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>

1 files changed, 7 insertions, 8 deletions

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);