Sounds like one of those goofy chicken/egg problems Magento Connect GUI is always coming up with. Cm_RedisSession is not a part of 1.9.0.1, therefore it can't be upgraded, therefore Magento Connect cannot upgrade it.
The cure is to do it through Magento ssh command line use of Connect with the following command:
./mage upgrade-all --force
And do it two or three times as it's known to time-out or do partial upgrades, missing a package here or there or miss upgrading a file here and there. It can be beautifully spotty that way.
Back up your database, your complete Magento application before playing with Magento Connect, it is truly one of the best ways of completely taking your live e-commerce website offline.
Or learn how to untar the install archive and do it manually for a complete file replacement upgrade without the coughing hacked half-hearted installs that Magento Connect is known so well for doing.