Glossary

To learn more about the various aspects of locks, check the wikipedia pages for locks and readers writer locks Here we briefly mention the main notions used in the documentation.

• Lock - a mechanism that prevents two or more threads or processes from running the same code at the same time.
• Readers writer lock - a mechanism that prevents two or more threads from having write (or write and read) access, while allowing multiple readers.

• Reentrant lock - a lock that can be acquired (and then released) multiple times, as in:

  with lock:
      with lock:
          ... # some code
  

• Mandatory lock (as opposed to advisory lock) - a lock that is enforced by the operating system, rather than by the cooperation between threads or processes

• Upgradable readers writer lock - a readers writer lock that can be upgraded from reader to writer (or downgraded from writer to reader) without losing the lock that is already held, as in:

  with rw_lock.read_lock():
      ... # read access
      with rw_lock.write_lock():
          ... # write access
      ... # read access
  

• Readers writer lock preference - describes the behaviour when multiple threads or processes are waiting for access. Some of the patterns are:
  • Reader preference - If lock is held by readers, then new readers will get immediate access. This can result in writers waiting forever (writer starvation).
  • Writer preference - If writer is waiting for a lock, then all the new readers (and writers) will be queued after it. This can result in readers waiting forever (reader starvation).
  • Phase fair - Lock that alternates between readers and writers.