Re: N2444/Boost: fast_pthread_once and atomicity w/ regard to threads

From:

"Chris Thomasson" <cristom@comcast.net>

Newsgroups:

comp.lang.c++.moderated

Date:

Sat, 26 Apr 2008 03:30:54 CST

Message-ID:

<gemdnexuCeUW0I_VnZ2dnUVZ_u2dnZ2d@comcast.com>

[added comp.programming.threads; is that okay?]

{ The clc++m moderator is seeing only clc++m. -mod }

<bachlipp@gmail.com> wrote in message
news:b9a4b2f7-95f8-4a51-a459-84ffd9576842@f36g2000hsa.googlegroups.com...

Dear C++ memory model experts,

yesterday I skimmed through the sources of the latest Boost.Thread
library, especially the POSIX Threads implementation of
boost::call_once(). It fundamentally changed in the past. The current
implementation quotes the ISO/IEC JTC1 SC22 WG21 N2444 document
(<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2444.html>)
and uses an algorithm by Mike Burrow published there.

[...]

AFAICT, this is not "100%" POSIX compliant. There is a rule... You cannot
read or write a memory location that might be updated by another thread
without a lock; simple. BTW, there are ways to implement a lock without
using a memory barrier:

http://blogs.sun.com/dave/resource/Asymmetric-Dekker-Synchronization.txt

http://groups.google.com/group/comp.programming.threads/browse_frm/thread/22b2736484af3ca6

You use an asymmetric model, and elect a "dominate" thread. Also, I am not
sure if this is anything all that new:

http://groups.google.com/group/comp.programming.threads/browse_frm/thread/98815a326299c723

Basically, you can do it like:
<crude code-sketch>
________________________________________________________________
template<typename T>
static T* once() {
  static T* g_object = NULL;
  static __thread bool g_flag = false;
  static pthread_mutex_t g_lock = PTHREAD_MUTEX_INITIALIZER;
  if (! g_flag) {
    pthread_mutex_lock(&g_lock);
    if (! g_object) {
      T* l_object;
      try {
        l_object = new T;
      } catch(...) {
        pthread_mutex_unlock(&g_lock);
        throw;
      }
      g_object = l_object;
    }
    pthread_mutex_unlock(&g_lock);
    g_flag = true;
  }
  return g_object;
}
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