mb();
> +		if (tx_dma_ring_space(priv) > MAX_SKB_FRAGS)
> +			netif_wake_queue(dev);

You should use netif_start_queue() rather than netif_wake_queue(), since
you know the TX scheduler is already active.

Ben.

> +	}
>  	return NETDEV_TX_OK;
>  }
>  

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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