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RDS: TCP: HA/Failover fixes

PATCH In review Sowmini Varadhan 2 participants 5 messages Nov 16, 2016 – Nov 17, 2016

Subsystems (3)
Networking Network drivers RDS - reliable datagram sockets
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Sowmini Varadhan

author
#0 lore raw

cover RDS: TCP: HA/Failover fixes

This series contains a set of fixes for bugs exposed when we ran the following in a loop between a test machine pair:

while (1); do

   # modprobe rds-tcp on test nodes
   # run rds-stress in bi-dir mode between test machine pair 
   # modprobe -r rds-tcp on test nodes
 done

rds-stress in bi-dir mode will cause both nodes to initiate RDS-TCP connections at almost the same instant, exposing the bugs fixed in this series.

Without the fixes, rds-stress reports sporadic packet drops, and packets arriving out of sequence. After the fixes,we have been able to run the test overnight, without any issues.

Each patch has a detailed description of the root-cause fixed by the patch.

Sowmini Varadhan (3):
RDS: TCP: set RDS_FLAG_RETRANSMITTED in cp_retrans list
RDS: TCP: Track peer's connection generation number
RDS: TCP: Force every connection to be initiated by numerically
smaller IP address

 net/rds/af_rds.c      |    4 ++++
 net/rds/connection.c  |    3 +++
 net/rds/message.c     |    1 +
 net/rds/rds.h         |    8 +++++++-
 net/rds/recv.c        |   36 ++++++++++++++++++++++++++++++++++++
 net/rds/send.c        |    9 +++++++--
 net/rds/tcp_connect.c |   14 +++++++++++++-
 net/rds/tcp_listen.c  |   29 ++++++++++++-----------------
 net/rds/tcp_send.c    |    3 +++
 9 files changed, 86 insertions(+), 21 deletions(-)
author
#1 lore raw

PATCH 1/3 RDS: TCP: set RDS_FLAG_RETRANSMITTED in cp_retrans list

As noted in rds_recv_incoming() sequence numbers on data packets can decreas for the failover case, and the Rx path is equipped to recover from this, if the RDS_FLAG_RETRANSMITTED is set on the rds header of an incoming message with a suspect sequence number.

The RDS_FLAG_RETRANSMITTED is predicated on the RDS_FLAG_RETRANSMITTED flag in the rds_message, so make sure the flag is set on messages queued for retransmission.

Signed-off-by Sowmini Varadhan <[email protected]>
Acked-by Santosh Shilimkar <[email protected]>
---
 net/rds/tcp_send.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
net/rds/tcp_send.c +3 -0
index 89d09b4..dcf4742 100644
@@ -100,6 +100,9 @@ int rds_tcp_xmit(struct rds_connection *conn, struct rds_message *rm,
 		set_bit(RDS_MSG_HAS_ACK_SEQ, &rm->m_flags);
 		tc->t_last_expected_una = rm->m_ack_seq + 1;
 
+		if (test_bit(RDS_MSG_RETRANSMITTED, &rm->m_flags))
+			rm->m_inc.i_hdr.h_flags |= RDS_FLAG_RETRANSMITTED;
+
 		rdsdebug("rm %p tcp nxt %u ack_seq %llu\n",
 			 rm, rds_tcp_snd_nxt(tc),
 			 (unsigned long long)rm->m_ack_seq);
Signature · 1 line
1.7.1
author
#2 lore raw

PATCH 2/3 RDS: TCP: Track peer's connection generation number

In reply to Sowmini Varadhan

The RDS transport has to be able to distinguish between
two types of failure events:
(a) when the transport fails (e.g., TCP connection reset) but the RDS socket/connection layer on both sides stays the same
(b) when the peer's RDS layer itself resets (e.g., due to module reload or machine reboot at the peer)
In case (a) both sides must reconnect and continue the RDS messaging without any message loss or disruption to the message sequence numbers, and this is achieved by rds_send_path_reset().

In case (b) we should reset all rds_connection state to the new incarnation of the peer. Examples of state that needs to be reset are next expected rx sequence number from, or messages to be retransmitted to, the new incarnation of the peer.

To achieve this, the RDS handshake probe added as part of commit 5916e2c1554f ("RDS: TCP: Enable multipath RDS for TCP") is enhanced so that sender and receiver of the RDS ping-probe will add a generation number as part of the RDS_EXTHDR_GEN_NUM extension header. Each peer stores local and remote generation numbers as part of each rds_connection. Changes in generation number will be detected via incoming handshake probe ping request or response and will allow the receiver to reset rds_connection state.

Signed-off-by Sowmini Varadhan <[email protected]>
Acked-by Santosh Shilimkar <[email protected]>
---
 net/rds/af_rds.c     |    4 ++++
 net/rds/connection.c |    2 ++
 net/rds/message.c    |    1 +
 net/rds/rds.h        |    8 +++++++-
 net/rds/recv.c       |   36 ++++++++++++++++++++++++++++++++++++
 net/rds/send.c       |    9 +++++++--
 6 files changed, 57 insertions(+), 3 deletions(-)
net/rds/af_rds.c +4 -0
index 6beaeb1..2ac1e61 100644
@@ -605,10 +605,14 @@ static void rds_exit(void)
 }
 module_exit(rds_exit);
 
+u32 rds_gen_num;
+
 static int rds_init(void)
 {
 	int ret;
 
+	net_get_random_once(&rds_gen_num, sizeof(rds_gen_num));
+
 	ret = rds_bind_lock_init();
 	if (ret)
 		goto out;
net/rds/connection.c +2 -0
index 13f459d..b86e188 100644
@@ -269,6 +269,8 @@ static void __rds_conn_path_init(struct rds_connection *conn,
 			kmem_cache_free(rds_conn_slab, conn);
 			conn = found;
 		} else {
+			conn->c_my_gen_num = rds_gen_num;
+			conn->c_peer_gen_num = 0;
 			hlist_add_head_rcu(&conn->c_hash_node, head);
 			rds_cong_add_conn(conn);
 			rds_conn_count++;
net/rds/message.c +1 -0
index 6cb9106..49bfb51 100644
@@ -42,6 +42,7 @@
 [RDS_EXTHDR_RDMA]	= sizeof(struct rds_ext_header_rdma),
 [RDS_EXTHDR_RDMA_DEST]	= sizeof(struct rds_ext_header_rdma_dest),
 [RDS_EXTHDR_NPATHS]	= sizeof(u16),
+[RDS_EXTHDR_GEN_NUM]	= sizeof(u32),
 };
net/rds/rds.h +7 -1
index 4121e18..ebbf909 100644
@@ -151,6 +151,9 @@ struct rds_connection {
 
 	struct rds_conn_path	c_path[RDS_MPATH_WORKERS];
 	wait_queue_head_t	c_hs_waitq; /* handshake waitq */
+
+	u32			c_my_gen_num;
+	u32			c_peer_gen_num;
 };
 
 static inline

@@ -243,7 +246,8 @@ struct rds_ext_header_rdma_dest {
 /* Extension header announcing number of paths.
  * Implicit length = 2 bytes.
  */
-#define RDS_EXTHDR_NPATHS	4
+#define RDS_EXTHDR_NPATHS	5
+#define RDS_EXTHDR_GEN_NUM	6
 
 #define __RDS_EXTHDR_MAX	16 /* for now */
 

@@ -338,6 +342,7 @@ static inline u32 rds_rdma_cookie_offset(rds_rdma_cookie_t cookie)
 #define RDS_MSG_RETRANSMITTED	5
 #define RDS_MSG_MAPPED		6
 #define RDS_MSG_PAGEVEC		7
+#define RDS_MSG_FLUSH		8
 
 struct rds_message {
 	atomic_t		m_refcount;

@@ -664,6 +669,7 @@ static inline void __rds_wake_sk_sleep(struct sock *sk)
 struct rds_message *rds_cong_update_alloc(struct rds_connection *conn);
 
 /* conn.c */
+extern u32 rds_gen_num;
 int rds_conn_init(void);
 void rds_conn_exit(void);
 struct rds_connection *rds_conn_create(struct net *net,
net/rds/recv.c +36 -0
index cbfabdf..9d0666e 100644
@@ -120,6 +120,36 @@ static void rds_recv_rcvbuf_delta(struct rds_sock *rs, struct sock *sk,
 	/* do nothing if no change in cong state */
 }
 
+static void rds_conn_peer_gen_update(struct rds_connection *conn,
+				     u32 peer_gen_num)
+{
+	int i;
+	struct rds_message *rm, *tmp;
+	unsigned long flags;
+
+	WARN_ON(conn->c_trans->t_type != RDS_TRANS_TCP);
+	if (peer_gen_num != 0) {
+		if (conn->c_peer_gen_num != 0 &&
+		    peer_gen_num != conn->c_peer_gen_num) {
+			for (i = 0; i < RDS_MPATH_WORKERS; i++) {
+				struct rds_conn_path *cp;
+
+				cp = &conn->c_path[i];
+				spin_lock_irqsave(&cp->cp_lock, flags);
+				cp->cp_next_tx_seq = 1;
+				cp->cp_next_rx_seq = 0;
+				list_for_each_entry_safe(rm, tmp,
+							 &cp->cp_retrans,
+							 m_conn_item) {
+					set_bit(RDS_MSG_FLUSH, &rm->m_flags);
+				}
+				spin_unlock_irqrestore(&cp->cp_lock, flags);
+			}
+		}
+		conn->c_peer_gen_num = peer_gen_num;
+	}
+}
+
 /*
  * Process all extension headers that come with this message.
  */

@@ -163,7 +193,9 @@ static void rds_recv_hs_exthdrs(struct rds_header *hdr,
 	union {
 		struct rds_ext_header_version version;
 		u16 rds_npaths;
+		u32 rds_gen_num;
 	} buffer;
+	u32 new_peer_gen_num = 0;
 
 	while (1) {
 		len = sizeof(buffer);

@@ -176,6 +208,9 @@ static void rds_recv_hs_exthdrs(struct rds_header *hdr,
 			conn->c_npaths = min_t(int, RDS_MPATH_WORKERS,
 					       buffer.rds_npaths);
 			break;
+		case RDS_EXTHDR_GEN_NUM:
+			new_peer_gen_num = buffer.rds_gen_num;
+			break;
 		default:
 			pr_warn_ratelimited("ignoring unknown exthdr type "
 					     "0x%x\n", type);

@@ -183,6 +218,7 @@ static void rds_recv_hs_exthdrs(struct rds_header *hdr,
 	}
 	/* if RDS_EXTHDR_NPATHS was not found, default to a single-path */
 	conn->c_npaths = max_t(int, conn->c_npaths, 1);
+	rds_conn_peer_gen_update(conn, new_peer_gen_num);
 }
 
 /* rds_start_mprds() will synchronously start multiple paths when appropriate.
net/rds/send.c +7 -2
index 896626b..77c8c6e 100644
@@ -259,8 +259,9 @@ int rds_send_xmit(struct rds_conn_path *cp)
 			 * connection.
 			 * Therefore, we never retransmit messages with RDMA ops.
 			 */
-			if (rm->rdma.op_active &&
-			    test_bit(RDS_MSG_RETRANSMITTED, &rm->m_flags)) {
+			if (test_bit(RDS_MSG_FLUSH, &rm->m_flags) ||
+			    (rm->rdma.op_active &&
+			    test_bit(RDS_MSG_RETRANSMITTED, &rm->m_flags))) {
 				spin_lock_irqsave(&cp->cp_lock, flags);
 				if (test_and_clear_bit(RDS_MSG_ON_CONN, &rm->m_flags))
 					list_move(&rm->m_conn_item, &to_be_dropped);

@@ -1209,6 +1210,10 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len)
 		rds_message_add_extension(&rm->m_inc.i_hdr,
 					  RDS_EXTHDR_NPATHS, &npaths,
 					  sizeof(npaths));
+		rds_message_add_extension(&rm->m_inc.i_hdr,
+					  RDS_EXTHDR_GEN_NUM,
+					  &cp->cp_conn->c_my_gen_num,
+					  sizeof(u32));
 	}
 	spin_unlock_irqrestore(&cp->cp_lock, flags);
Signature · 1 line
1.7.1
author
#3 lore raw

PATCH 3/3 RDS: TCP: Force every connection to be initiated by numerically smaller IP address

In reply to Sowmini Varadhan

When 2 RDS peers initiate an RDS-TCP connection simultaneously, there is a potential for "duelling syns" on either/both sides. See commit 241b271952eb ("RDS-TCP: Reset tcp callbacks if re-using an outgoing socket in rds_tcp_accept_one()") for a description of this condition, and the arbitration logic which ensures that the numerically large IP address in the TCP connection is bound to the RDS_TCP_PORT ("canonical ordering").

The rds_connection should not be marked as RDS_CONN_UP until the arbitration logic has converged for the following reason. The sender may start transmitting RDS datagrams as soon as RDS_CONN_UP is set, and since the sender removes all datagrams from the rds_connection's cp_retrans queue based on TCP acks. If the TCP ack was sent from a tcp socket that got reset as part of duel aribitration (but before data was delivered to the receivers RDS socket layer), the sender may end up prematurely freeing the datagram, and the datagram is no longer reliably deliverable.

This patch remedies that condition by making sure that, upon receipt of 3WH completion state change notification of TCP_ESTABLISHED in rds_tcp_state_change, we mark the rds_connection as RDS_CONN_UP if, and only if, the IP addresses and ports for the connection are canonically ordered. In all other cases, rds_tcp_state_change will force an rds_conn_path_drop(), and rds_queue_reconnect() on both peers will restart the connection to ensure canonical ordering.

A side-effect of enforcing this condition in rds_tcp_state_change() is that rds_tcp_accept_one_path() can now be refactored for simplicity. It is also no longer possible to encounter an RDS_CONN_UP connection in the arbitration logic in rds_tcp_accept_one().

Signed-off-by Sowmini Varadhan <[email protected]>
Acked-by Santosh Shilimkar <[email protected]>
---
 net/rds/connection.c  |    1 +
 net/rds/tcp_connect.c |   14 +++++++++++++-
 net/rds/tcp_listen.c  |   29 ++++++++++++-----------------
 3 files changed, 26 insertions(+), 18 deletions(-)
net/rds/connection.c +1 -0
index b86e188..fe9d31c 100644
@@ -683,6 +683,7 @@ void rds_conn_path_connect_if_down(struct rds_conn_path *cp)
 	    !test_and_set_bit(RDS_RECONNECT_PENDING, &cp->cp_flags))
 		queue_delayed_work(rds_wq, &cp->cp_conn_w, 0);
 }
+EXPORT_SYMBOL_GPL(rds_conn_path_connect_if_down);
 
 void rds_conn_connect_if_down(struct rds_connection *conn)
 {
net/rds/tcp_connect.c +13 -1
index 05f61c5..d6839d9 100644
@@ -60,7 +60,19 @@ void rds_tcp_state_change(struct sock *sk)
 	case TCP_SYN_RECV:
 		break;
 	case TCP_ESTABLISHED:
-		rds_connect_path_complete(cp, RDS_CONN_CONNECTING);
+		/* Force the peer to reconnect so that we have the
+		 * TCP ports going from <smaller-ip>.<transient> to
+		 * <larger-ip>.<RDS_TCP_PORT>. We avoid marking the
+		 * RDS connection as RDS_CONN_UP until the reconnect,
+		 * to avoid RDS datagram loss.
+		 */
+		if (cp->cp_conn->c_laddr > cp->cp_conn->c_faddr &&
+		    rds_conn_path_transition(cp, RDS_CONN_CONNECTING,
+					     RDS_CONN_ERROR)) {
+			rds_conn_path_drop(cp);
+		} else {
+			rds_connect_path_complete(cp, RDS_CONN_CONNECTING);
+		}
 		break;
 	case TCP_CLOSE_WAIT:
 	case TCP_CLOSE:
net/rds/tcp_listen.c +12 -17
index c9c4968..f74bab3 100644
@@ -83,25 +83,20 @@ struct rds_tcp_connection *rds_tcp_accept_one_path(struct rds_connection *conn)
 {
 	int i;
 	bool peer_is_smaller = (conn->c_faddr < conn->c_laddr);
-	int npaths = conn->c_npaths;
-
-	if (npaths <= 1) {
-		struct rds_conn_path *cp = &conn->c_path[0];
-		int ret;
-
-		ret = rds_conn_path_transition(cp, RDS_CONN_DOWN,
-					       RDS_CONN_CONNECTING);
-		if (!ret)
-			rds_conn_path_transition(cp, RDS_CONN_ERROR,
-						 RDS_CONN_CONNECTING);
-		return cp->cp_transport_data;
-	}
+	int npaths = max_t(int, 1, conn->c_npaths);
 
-	/* for mprds, paths with cp_index > 0 MUST be initiated by the peer
+	/* for mprds, all paths MUST be initiated by the peer
 	 * with the smaller address.
 	 */
-	if (!peer_is_smaller)
+	if (!peer_is_smaller) {
+		/* Make sure we initiate at least one path if this
+		 * has not already been done; rds_start_mprds() will
+		 * take care of additional paths, if necessary.
+		 */
+		if (npaths == 1)
+			rds_conn_path_connect_if_down(&conn->c_path[0]);
 		return NULL;
+	}
 
 	for (i = 0; i < npaths; i++) {
 		struct rds_conn_path *cp = &conn->c_path[i];

@@ -171,8 +166,8 @@ int rds_tcp_accept_one(struct socket *sock)
 	mutex_lock(&rs_tcp->t_conn_path_lock);
 	cp = rs_tcp->t_cpath;
 	conn_state = rds_conn_path_state(cp);
-	if (conn_state != RDS_CONN_CONNECTING && conn_state != RDS_CONN_UP &&
-	    conn_state != RDS_CONN_ERROR)
+	WARN_ON(conn_state == RDS_CONN_UP);
+	if (conn_state != RDS_CONN_CONNECTING && conn_state != RDS_CONN_ERROR)
 		goto rst_nsk;
 	if (rs_tcp->t_sock) {
 		/* Need to resolve a duelling SYN between peers.
Signature · 1 line
1.7.1
maintainer
#4 lore raw
In reply to Sowmini Varadhan

From: Sowmini Varadhan <[email protected]>
Date: Wed, 16 Nov 2016 13:29:47 -0800

Quoted 13 lines

This series contains a set of fixes for bugs exposed when we ran the following in a loop between a test machine pair:

while (1); do

   # modprobe rds-tcp on test nodes
   # run rds-stress in bi-dir mode between test machine pair 
   # modprobe -r rds-tcp on test nodes
 done

rds-stress in bi-dir mode will cause both nodes to initiate RDS-TCP connections at almost the same instant, exposing the bugs fixed in this series.

Quoted 6 lines

Without the fixes, rds-stress reports sporadic packet drops, and packets arriving out of sequence. After the fixes,we have been able to run the test overnight, without any issues.

Each patch has a detailed description of the root-cause fixed by the patch.

Series applied, thank you.