Reader/Writer Lock - Introduction

Okay, enough procrastination; let's finish the thread synchronization object library. We've been over events, conditions, mutexes, semaphores, and atomic functions; that leaves us with only one left.

Suppose we have a linked list of something or other, which is accessed by multiple threads. Clearly it would be disastrous if two threads tried to add new entries to the end of the list at the same time. It would also be disastrous if one thread was iterating through the list (reading, only) while another thread modified the list in some way, such as inserting or deleting an element. Protecting the list with a mutex would handily solve the problem, as there would never be more than one thread accessing the list at once.

But consider what the consequence of two threads iterating (and reading from) the list simultaneously would be: nothing. So long as neither thread modifies the list, two threads can read from it without any problems, whatsoever. However, if the list is protected by a mutex, this is impossible, as only one thread can ever hold the mutex at a time; this is wasteful, particularly if the reads take a long time to execute.

The solution is the last kind of thread synchronization object: the reader/writer lock (also called a single writer, multiple reader guard). This type of lock can be locked two ways: for reading or for writing. When a thread has the lock for reading, other threads may enter the lock for reading simultaneously, but threads trying to enter the lock for writing must wait until all readers have left the lock. However, while a thread has the lock for writing, no more threads may enter the lock, regardless of whether they want read or write access.

A practical example of where this would be useful is in a database. Multiple threads may read from the database concurrently, but if a thread wants to write to the database, it must wait until it has exclusive access. Because reading from disk can be time-consuming, overlapping reads can significantly increase performance.