Problem

1. In at what instant is the point-of-no-return reached? That is, when is the atomic commit actually performed?

2. Give the full algorithm for whether an attempt to lock a file should succeed or fail. Consider both read and write locks, and the possibility that the file was unlocked, read locked, or write locked.

3. Systems that use locking for concurrency control usually distinguish read locks from write locks. What should happen if a process has already acquired a read lock and now wants to change it into a write lock? What about changing a write lock into a read lock?