xtrabackup 2.0.4 can't unlock tables

Server version: 5.1.61-log Source distribution
xtrabackup 2.0.4

backup log from xtrabackup:

[01] xtrabackup: Page 170 seems double write buffer. passing the check.
[01] xtrabackup: Page 171 seems double write buffer. passing the check.
[01] xtrabackup: Page 172 seems double write buffer. passing the check.

130228 03:00:16 innobackupex-1.5.1: Continuing after ibbackup has suspended

130228 03:00:16 innobackupex-1.5.1: Connected to database with mysql child process (pid=388)
130228 03:00:18 innobackupex-1.5.1: Starting to lock all tables…

log scanned up to (870511098952)
log scanned up to (870511285007)
[01] …done
[01] Streaming ./aaa.ibd
[01] …done
[01] Streaming ./bbb.ibd

130228 03:00:28 innobackupex-1.5.1: All tables locked and flushed to disk

log scanned up to (870511285007)

130228 03:00:30 innobackupex-1.5.1: Starting to backup .frm, .MRG, .MYD, .MYI,
innobackupex-1.5.1: .TRG, .TRN, .ARM, .ARZ, .CSM, .CSV and .opt files in
innobackupex-1.5.1: subdirectories of

130228 03:00:33 innobackupex-1.5.1: Finished backing up .frm, .MRG, .MYD, .MYI, .TRG, .TRN, .ARM, .ARZ, .CSV, .CSM and .opt files

innobackupex-1.5.1: Resuming ibbackup

log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
[01] …done
[01] Streaming ./xx.ibd
[01] …done
[01] Streaming ./yy.ibd

log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)
log scanned up to (870511289801)

Waiting for a long time,no unlock tables happend,so you will see readlock from show processlist;
mysql> show processlist;
±------±------------±--------------------±------±------ ------±------±-------------------------------------------- ---- ±------------------+
| Id | User | Host | db | Command | Time | State | Info |
±------±------------±--------------------±------±------ ------±------±-------------------------------------------- -----±------------------+
| 1 | system user | | NULL | Connect | 85045 | Waiting for master to send event | NULL |
| 2 | system user | | NULL | Connect | 23401 | Waiting for release of readlock | NULL

what happend?
thank you.