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>

4 files changed, 18 insertions, 29 deletions

diff --git a/net/rds/connection.c b/net/rds/connection.c
index 88bcaf3f3e16..56aebe444ad3 100644
--- a/net/rds/connection.c
+++ b/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,
diff --git a/net/rds/rds.h b/net/rds/rds.h
index e81d7e478474..c3a668b9cc14 100644
--- a/net/rds/rds.h
+++ b/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;
diff --git a/net/rds/send.c b/net/rds/send.c
index 8a0647af5d95..d4feec6ad09c 100644
--- a/net/rds/send.c
+++ b/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);
diff --git a/net/rds/stats.c b/net/rds/stats.c
index c66d95d9c262..b77be8be33ba 100644
--- a/net/rds/stats.c
+++ b/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",