Re: [PATCH v3 1/3] can: kvaser_usb: Fix tx queue start/stop race conditions

[Marc Kleine-Budde]

On 03/11/2015 06:37 PM, Ahmed S. Darwish wrote:
> From: Ahmed S. Darwish <ahmed.darwish@xxxxxxxxx>
> 
> A number of tx queue wake-up events went missing due to the
> outlined scenario below. Start state is a pool of 16 tx URBs,
> active tx_urbs count = 15, with the netdev tx queue open.
> 
> start_xmit()                             tx_acknowledge()
> ............                             ................
> atomic_inc(&tx_urbs);
> if (atomic_read(&tx_urbs) >= 16) {
>                         URB completion IRQ!
>                         -->
>                                          atomic_dec(&tx_urbs);
>                                          netif_wake_queue();
>                                          return;
>                         <--
>                         end of IRQ!
>     netif_stop_queue();
> }
> 
> At the end, the correct state expected is a 15 tx_urbs count
> value with the tx queue state _open_. Due to the race, we get
> the same tx_urbs value but with the tx queue state _stopped_.
> The wake-up event is completely lost.
> 
> Thus avoid hand-rolled concurrency mechanisms and use a proper
> lock for contexts protection.
> 
> Signed-off-by: Ahmed S. Darwish <ahmed.darwish@xxxxxxxxx>
> ---
>  drivers/net/can/usb/kvaser_usb.c | 83 ++++++++++++++++++++++++----------------
>  1 file changed, 51 insertions(+), 32 deletions(-)
> 
> changelog-v3: Added missing spin_lock_init(). With all kernel
> lock debugging options set, I've been running my test suite
> for an hour now without apparent problems in dmesg so far.
> 
> changelog-v2: Put bugfix patch at the start of the series.
> 
> diff --git a/drivers/net/can/usb/kvaser_usb.c b/drivers/net/can/usb/kvaser_usb.c
> index a316fa4..e97a08c 100644
> --- a/drivers/net/can/usb/kvaser_usb.c
> +++ b/drivers/net/can/usb/kvaser_usb.c
> @@ -14,6 +14,7 @@
>   * Copyright (C) 2015 Valeo S.A.
>   */
>  
> +#include <linux/spinlock.h>
>  #include <linux/kernel.h>
>  #include <linux/completion.h>
>  #include <linux/module.h>
> @@ -467,10 +468,11 @@ struct kvaser_usb {
>  struct kvaser_usb_net_priv {
>  	struct can_priv can;
>  
> -	atomic_t active_tx_urbs;
> -	struct usb_anchor tx_submitted;
> +	spinlock_t tx_contexts_lock;
> +	int active_tx_contexts;
>  	struct kvaser_usb_tx_urb_context tx_contexts[MAX_TX_URBS];
>  
> +	struct usb_anchor tx_submitted;
>  	struct completion start_comp, stop_comp;
>  
>  	struct kvaser_usb *dev;
> @@ -694,6 +696,7 @@ static void kvaser_usb_tx_acknowledge(const struct kvaser_usb *dev,
>  	struct kvaser_usb_net_priv *priv;
>  	struct sk_buff *skb;
>  	struct can_frame *cf;
> +	unsigned long flags;
>  	u8 channel, tid;
>  
>  	channel = msg->u.tx_acknowledge_header.channel;
> @@ -737,12 +740,15 @@ static void kvaser_usb_tx_acknowledge(const struct kvaser_usb *dev,
>  
>  	stats->tx_packets++;
>  	stats->tx_bytes += context->dlc;
> -	can_get_echo_skb(priv->netdev, context->echo_index);
>  
> -	context->echo_index = MAX_TX_URBS;
> -	atomic_dec(&priv->active_tx_urbs);
> +	spin_lock_irqsave(&priv->tx_contexts_lock, flags);
>  
> +	can_get_echo_skb(priv->netdev, context->echo_index);
> +	context->echo_index = MAX_TX_URBS;
> +	--priv->active_tx_contexts;
>  	netif_wake_queue(priv->netdev);
> +
> +	spin_unlock_irqrestore(&priv->tx_contexts_lock, flags);

I think it should be possible to move tun unlock before waking up the queue?

>  }
>  
>  static void kvaser_usb_simple_msg_callback(struct urb *urb)
> @@ -803,17 +809,6 @@ static int kvaser_usb_simple_msg_async(struct kvaser_usb_net_priv *priv,
>  	return 0;
>  }
>  
> -static void kvaser_usb_unlink_tx_urbs(struct kvaser_usb_net_priv *priv)
> -{
> -	int i;
> -
> -	usb_kill_anchored_urbs(&priv->tx_submitted);
> -	atomic_set(&priv->active_tx_urbs, 0);
> -
> -	for (i = 0; i < MAX_TX_URBS; i++)
> -		priv->tx_contexts[i].echo_index = MAX_TX_URBS;
> -}
> -
>  static void kvaser_usb_rx_error_update_can_state(struct kvaser_usb_net_priv *priv,
>  						 const struct kvaser_usb_error_summary *es,
>  						 struct can_frame *cf)
> @@ -1515,6 +1510,24 @@ error:
>  	return err;
>  }
>  
> +static void kvaser_usb_reset_tx_urb_contexts(struct kvaser_usb_net_priv *priv)
> +{
> +	int i;
> +
> +	priv->active_tx_contexts = 0;
> +	for (i = 0; i < MAX_TX_URBS; i++)
> +		priv->tx_contexts[i].echo_index = MAX_TX_URBS;
> +}
> +
> +/* This method might sleep. Do not call it in the atomic context
> + * of URB completions.
> + */
> +static void kvaser_usb_unlink_tx_urbs(struct kvaser_usb_net_priv *priv)
> +{
> +	usb_kill_anchored_urbs(&priv->tx_submitted);
> +	kvaser_usb_reset_tx_urb_contexts(priv);
> +}
> +
>  static void kvaser_usb_unlink_all_urbs(struct kvaser_usb *dev)
>  {
>  	int i;
> @@ -1634,6 +1647,7 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
>  	struct kvaser_msg *msg;
>  	int i, err, ret = NETDEV_TX_OK;
>  	u8 *msg_tx_can_flags = NULL;		/* GCC */
> +	unsigned long flags;
>  
>  	if (can_dropped_invalid_skb(netdev, skb))
>  		return NETDEV_TX_OK;
> @@ -1687,12 +1701,21 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
>  	if (cf->can_id & CAN_RTR_FLAG)
>  		*msg_tx_can_flags |= MSG_FLAG_REMOTE_FRAME;
>  
> +	spin_lock_irqsave(&priv->tx_contexts_lock, flags);
>  	for (i = 0; i < ARRAY_SIZE(priv->tx_contexts); i++) {
>  		if (priv->tx_contexts[i].echo_index == MAX_TX_URBS) {
>  			context = &priv->tx_contexts[i];
> +
> +			context->echo_index = i;
> +			can_put_echo_skb(skb, netdev, context->echo_index);
> +			++priv->active_tx_contexts;
> +			if (priv->active_tx_contexts >= MAX_TX_URBS)
> +				netif_stop_queue(netdev);
> +
>  			break;
>  		}
>  	}
> +	spin_unlock_irqrestore(&priv->tx_contexts_lock, flags);
>  
>  	/* This should never happen; it implies a flow control bug */
>  	if (!context) {
> @@ -1704,7 +1727,6 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
>  	}
>  
>  	context->priv = priv;
> -	context->echo_index = i;
>  	context->dlc = cf->can_dlc;
>  
>  	msg->u.tx_can.tid = context->echo_index;
> @@ -1716,18 +1738,17 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
>  			  kvaser_usb_write_bulk_callback, context);
>  	usb_anchor_urb(urb, &priv->tx_submitted);
>  
> -	can_put_echo_skb(skb, netdev, context->echo_index);
> -
> -	atomic_inc(&priv->active_tx_urbs);
> -
> -	if (atomic_read(&priv->active_tx_urbs) >= MAX_TX_URBS)
> -		netif_stop_queue(netdev);
> -
>  	err = usb_submit_urb(urb, GFP_ATOMIC);
>  	if (unlikely(err)) {
> +		spin_lock_irqsave(&priv->tx_contexts_lock, flags);
> +
>  		can_free_echo_skb(netdev, context->echo_index);
> +		context->echo_index = MAX_TX_URBS;
> +		--priv->active_tx_contexts;
> +		netif_wake_queue(netdev);
> +
> +		spin_unlock_irqrestore(&priv->tx_contexts_lock, flags);

Same here?

>  
> -		atomic_dec(&priv->active_tx_urbs);
>  		usb_unanchor_urb(urb);
>  
>  		stats->tx_dropped++;
> @@ -1854,7 +1875,7 @@ static int kvaser_usb_init_one(struct usb_interface *intf,
>  	struct kvaser_usb *dev = usb_get_intfdata(intf);
>  	struct net_device *netdev;
>  	struct kvaser_usb_net_priv *priv;
> -	int i, err;
> +	int err;
>  
>  	err = kvaser_usb_send_simple_msg(dev, CMD_RESET_CHIP, channel);
>  	if (err)
> @@ -1868,19 +1889,17 @@ static int kvaser_usb_init_one(struct usb_interface *intf,
>  
>  	priv = netdev_priv(netdev);
>  
> +	init_usb_anchor(&priv->tx_submitted);
>  	init_completion(&priv->start_comp);
>  	init_completion(&priv->stop_comp);
>  
> -	init_usb_anchor(&priv->tx_submitted);
> -	atomic_set(&priv->active_tx_urbs, 0);
> -
> -	for (i = 0; i < ARRAY_SIZE(priv->tx_contexts); i++)
> -		priv->tx_contexts[i].echo_index = MAX_TX_URBS;
> -
>  	priv->dev = dev;
>  	priv->netdev = netdev;
>  	priv->channel = channel;
>  
> +	spin_lock_init(&priv->tx_contexts_lock);
> +	kvaser_usb_reset_tx_urb_contexts(priv);
> +
>  	priv->can.state = CAN_STATE_STOPPED;
>  	priv->can.clock.freq = CAN_USB_CLOCK;
>  	priv->can.bittiming_const = &kvaser_usb_bittiming_const;
> 

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
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