MySQL 5.6 slave Slave_IO_State is in system lock state (no replies)

I have the same problem as
http://forums.mysql.com/read.php?22,598354,598354

version 5.6.15-log, master-master replication, the standby slave is not able to keep up. There's no obvious bottlenecks in CPU, mem, or disk.

Replication threads:
| 6336900 | thread/sql/one_connection | FOREGROUND | 6336881 | repl | 10.55.197.120 | NULL | Binlog Dump | 1623477 | Master has sent all binlog to slave; waiting for binlog to be up | NULL | NULL | NULL | YES |

| 1 | thread/sql/main | BACKGROUND | NULL | NULL | NULL | NULL | NULL | 9588678 | System lock | INTERNAL DDL LOG RECOVER IN PROGRESS | NULL | NULL | YES |