On 5/1/2016 4:10 PM, Sowmini Varadhan wrote:
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 sequencee:
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.varad...@oracle.com>
---
Mostly looks correct. A question below.
net/rds/tcp.c | 2 +-
net/rds/tcp_listen.c | 40 ++++++++++++++++++++++++----------------
2 files changed, 25 insertions(+), 17 deletions(-)
diff --git a/net/rds/tcp.c b/net/rds/tcp.c
index 61ed2a8..9134544 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 0936a4a..0896187 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));
Would it be good to check the return value of rds_conn_transition()
since if CONN is already UP above will fail and then send message
might again race and we will let message through even though passive
hasn't finished its connection.
+ 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;
