IB/ipath: Fix race with ACK retry timeout list management

When an ACK is received, the QP is removed from the timeout list and
then if there are still pending send WQEs, the QP is put back on the
timeout list. It is possible that another post send has put the QP on
the timeout list thus, a check needs to be made before trying to do it
again or the list is corrupted.
Signed-off-by: NRalph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: NRoland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/ipath/ipath_rc.c

@@ -959,8 +959,9 @@ static int do_rc_ack(struct ipath_qp *qp, u32 aeth, u32 psn, int opcode,
 		/* If this is a partial ACK, reset the retransmit timer. */
 		if (qp->s_last != qp->s_tail) {
 			spin_lock(&dev->pending_lock);
-			list_add_tail(&qp->timerwait,
-				      &dev->pending[dev->pending_index]);
+			if (list_empty(&qp->timerwait))
+				list_add_tail(&qp->timerwait,
+					&dev->pending[dev->pending_index]);
 			spin_unlock(&dev->pending_lock);
 			/*
 			 * If we get a partial ACK for a resent operation,