libceph: flush msgr queue during mon_client shutdown

We need to flush the msgr workqueue during mon_client shutdown to
ensure that any work affecting our embedded ceph_connection is
finished so that we can be safely destroyed.

Previously, we were flushing the work queue after osd_client
shutdown and before mon_client shutdown to ensure that any osd
connection refs to authorizers are flushed.  Remove the redundant
flush, and document in the comment that the mon_client flush is
needed to cover that case as well.
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
(cherry picked from commit f3dea7ed)

net/ceph/ceph_common.c

@@ -504,13 +504,6 @@ void ceph_destroy_client(struct ceph_client *client)
 	/* unmount */
 	ceph_osdc_stop(&client->osdc);
 
-	/*
-	 * make sure osd connections close out before destroying the
-	 * auth module, which is needed to free those connections'
-	 * ceph_authorizers.
-	 */
-	ceph_msgr_flush();
-
 	ceph_monc_stop(&client->monc);
 
 	ceph_debugfs_client_cleanup(client);

net/ceph/mon_client.c

@@ -847,6 +847,14 @@ void ceph_monc_stop(struct ceph_mon_client *monc)
 
 	mutex_unlock(&monc->mutex);
 
+	/*
+	 * flush msgr queue before we destroy ourselves to ensure that:
+	 *  - any work that references our embedded con is finished.
+	 *  - any osd_client or other work that may reference an authorizer
+	 *    finishes before we shut down the auth subsystem.
+	 */
+	ceph_msgr_flush();
+
 	ceph_auth_destroy(monc->auth);
 
 	ceph_msg_put(monc->m_auth);