Is there a global named reader/writer lock?

Question

I have multiple asp.net web apps serving a set of files. Periodically, one will update the file before serving it, but it can\'t update the file if it is in use.

I c

Answer 1

I appreciate Jim Mischel's fine answer, but I see opportunity for performance improvement by avoiding Thread.Sleep() and avoiding lock contention when multiple readers try to acquire at the same time!

Initialization

  Mutex writer = new Mutex(false, "Global\\MyWriterMutex");
  Semaphore readers = new Semaphore(int.MaxValue, int.MaxValue, "Global\\MyReadersSemaphore");
  EventWaitHandle readAllowed = new EventWaitHandle(true, EventResetMode.ManualReset, "Global\\MyReadAllowedEvent");
  EventWaitHandle readFinished = new EventWaitHandle(false, EventResetMode.ManualReset, "Global\\MyReadFinishedEvent");

Reader

  while (true)
  {
    // signal that I'm reading 
    readers.WaitOne();

    // check whether I'm actually allowed to read
    if (readAllowed.WaitOne(0))
    {
      break; // great!
    }

    // oops, nevermind, signal that I'm not reading
    readers.Release();
    readFinished.Set();

    // block until it's ok to read
    readAllowed.WaitOne();
  }

  try
  {
    readData();
  }
  finally
  {
    // signal that I'm no longer reading
    readers.Release();
    readFinished.Set();
  }

Writer

  // block until I am the only writer
  try
  {
    writer.WaitOne();
  }
  catch (AbandonedMutexException)
  {
    // The mutex was abandoned in another process, but it was still acquired
  }

  // signal that readers need to cease
  readAllowed.Reset();

  // loop until there are no readers
  int readerCount = -1;
  while (readerCount != 0)
  {
    // wipe the knowledge that a reader recently finished
    readFinished.Reset();

    // check if there is a reader
    readers.WaitOne();
    readerCount = int.MaxValue - (readers.Release() + 1);
    if (readerCount > 0)
    {
      // block until some reader finishes
      readFinished.WaitOne();
    }
  }

  try
  {
    writeData();
  }
  finally
  {
    // signal that readers may continue, and I am no longer the writer
    readAllowed.Set();
    writer.ReleaseMutex();
  }