我正在使用redis运行magento商店进行会话。有时我会收到以下错误：
2014-08-04T14:18:36+00:00 ERR (3): Warning: session_start(): user session functions not defined in /var/www/homegourmet/src/app/code/core/Mage/Core/Model/Session/Abstract/Varien.php on line 125
2014-08-04T14:18:37+00:00 ERR (3): Warning: session_start(): user session functions not defined in /var/www/homegourmet/src/app/code/core/Mage/Core/Model/Session/Abstract/Varien.php on line 125
@ line125：
session_start();
对于redis我使用的是默认设置。同样在不同的DB，同一个实例中，我正在运行magento缓存。我正在使用php-redis模块。
我的redis-session模块配置：
<session_save><![CDATA[db]]></session_save>
<redis_session> <!-- All options seen here are the defaults -->
<host>127.0.0.1</host> <!-- Specify an absolute path if using a unix socket -->
<port>6379</port>
<timeout>120</timeout> <!-- This is the Redis connection timeout, not the locking timeout -->
<db>1</db> <!-- Redis database number; protection from accidental loss is improved by using a unique DB number for sessions -->
<compression_threshold>2048</compression_threshold> <!-- Set to 0 to disable compression (recommended when suhosin.session.encrypt=on); known bug with strings over 64k: https://github.com/colinmollenhour/Cm_Cache_Backend_Redis/issues/18 -->
<compression_lib>lzf</compression_lib> <!-- gzip, lzf or snappy -->
<log_level>4</log_level> <!-- 0 (emergency: system is unusable), 4 (warning; additional information, recommended), 5 (notice: normal but significant condition), 6 (info: informational messages), 7 (debug: the most information for development/testing) -->
<max_concurrency>6</max_concurrency> <!-- maximum number of processes that can wait for a lock on one session; for large production clusters, set this to at least 10% of the number of PHP processes -->
<break_after_frontend>30</break_after_frontend> <!-- seconds to wait for a session lock in the frontend; not as critical as admin -->
<break_after_adminhtml>120</break_after_adminhtml>
<bot_lifetime>7200</bot_lifetime> <!-- Bots get shorter session lifetimes. 0 to disable -->
</redis_session>
我如何解决这个问题？