Magento的/ redis的会话。 警告：session_start（）：未定义用户会话功能

I am running magento shop with redis for sessions. Sometimes I am getting following error:

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();

For redis I am using default settings. Also in different DB, same instance, I am running magento cache. I am using php-redis module.

My config for redis-session module:

        <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>

How I can solve this issue?