RDS: Fix corrupted rds_mrs

On second look at this bug (OFED #2002), it seems that the
collision is not with the retransmission queue (packet acked
by the peer), but with the local send completion.  A theoretical
sequence of events (from time t0 to t3) is thought to be as
follows,

Thread #1
t0:
    sock_release
    rds_release
    rds_send_drop_to /* wait on send completion */
t2:
    rds_rdma_drop_keys()   /* destroy & free all mrs */

Thread #2
t1:
    rds_ib_send_cq_comp_handler
    rds_ib_send_unmap_rm
    rds_message_unmapped   /* wake up #1 @ t0 */
t3:
    rds_message_put
    rds_message_purge
    rds_mr_put   /* memory corruption detected */

The problem with the rds_rdma_drop_keys() is it could
remove a mr's refcount more than its due (i.e. repeatedly
as long as it still remains in the tree (mr->r_refcount > 0)).
Theoretically it should remove only one reference - reference
by the tree.

        /* Release any MRs associated with this socket */
        while ((node = rb_first(&rs->rs_rdma_keys))) {
                mr = container_of(node, struct rds_mr, r_rb_node);
                if (mr->r_trans == rs->rs_transport)
                        mr->r_invalidate = 0;
                rds_mr_put(mr);
        }

I think the correct way of doing it is to remove the mr from
the tree and rds_destroy_mr it first, then a rds_mr_put()
to decrement its reference count by one.  Whichever thread
holds the last reference will free the mr via rds_mr_put().
Signed-off-by: Tina Yang <tina.yang@oracle.com>
Signed-off-by: Andy Grover <andy.grover@oracle.com>

net/rds/rdma.c

@@ -130,14 +130,22 @@ void rds_rdma_drop_keys(struct rds_sock *rs)
 {
 	struct rds_mr *mr;
 	struct rb_node *node;
+	unsigned long flags;
 
 	/* Release any MRs associated with this socket */
+	spin_lock_irqsave(&rs->rs_rdma_lock, flags);
 	while ((node = rb_first(&rs->rs_rdma_keys))) {
 		mr = container_of(node, struct rds_mr, r_rb_node);
 		if (mr->r_trans == rs->rs_transport)
 			mr->r_invalidate = 0;
+		rb_erase(&mr->r_rb_node, &rs->rs_rdma_keys);
+		RB_CLEAR_NODE(&mr->r_rb_node);
+		spin_unlock_irqrestore(&rs->rs_rdma_lock, flags);
+		rds_destroy_mr(mr);
 		rds_mr_put(mr);
+		spin_lock_irqsave(&rs->rs_rdma_lock, flags);
 	}
+	spin_unlock_irqrestore(&rs->rs_rdma_lock, flags);
 
 	if (rs->rs_transport && rs->rs_transport->flush_mrs)
 		rs->rs_transport->flush_mrs();