rbd: define common queue_con_delay()

This patch defines a single function, queue_con_delay() to call
queue_delayed_work() for a connection.  It basically generalizes
what was previously queue_con() by adding the delay argument.
queue_con() is now a simple helper that passes 0 for its delay.
queue_con_delay() returns 0 if it queued work or an errno if it
did not for some reason.

If con_work() finds the BACKOFF flag set for a connection, it now
calls queue_con_delay() to handle arranging to start again after a
delay.

Note about connection reference counts:  con_work() only ever gets
called as a work item function.  At the time that work is scheduled,
a reference to the connection is acquired, and the corresponding
con_work() call is then responsible for dropping that reference
before it returns.

Previously, the backoff handling inside con_work() silently handed
off its reference to delayed work it scheduled.  Now that
queue_con_delay() is used, a new reference is acquired for the
newly-scheduled work, and the original reference is dropped by the
con->ops->put() call at the end of the function.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>

net/ceph/messenger.c

@@ -2244,22 +2244,33 @@ static int try_read(struct ceph_connection *con)
 
 
 /*
- * Atomically queue work on a connection.  Bump @con reference to
- * avoid races with connection teardown.
+ * Atomically queue work on a connection after the specified delay.
+ * Bump @con reference to avoid races with connection teardown.
+ * Returns 0 if work was queued, or an error code otherwise.
  */
-static void queue_con(struct ceph_connection *con)
+static int queue_con_delay(struct ceph_connection *con, unsigned long delay)
 {
 	if (!con->ops->get(con)) {
-		dout("queue_con %p ref count 0\n", con);
-		return;
+		dout("%s %p ref count 0\n", __func__, con);
+
+		return -ENOENT;
 	}
 
-	if (!queue_delayed_work(ceph_msgr_wq, &con->work, 0)) {
-		dout("queue_con %p - already queued\n", con);
+	if (!queue_delayed_work(ceph_msgr_wq, &con->work, delay)) {
+		dout("%s %p - already queued\n", __func__, con);
 		con->ops->put(con);
-	} else {
-		dout("queue_con %p\n", con);
+
+		return -EBUSY;
 	}
+
+	dout("%s %p %lu\n", __func__, con, delay);
+
+	return 0;
+}
+
+static void queue_con(struct ceph_connection *con)
+{
+	(void) queue_con_delay(con, 0);
 }
 
 /*
@@ -2294,14 +2305,11 @@ static void con_work(struct work_struct *work)
 
 	if (test_and_clear_bit(CON_FLAG_BACKOFF, &con->flags)) {
 		dout("con_work %p backing off\n", con);
-		if (queue_delayed_work(ceph_msgr_wq, &con->work,
-				       round_jiffies_relative(con->delay))) {
-			dout("con_work %p backoff %lu\n", con, con->delay);
-			mutex_unlock(&con->mutex);
-			return;
-		} else {
+		ret = queue_con_delay(con, round_jiffies_relative(con->delay));
+		if (ret) {
 			dout("con_work %p FAILED to back off %lu\n", con,
 			     con->delay);
+			BUG_ON(ret == -ENOENT);
 			set_bit(CON_FLAG_BACKOFF, &con->flags);
 		}
 		goto done;