RE: [PATCH v2] avoid race condition between start_xmit and cm_rep_handler

Inside of start_xmit() the call to check if the connection is up and the queueing of the packets for later transmission is not atomic which leaves a window where cm_rep_handler can run, set the connection up, dequeue pending packets and leave the subsequently queued packets by start_xmit() sitting on neigh->queue until they're dropped when the connection is torn down. This only applies to connected mode. These dropped packets can really upset TCP, for example, and cause multi-minute delays in transmission for open connections.

Here's the code in start_xmit where we check to see if the connection is up:

   if (ipoib_cm_get(neigh)) {
           if (ipoib_cm_up(neigh)) {
                   ipoib_cm_send(dev, skb, ipoib_cm_get(neigh));
                   goto unref;
           }
   }

The race occurs if cm_rep_handler execution occurs after the above connection check (specifically if it gets to the point where it acquires priv->lock to dequeue pending skb's) but before the below code snippet in start_xmit where packets are queued.

   if (skb_queue_len(&neigh->queue) < IPOIB_MAX_PATH_REC_QUEUE) {
           push_pseudo_header(skb, phdr->hwaddr);
           spin_lock_irqsave(&priv->lock, flags);
           __skb_queue_tail(&neigh->queue, skb);
           spin_unlock_irqrestore(&priv->lock, flags);
   } else {
           ++dev->stats.tx_dropped;
           dev_kfree_skb_any(skb);
   }

The patch re-checks ipoib_cm_up with priv->lock held to avoid this race condition. Since odds are the conn should be up most of the time (and thus the connection *not* down most of the time) we don't hold the lock for the first check attempt to avoid a slowdown from unecessary locking for the majority of the packets transmitted during the connection's life.

Signed-off-by: Aaron Knister aaron.s.knister@nasa.gov

Reviewed-by: Alex Estrin alex.estrin@intel.com

---

drivers/infiniband/ulp/ipoib/ipoib_main.c | 46 ++++++++++++++++++++++++++-

1 file changed, 39 insertions(+), 7 deletions(-)

diff --git a/drivers/infiniband/ulp/ipoib/ipoib_main.c b/drivers/infiniband/ulp/ipoib/ipoib_main.c index 26cde95b..a950c916 100644 --- a/drivers/infiniband/ulp/ipoib/ipoib_main.c +++ b/drivers/infiniband/ulp/ipoib/ipoib_main.c @@ -1093,6 +1093,21 @@ static void unicast_arp_send(struct sk_buff *skb, struct net_device *dev, spin_unlock_irqrestore(&priv->lock, flags); }

+static bool defer_neigh_skb(struct sk_buff *skb,

• 	    struct net_device *dev,
  

• 	    struct ipoib_neigh *neigh,
  

• 	    struct ipoib_pseudo_header *phdr)
  

+{

• if (skb_queue_len(&neigh->queue) < IPOIB_MAX_PATH_REC_QUEUE) {

• push_pseudo_header(skb, phdr->hwaddr);
  

• __skb_queue_tail(&neigh->queue, skb);
  

• return true;
  

• }
• return false;

+}

static netdev_tx_t ipoib_start_xmit(struct sk_buff *skb, struct net_device *dev) { struct ipoib_dev_priv *priv = ipoib_priv(dev); @@ -1101,6 +1116,7 @@ static netdev_tx_t ipoib_start_xmit(struct sk_buff *skb, struct net_device *dev) struct ipoib_pseudo_header *phdr; struct ipoib_header *header; unsigned long flags;

• bool deferred_pkt = true;

  phdr = (struct ipoib_pseudo_header *) skb->data; skb_pull(skb, sizeof(*phdr));

@@ -1160,6 +1176,23 @@ static netdev_tx_t ipoib_start_xmit(struct sk_buff *skb, struct net_device *dev) ipoib_cm_send(dev, skb, ipoib_cm_get(neigh)); goto unref; }

• /*
  

•  * Re-check ipoib_cm_up with priv->lock held to avoid
  

•  * race condition between start_xmit and skb_dequeue in
  

•  * cm_rep_handler. Since odds are the conn should be up
  

•  * most of the time, we don't hold the lock for the
  

•  * first check above
  

•  */
  

• spin_lock_irqsave(&priv->lock, flags);
  

• if (ipoib_cm_up(neigh)) {
  

• 	spin_unlock_irqrestore(&priv->lock, flags);
  

• 	ipoib_cm_send(dev, skb, ipoib_cm_get(neigh));
  

• } else {
  

• 	deferred_pkt = defer_neigh_skb(skb, dev, neigh, phdr);
  

• 	spin_unlock_irqrestore(&priv->lock, flags);
  

• }
  

• goto unref;
  

  } else if (neigh->ah && neigh->ah->valid) { neigh->ah->last_send = rn->send(dev, skb, neigh->ah->ah, IPOIB_QPN(phdr->hwaddr));

@@ -1168,17 +1201,16 @@ static netdev_tx_t ipoib_start_xmit(struct sk_buff *skb, struct net_device *dev) neigh_refresh_path(neigh, phdr->hwaddr, dev); }

• if (skb_queue_len(&neigh->queue) < IPOIB_MAX_PATH_REC_QUEUE) {

• push_pseudo_header(skb, phdr->hwaddr);
  

• spin_lock_irqsave(&priv->lock, flags);
  

• __skb_queue_tail(&neigh->queue, skb);
  

• spin_unlock_irqrestore(&priv->lock, flags);
  

• } else {

• spin_lock_irqsave(&priv->lock, flags);
• deferred_pkt = defer_neigh_skb(skb, dev, neigh, phdr);
• spin_unlock_irqrestore(&priv->lock, flags);

+unref:

• if (!deferred_pkt) { ++dev->stats.tx_dropped; dev_kfree_skb_any(skb); }

-unref: ipoib_neigh_put(neigh);

return NETDEV_TX_OK;

2.12.3