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 --- 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;