yes,If user A commits then RBS header will be updated.Ok when new transcation goes to same datablock Oracle looks at datablock header and notices that non-commited transaction was working on that block so Oracle goes to the corresponding RBS for more information, finds out the transaction did got commited then marks the datablock header as commited.At this time oracle understand that block is commited and at this stage from where it will take data from data segment or from rollback segment..I have doubt here...

If it is from data segment we wont get ora 0155 error beacuse new data is available there but if it from rbs then we will get.Please clear ..