Reader Writer Locks

Motivation

• locks provide exclusive access to a piece of shared data
• gets rid of data races, makes reasoning about the shared data easier
• does it mean we should always protect accesses to shared data with a lock?
• what if all accesses are reads?
• what if some accesses are reads and some are writes?
• do writes need to be protected?
• do reads need to be protected?

Reader Writer Locks

• a sychronization primitive that allow for concurrent read accesses and exclusive write access
• concurrent reads: multiple readers can acquire the read lock at the same time, when read is going on no writes are allowed
• exclusive write: only one writer can acquire the write lock at a time, when write is going on, no reads or other writes are allowed
• APIs
• read_acquire, read_release, write_acquire, write_release
• when can a read_acquire succeed?
• when there's no ongoing write
• when there are ongoing reads (should this always be true?)
• when can a write_acquire succeed?
• when there's no ongoing read and write
• read-preferring vs write-preferring
• how might we implement a write-preferring reader writer lock with a monitor?
• what conditions do we need?
• how many condition variables do we need?