[PATCH v2] avoid race condition between start_xmit and cm_rep_handler

Aaron Knister
2018-08-17T21:31:26Z

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;
--
2.12.3