[PATCH 3.16 210/328] IB/ipoib: Avoid a race condition between start_xmit and cm_rep_handler

From: Ben Hutchings
Date: Sun Dec 09 2018 - 17:11:40 EST

3.16.62-rc1 review patch. If anyone has any objections, please let me know.


From: Aaron Knister <aaron.s.knister@xxxxxxxx>

commit 816e846c2eb9129a3e0afa5f920c8bbc71efecaa upstream.

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

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 {

The patch acquires the netif tx lock in cm_rep_handler for the section
where it sets the connection up and dequeues and retransmits deferred

Fixes: 839fcaba355a ("IPoIB: Connected mode experimental support")
Signed-off-by: Aaron Knister <aaron.s.knister@xxxxxxxx>
Tested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
drivers/infiniband/ulp/ipoib/ipoib_cm.c | 2 ++
1 file changed, 2 insertions(+)

--- a/drivers/infiniband/ulp/ipoib/ipoib_cm.c
+++ b/drivers/infiniband/ulp/ipoib/ipoib_cm.c
@@ -996,12 +996,14 @@ static int ipoib_cm_rep_handler(struct i


+ netif_tx_lock_bh(p->dev);
set_bit(IPOIB_FLAG_OPER_UP, &p->flags);
if (p->neigh)
while ((skb = __skb_dequeue(&p->neigh->queue)))
__skb_queue_tail(&skqueue, skb);
+ netif_tx_unlock_bh(p->dev);

while ((skb = __skb_dequeue(&skqueue))) {
skb->dev = p->dev;