RDS: Connection handling

While arguably the fact that the underlying transport needs a
connection to convey RDS's datagrame reliably is not important
to rds proper, the transports implemented so far (IB and TCP)
have both been connection-oriented, and so the connection
state machine-related code is in the common rds code.

This patch also includes several work items, to handle connecting,
sending, receiving, and shutdown.
Signed-off-by: Andy Grover <andy.grover@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/threads.c

+/*
+ * Copyright (c) 2006 Oracle.  All rights reserved.
+ *
+ * This software is available to you under a choice of one of two
+ * licenses.  You may choose to be licensed under the terms of the GNU
+ * General Public License (GPL) Version 2, available from the file
+ * COPYING in the main directory of this source tree, or the
+ * OpenIB.org BSD license below:
+ *
+ *     Redistribution and use in source and binary forms, with or
+ *     without modification, are permitted provided that the following
+ *     conditions are met:
+ *
+ *      - Redistributions of source code must retain the above
+ *        copyright notice, this list of conditions and the following
+ *        disclaimer.
+ *
+ *      - Redistributions in binary form must reproduce the above
+ *        copyright notice, this list of conditions and the following
+ *        disclaimer in the documentation and/or other materials
+ *        provided with the distribution.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ */
+#include <linux/kernel.h>
+#include <linux/random.h>
+
+#include "rds.h"
+
+/*
+ * All of connection management is simplified by serializing it through
+ * work queues that execute in a connection managing thread.
+ *
+ * TCP wants to send acks through sendpage() in response to data_ready(),
+ * but it needs a process context to do so.
+ *
+ * The receive paths need to allocate but can't drop packets (!) so we have
+ * a thread around to block allocating if the receive fast path sees an
+ * allocation failure.
+ */
+
+/* Grand Unified Theory of connection life cycle:
+ * At any point in time, the connection can be in one of these states:
+ * DOWN, CONNECTING, UP, DISCONNECTING, ERROR
+ *
+ * The following transitions are possible:
+ *  ANY		  -> ERROR
+ *  UP		  -> DISCONNECTING
+ *  ERROR	  -> DISCONNECTING
+ *  DISCONNECTING -> DOWN
+ *  DOWN	  -> CONNECTING
+ *  CONNECTING	  -> UP
+ *
+ * Transition to state DISCONNECTING/DOWN:
+ *  -	Inside the shutdown worker; synchronizes with xmit path
+ *	through c_send_lock, and with connection management callbacks
+ *	via c_cm_lock.
+ *
+ *	For receive callbacks, we rely on the underlying transport
+ *	(TCP, IB/RDMA) to provide the necessary synchronisation.
+ */
+struct workqueue_struct *rds_wq;
+
+void rds_connect_complete(struct rds_connection *conn)
+{
+	if (!rds_conn_transition(conn, RDS_CONN_CONNECTING, RDS_CONN_UP)) {
+		printk(KERN_WARNING "%s: Cannot transition to state UP, "
+				"current state is %d\n",
+				__func__,
+				atomic_read(&conn->c_state));
+		atomic_set(&conn->c_state, RDS_CONN_ERROR);
+		queue_work(rds_wq, &conn->c_down_w);
+		return;
+	}
+
+	rdsdebug("conn %p for %pI4 to %pI4 complete\n",
+	  conn, &conn->c_laddr, &conn->c_faddr);
+
+	conn->c_reconnect_jiffies = 0;
+	set_bit(0, &conn->c_map_queued);
+	queue_delayed_work(rds_wq, &conn->c_send_w, 0);
+	queue_delayed_work(rds_wq, &conn->c_recv_w, 0);
+}
+
+/*
+ * This random exponential backoff is relied on to eventually resolve racing
+ * connects.
+ *
+ * If connect attempts race then both parties drop both connections and come
+ * here to wait for a random amount of time before trying again.  Eventually
+ * the backoff range will be so much greater than the time it takes to
+ * establish a connection that one of the pair will establish the connection
+ * before the other's random delay fires.
+ *
+ * Connection attempts that arrive while a connection is already established
+ * are also considered to be racing connects.  This lets a connection from
+ * a rebooted machine replace an existing stale connection before the transport
+ * notices that the connection has failed.
+ *
+ * We should *always* start with a random backoff; otherwise a broken connection
+ * will always take several iterations to be re-established.
+ */
+static void rds_queue_reconnect(struct rds_connection *conn)
+{
+	unsigned long rand;
+
+	rdsdebug("conn %p for %pI4 to %pI4 reconnect jiffies %lu\n",
+	  conn, &conn->c_laddr, &conn->c_faddr,
+	  conn->c_reconnect_jiffies);
+
+	set_bit(RDS_RECONNECT_PENDING, &conn->c_flags);
+	if (conn->c_reconnect_jiffies == 0) {
+		conn->c_reconnect_jiffies = rds_sysctl_reconnect_min_jiffies;
+		queue_delayed_work(rds_wq, &conn->c_conn_w, 0);
+		return;
+	}
+
+	get_random_bytes(&rand, sizeof(rand));
+	rdsdebug("%lu delay %lu ceil conn %p for %pI4 -> %pI4\n",
+		 rand % conn->c_reconnect_jiffies, conn->c_reconnect_jiffies,
+		 conn, &conn->c_laddr, &conn->c_faddr);
+	queue_delayed_work(rds_wq, &conn->c_conn_w,
+			   rand % conn->c_reconnect_jiffies);
+
+	conn->c_reconnect_jiffies = min(conn->c_reconnect_jiffies * 2,
+					rds_sysctl_reconnect_max_jiffies);
+}
+
+void rds_connect_worker(struct work_struct *work)
+{
+	struct rds_connection *conn = container_of(work, struct rds_connection, c_conn_w.work);
+	int ret;
+
+	clear_bit(RDS_RECONNECT_PENDING, &conn->c_flags);
+	if (rds_conn_transition(conn, RDS_CONN_DOWN, RDS_CONN_CONNECTING)) {
+		ret = conn->c_trans->conn_connect(conn);
+		rdsdebug("conn %p for %pI4 to %pI4 dispatched, ret %d\n",
+			conn, &conn->c_laddr, &conn->c_faddr, ret);
+
+		if (ret) {
+			if (rds_conn_transition(conn, RDS_CONN_CONNECTING, RDS_CONN_DOWN))
+				rds_queue_reconnect(conn);
+			else
+				rds_conn_error(conn, "RDS: connect failed\n");
+		}
+	}
+}
+
+void rds_shutdown_worker(struct work_struct *work)
+{
+	struct rds_connection *conn = container_of(work, struct rds_connection, c_down_w);
+
+	/* shut it down unless it's down already */
+	if (!rds_conn_transition(conn, RDS_CONN_DOWN, RDS_CONN_DOWN)) {
+		/*
+		 * Quiesce the connection mgmt handlers before we start tearing
+		 * things down. We don't hold the mutex for the entire
+		 * duration of the shutdown operation, else we may be
+		 * deadlocking with the CM handler. Instead, the CM event
+		 * handler is supposed to check for state DISCONNECTING
+		 */
+		mutex_lock(&conn->c_cm_lock);
+		if (!rds_conn_transition(conn, RDS_CONN_UP, RDS_CONN_DISCONNECTING)
+		 && !rds_conn_transition(conn, RDS_CONN_ERROR, RDS_CONN_DISCONNECTING)) {
+			rds_conn_error(conn, "shutdown called in state %d\n",
+					atomic_read(&conn->c_state));
+			mutex_unlock(&conn->c_cm_lock);
+			return;
+		}
+		mutex_unlock(&conn->c_cm_lock);
+
+		mutex_lock(&conn->c_send_lock);
+		conn->c_trans->conn_shutdown(conn);
+		rds_conn_reset(conn);
+		mutex_unlock(&conn->c_send_lock);
+
+		if (!rds_conn_transition(conn, RDS_CONN_DISCONNECTING, RDS_CONN_DOWN)) {
+			/* This can happen - eg when we're in the middle of tearing
+			 * down the connection, and someone unloads the rds module.
+			 * Quite reproduceable with loopback connections.
+			 * Mostly harmless.
+			 */
+			rds_conn_error(conn,
+				"%s: failed to transition to state DOWN, "
+				"current state is %d\n",
+				__func__,
+				atomic_read(&conn->c_state));
+			return;
+		}
+	}
+
+	/* Then reconnect if it's still live.
+	 * The passive side of an IB loopback connection is never added
+	 * to the conn hash, so we never trigger a reconnect on this
+	 * conn - the reconnect is always triggered by the active peer. */
+	cancel_delayed_work(&conn->c_conn_w);
+	if (!hlist_unhashed(&conn->c_hash_node))
+		rds_queue_reconnect(conn);
+}
+
+void rds_send_worker(struct work_struct *work)
+{
+	struct rds_connection *conn = container_of(work, struct rds_connection, c_send_w.work);
+	int ret;
+
+	if (rds_conn_state(conn) == RDS_CONN_UP) {
+		ret = rds_send_xmit(conn);
+		rdsdebug("conn %p ret %d\n", conn, ret);
+		switch (ret) {
+		case -EAGAIN:
+			rds_stats_inc(s_send_immediate_retry);
+			queue_delayed_work(rds_wq, &conn->c_send_w, 0);
+			break;
+		case -ENOMEM:
+			rds_stats_inc(s_send_delayed_retry);
+			queue_delayed_work(rds_wq, &conn->c_send_w, 2);
+		default:
+			break;
+		}
+	}
+}
+
+void rds_recv_worker(struct work_struct *work)
+{
+	struct rds_connection *conn = container_of(work, struct rds_connection, c_recv_w.work);
+	int ret;
+
+	if (rds_conn_state(conn) == RDS_CONN_UP) {
+		ret = conn->c_trans->recv(conn);
+		rdsdebug("conn %p ret %d\n", conn, ret);
+		switch (ret) {
+		case -EAGAIN:
+			rds_stats_inc(s_recv_immediate_retry);
+			queue_delayed_work(rds_wq, &conn->c_recv_w, 0);
+			break;
+		case -ENOMEM:
+			rds_stats_inc(s_recv_delayed_retry);
+			queue_delayed_work(rds_wq, &conn->c_recv_w, 2);
+		default:
+			break;
+		}
+	}
+}
+
+void rds_threads_exit(void)
+{
+	destroy_workqueue(rds_wq);
+}
+
+int __init rds_threads_init(void)
+{
+	rds_wq = create_singlethread_workqueue("krdsd");
+	if (rds_wq == NULL)
+		return -ENOMEM;
+
+	return 0;
+}