RDS:TCP: Synchronize rds_tcp_accept_one with rds_send_xmit when resetting t_sock

There is a race condition between rds_send_xmit -> rds_tcp_xmit
and the code that deals with resolution of duelling syns added
by commit 241b271952eb ("RDS-TCP: Reset tcp callbacks if re-using an
outgoing socket in rds_tcp_accept_one()").

Specifically, we may end up derefencing a null pointer in rds_send_xmit
if we have the interleaving sequence:
           rds_tcp_accept_one                  rds_send_xmit

                                             conn is RDS_CONN_UP, so
    					 invoke rds_tcp_xmit

                                             tc = conn->c_transport_data
        rds_tcp_restore_callbacks
            /* reset t_sock */
    					 null ptr deref from tc->t_sock

The race condition can be avoided without adding the overhead of
additional locking in the xmit path: have rds_tcp_accept_one wait
for rds_tcp_xmit threads to complete before resetting callbacks.
The synchronization can be done in the same manner as rds_conn_shutdown().
First set the rds_conn_state to something other than RDS_CONN_UP
(so that new threads cannot get into rds_tcp_xmit()), then wait for
RDS_IN_XMIT to be cleared in the conn->c_flags indicating that any
threads in rds_tcp_xmit are done.

Fixes: 241b271952eb ("RDS-TCP: Reset tcp callbacks if re-using an
outgoing socket in rds_tcp_accept_one()")
Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 25 insertions, 17 deletions

diff --git a/net/rds/tcp.c b/net/rds/tcp.c
index 61ed2a8764ba..9134544941c2 100644
--- a/net/rds/tcp.c
+++ b/net/rds/tcp.c
@@ -127,7 +127,7 @@ void rds_tcp_restore_callbacks(struct socket *sock,
 
 /*
  * This is the only path that sets tc->t_sock.  Send and receive trust that
- * it is set.  The RDS_CONN_CONNECTED bit protects those paths from being
+ * it is set.  The RDS_CONN_UP bit protects those paths from being
  * called while it isn't set.
  */
 void rds_tcp_set_callbacks(struct socket *sock, struct rds_connection *conn)
diff --git a/net/rds/tcp_listen.c b/net/rds/tcp_listen.c
index 0936a4a32b47..0896187243d6 100644
--- a/net/rds/tcp_listen.c
+++ b/net/rds/tcp_listen.c
@@ -115,24 +115,32 @@ int rds_tcp_accept_one(struct socket *sock)
          * rds_tcp_state_change() will do that cleanup
          */
         rs_tcp = (struct rds_tcp_connection *)conn->c_transport_data;
-        if (rs_tcp->t_sock &&
-            ntohl(inet->inet_saddr) < ntohl(inet->inet_daddr)) {
-                struct sock *nsk = new_sock->sk;
-
-                nsk->sk_user_data = NULL;
-                nsk->sk_prot->disconnect(nsk, 0);
-                tcp_done(nsk);
-                new_sock = NULL;
-                ret = 0;
-                goto out;
-        } else if (rs_tcp->t_sock) {
-                rds_tcp_restore_callbacks(rs_tcp->t_sock, rs_tcp);
-                conn->c_outgoing = 0;
-        }
-
         rds_conn_transition(conn, RDS_CONN_DOWN, RDS_CONN_CONNECTING);
+        if (rs_tcp->t_sock) {
+                /* Need to resolve a duelling SYN between peers.
+                 * We have an outstanding SYN to this peer, which may
+                 * potentially have transitioned to the RDS_CONN_UP state,
+                 * so we must quiesce any send threads before resetting
+                 * c_transport_data.
+                 */
+                wait_event(conn->c_waitq,
+                           !test_bit(RDS_IN_XMIT, &conn->c_flags));
+                if (ntohl(inet->inet_saddr) < ntohl(inet->inet_daddr)) {
+                        struct sock *nsk = new_sock->sk;
+
+                        nsk->sk_user_data = NULL;
+                        nsk->sk_prot->disconnect(nsk, 0);
+                        tcp_done(nsk);
+                        new_sock = NULL;
+                        ret = 0;
+                        goto out;
+                } else if (rs_tcp->t_sock) {
+                        rds_tcp_restore_callbacks(rs_tcp->t_sock, rs_tcp);
+                        conn->c_outgoing = 0;
+                }
+        }
         rds_tcp_set_callbacks(new_sock, conn);
-        rds_connect_complete(conn);
+        rds_connect_complete(conn); /* marks RDS_CONN_UP */
         new_sock = NULL;
         ret = 0;