Solving Magento after upgarde

It has been observed that, after upgrading there are some common well known issue. Recently I upgraded my one of the store  to Magento version 1.9.2.1.I face issue like, I can't able to login either back-end or front-end. So I come up with solution as below,

1) Manually remove the var/cache

2) Manually remove the var/session

3) Manually remove the var/ locks

4) Adding form key to register.phtml

<input type="hidden" name="form_key" value="<?php echo Mage::getSingleton('core/session')->getFormKey() ?>" />

The default register.phtml files, we can find at below location,

1) app/design/frontend/base/default/template/customer/form/register.phtml
2) app/design/frontend/base/default/template/persistent/customer/form/register.phtml

Then,  refresh cache and re-index solved my issue.