RDS: Change send lock from a mutex to a spinlock

This change allows us to call rds_send_xmit() from a tasklet,
which is crucial to our new operating model.

* Change c_send_lock to a spinlock
* Update stats fields "sem_" to "_lock"
* Remove unneeded rds_conn_is_sending()

About locking between shutdown and send -- send checks if the
connection is up. Shutdown puts the connection into
DISCONNECTING. After this, all threads entering send will exit
immediately. However, a thread could be *in* send_xmit(), so
shutdown acquires the c_send_lock to ensure everyone is out
before proceeding with connection shutdown.
Signed-off-by: Andy Grover <andy.grover@oracle.com>

net/rds/connection.c

@@ -62,18 +62,6 @@ static struct hlist_head *rds_conn_bucket(__be32 laddr, __be32 faddr)
 		var |= RDS_INFO_CONNECTION_FLAG_##suffix;	\
 } while (0)
 
-static inline int rds_conn_is_sending(struct rds_connection *conn)
-{
-	int ret = 0;
-
-	if (!mutex_trylock(&conn->c_send_lock))
-		ret = 1;
-	else
-		mutex_unlock(&conn->c_send_lock);
-
-	return ret;
-}
-
 static struct rds_connection *rds_conn_lookup(struct hlist_head *head,
 					      __be32 laddr, __be32 faddr,
 					      struct rds_transport *trans)
@@ -158,7 +146,7 @@ static struct rds_connection *__rds_conn_create(__be32 laddr, __be32 faddr,
 	spin_lock_init(&conn->c_lock);
 	conn->c_next_tx_seq = 1;
 
-	mutex_init(&conn->c_send_lock);
+	spin_lock_init(&conn->c_send_lock);
 	INIT_LIST_HEAD(&conn->c_send_queue);
 	INIT_LIST_HEAD(&conn->c_retrans);
 
@@ -283,10 +271,12 @@ void rds_conn_shutdown(struct rds_connection *conn)
 		}
 		mutex_unlock(&conn->c_cm_lock);
 
-		mutex_lock(&conn->c_send_lock);
+		/* verify everybody's out of rds_send_xmit() */
+		spin_lock_irq(&conn->c_send_lock);
+		spin_unlock_irq(&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
@@ -476,7 +466,7 @@ static int rds_conn_info_visitor(struct rds_connection *conn,
 	cinfo->flags = 0;
 
 	rds_conn_info_set(cinfo->flags,
-			  rds_conn_is_sending(conn), SENDING);
+			  spin_is_locked(&conn->c_send_lock), SENDING);
 	/* XXX Future: return the state rather than these funky bits */
 	rds_conn_info_set(cinfo->flags,
 			  atomic_read(&conn->c_state) == RDS_CONN_CONNECTING,

net/rds/rds.h

@@ -91,7 +91,7 @@ struct rds_connection {
 	struct rds_cong_map	*c_lcong;
 	struct rds_cong_map	*c_fcong;
 
-	struct mutex		c_send_lock;	/* protect send ring */
+	spinlock_t		c_send_lock;	/* protect send ring */
 	struct rds_message	*c_xmit_rm;
 	unsigned long		c_xmit_sg;
 	unsigned int		c_xmit_hdr_off;
@@ -548,8 +548,8 @@ struct rds_statistics {
 	uint64_t	s_recv_ping;
 	uint64_t	s_send_queue_empty;
 	uint64_t	s_send_queue_full;
-	uint64_t	s_send_sem_contention;
-	uint64_t	s_send_sem_queue_raced;
+	uint64_t	s_send_lock_contention;
+	uint64_t	s_send_lock_queue_raced;
 	uint64_t	s_send_immediate_retry;
 	uint64_t	s_send_delayed_retry;
 	uint64_t	s_send_drop_acked;

net/rds/send.c

@@ -116,19 +116,18 @@ int rds_send_xmit(struct rds_connection *conn)
 	int was_empty = 0;
 	LIST_HEAD(to_be_dropped);
 
+	if (!rds_conn_up(conn))
+		goto out;
+
 	/*
 	 * sendmsg calls here after having queued its message on the send
 	 * queue.  We only have one task feeding the connection at a time.  If
 	 * another thread is already feeding the queue then we back off.  This
 	 * avoids blocking the caller and trading per-connection data between
 	 * caches per message.
-	 *
-	 * The sem holder will issue a retry if they notice that someone queued
-	 * a message after they stopped walking the send queue but before they
-	 * dropped the sem.
 	 */
-	if (!mutex_trylock(&conn->c_send_lock)) {
-		rds_stats_inc(s_send_sem_contention);
+	if (!spin_trylock_irqsave(&conn->c_send_lock, flags)) {
+		rds_stats_inc(s_send_lock_contention);
 		ret = -ENOMEM;
 		goto out;
 	}
@@ -346,7 +345,7 @@ int rds_send_xmit(struct rds_connection *conn)
 	 * stop processing the loop when the transport hasn't taken
 	 * responsibility for forward progress.
 	 */
-	mutex_unlock(&conn->c_send_lock);
+	spin_unlock_irqrestore(&conn->c_send_lock, flags);
 
 	if (send_quota == 0 && !was_empty) {
 		/* We exhausted the send quota, but there's work left to
@@ -360,7 +359,7 @@ int rds_send_xmit(struct rds_connection *conn)
 		 * spin lock */
 		spin_lock_irqsave(&conn->c_lock, flags);
 		if (!list_empty(&conn->c_send_queue)) {
-			rds_stats_inc(s_send_sem_queue_raced);
+			rds_stats_inc(s_send_lock_queue_raced);
 			ret = -EAGAIN;
 		}
 		spin_unlock_irqrestore(&conn->c_lock, flags);

net/rds/stats.c

@@ -57,8 +57,8 @@ static const char *const rds_stat_names[] = {
 	"recv_ping",
 	"send_queue_empty",
 	"send_queue_full",
-	"send_sem_contention",
-	"send_sem_queue_raced",
+	"send_lock_contention",
+	"send_lock_queue_raced",
 	"send_immediate_retry",
 	"send_delayed_retry",
 	"send_drop_acked",