netdev - RE: [PATCH net-next 1/4] net: systemport: fix transmit locking scheme

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Date:	Fri, 6 Jun 2014 09:29:55 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Florian Fainelli' <f.fainelli@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	"davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCH net-next 1/4] net: systemport: fix transmit locking
 scheme

From: Florian Fainelli
> Our transmit locking scheme did not account for the TX ring full
> interrupt. If a TX ring full interrupt fires while we are attempting to
> transmit, we will cause a deadlock to occur. Fix this by making sure
> that we properly disable interrupts while acquiring the spinlock.

Presumably you mean a 'TX complete' interrupt?

It ought to be possible to arrange the logic so that you don't
need to disable interrupts for the entirety of the tx reclaim 
and tx setup code.

If you have an absolutely accurate count of the number of
tx descriptors required, even the tx setup code probably only needs
the ring's lock while it advances the write pointer.
It can fill in the ring entries after releasing the lock.

	David

> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> ---
>  drivers/net/ethernet/broadcom/bcmsysport.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/ethernet/broadcom/bcmsysport.c b/drivers/net/ethernet/broadcom/bcmsysport.c
> index 25982b02f0ea..4dfc93fe9744 100644
> --- a/drivers/net/ethernet/broadcom/bcmsysport.c
> +++ b/drivers/net/ethernet/broadcom/bcmsysport.c
> @@ -637,10 +637,11 @@ static unsigned int bcm_sysport_tx_reclaim(struct bcm_sysport_priv *priv,
>  					   struct bcm_sysport_tx_ring *ring)
>  {
>  	unsigned int released;
> +	unsigned long flags;
> 
> -	spin_lock(&ring->lock);
> +	spin_lock_irqsave(&ring->lock, flags);
>  	released = __bcm_sysport_tx_reclaim(priv, ring);
> -	spin_unlock(&ring->lock);
> +	spin_unlock_irqrestore(&ring->lock, flags);
> 
>  	return released;
>  }
> @@ -822,6 +823,7 @@ static netdev_tx_t bcm_sysport_xmit(struct sk_buff *skb,
>  	struct netdev_queue *txq;
>  	struct dma_desc *desc;
>  	unsigned int skb_len;
> +	unsigned long flags;
>  	dma_addr_t mapping;
>  	u32 len_status;
>  	u16 queue;
> @@ -831,8 +833,8 @@ static netdev_tx_t bcm_sysport_xmit(struct sk_buff *skb,
>  	txq = netdev_get_tx_queue(dev, queue);
>  	ring = &priv->tx_rings[queue];
> 
> -	/* lock against tx reclaim in BH context */
> -	spin_lock(&ring->lock);
> +	/* lock against tx reclaim in BH context and TX ring full interrupt */
> +	spin_lock_irqsave(&ring->lock, flags);
>  	if (unlikely(ring->desc_count == 0)) {
>  		netif_tx_stop_queue(txq);
>  		netdev_err(dev, "queue %d awake and ring full!\n", queue);
> @@ -914,7 +916,7 @@ static netdev_tx_t bcm_sysport_xmit(struct sk_buff *skb,
> 
>  	ret = NETDEV_TX_OK;
>  out:
> -	spin_unlock(&ring->lock);
> +	spin_unlock_irqrestore(&ring->lock, flags);
>  	return ret;
>  }
> 
> --
> 1.9.1
> 
> --
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