Re: atomically thread-safe Meyers singleton impl (fixed)...

From:

"Chris M. Thomasson" <no@spam.invalid>

Newsgroups:

comp.lang.c++,comp.programming.threads

Date:

Wed, 30 Jul 2008 06:51:14 -0700

Message-ID:

<0i_jk.6451$1N1.4713@newsfe07.iad>

"Dmitriy V'jukov" <dvyukov@gmail.com> wrote in message
news:c7877461-e97b-40cf-93ff-0523f290fe22@u12g2000prd.googlegroups.com...

On Jul 30, 8:44 am, "Chris Thomasson" <x...@xxx.xxx> wrote:

template<typename T>
struct singleton {
static T* instance() {
static T* volatile this_ptr = NULL;

I think here is a little problem. this_ptr is initialized dynamically,
and this initialization is not thread-safe. So some thread can
overwrite pointer in this_ptr with NULL.
You have to made this_ptr global, not function local, so it will be
initialized with NULL statically before any user code is executed.

Okay. However, by that logic and POSIX compiler magic aside for a moment,
the following is not "technically" thread-safe:

void foo() {
  static pthread_mutex_t this_mtx = PTHREAD_MUTEX_INITIALIZER;
  pthread_mutex_lock(&this_mtx);
  [...];
  pthread_mutex_unlock(&this_mtx);
}

unless the `pthread_mutex_lock()' and `pthread_mutex_unlock()' function
calls explicitly handle the condition you describe; where am I going wrong?

or even:

void foo() {
  static atomic_word volatile this_mtx = 0;

  while (ATOMIC_SWAP(&this_mtx, 1)) {
    sched_yield();
  }

  [...];

  ATOMIC_SWAP(&this_mtx, 0);
}

Could the above deadlock? Or encounter multiple threads within the
critical-section?

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