mutexes: "food for thought"

[Alexander Terekhov]

Hi Ross,

I'm sorta busy at the moment. Here's a version without DCSI.

More next week.

// doesn't provide "POSIX-safety" with respect to destruction
class swap_based_mutex { // noncopyable 

  atomic<int>      m_lock_status; // 0: free, 1/-1: locked/contention
  auto_reset_event m_retry_event; // bin.sema/gate

public:

  // ctor/dtor [w/o lazy event init] 

  void lock() throw() {
    if (m_lock_status.swap(1, msync::acq)) 
      while (m_lock_status.swap(-1, msync::acq))
        m_retry_event.wait();
  } 

  bool trylock() throw() {
    return !m_lock_status.swap(1, msync::acq) ? 
      true : !m_lock_status.swap(-1, msync::acq);
  }

  bool timedlock(absolute_timeout const & timeout) throw() {
    if (m_lock_status.swap(1, msync::acq)) {
      while (m_lock_status.swap(-1, msync::acq))
        if (!m_retry_event.timedwait(timeout)) 
          return false;
    }  return true;
  }

  void unlock() throw() {
    if (m_lock_status.swap(0, msync::rel) < 0) 
      m_retry_event.set();
  } 
};

As for Ulrich's "take 2", my "take 3" can be found here:

http://listman.redhat.com/archives/phil-list/2003-October/msg00030.html
(Subject: Mutex, Take 3 (for "dumb" futex))

regards,
alexander.

P.S. "POSIX-safety" with respect to destruction:

http://lists.boost.org/MailArchives/boost/msg67616.php
http://lists.boost.org/MailArchives/boost/msg67651.php
http://lists.boost.org/MailArchives/boost/msg67667.php

regards,
alexander.