Question on Lock in In-Doubt Transaction

Dear All,
Could you kindly explain me why distributed transaction imposes a lock on read also with write? During Read, Select statement also uses rollback segment.
But in the document “Oracle9i - Database Administrator’s Guide Release 2 (9.2) - March 2002 Part No. A96521-01” at page 31-16, it has been written that
“the data is locked for both reads and writes. Oracle blocks reads because it cannot determine which version of the data to display for a query.”
So could we come to a conclusion that distributed transaction does not use rollback segment.
But at page 31-18, under manual resolution of In-Doubt Transaction, the first point is that the in-doubt transaction has locks on critical data or rollback segment.
Therefore, rollback segment is using during distributed transaction. So why does Oracle impose a lock on READ the data?
Regards,
Student

Redo logs are effectively write-only under normal circumstances. The database NEVER goes back to read the redo log unless it is in crash recovery mode.
Therefore, even though the information has been recorded in the online redo log, that information can not be used to roll back a failed in-doubt transaction. The rollback information MUST come from the rollback segment or from some other online location.
(I am not taking this as an argument - it's helping me as well.)

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