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

"Chris M. Thomasson" <no@spam.invalid>
Wed, 30 Jul 2008 07:21:30 -0700
"Anthony Williams" <> wrote in message

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

"Anthony Williams" <> wrote in message

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


The algorithm used by boost::call_once on pthreads platforms is
described here:

It doesn't use a
lock unless it has to and is portable across threads and win32

The code I posted does not use a lock unless it absolutely has to
because it attempts to efficiently take advantage of the double
checked locking pattern.

Oh yes, I realise that: the code for call_once is similar. However, it
attempts to avoid contention on the mutex by using thread-local
storage. If you have atomic ops, you can go even further in
eliminating the mutex, e.g. using compare_exchange and fetch_add.


Before I reply to your entire post I should point out that:

the Boost mechanism is not 100% portable, but is elegant in

Yes. If you look at the whole thread, you'll see a comment by me there
where I admit as much.

Does the following line:

__thread fast_pthread_once_t _fast_pthread_once_per_thread_epoch;

explicitly set `_fast_pthread_once_per_thread_epoch' to zero? If so, is it

It uses a similar technique that a certain distributed
reference counting algorithm I created claims:

I wasn't aware that you were using something similar in vZOOM.

Humm, now that I think about it, it seems like I am totally mistaken. The
"most portable" version of vZOOM relies on an assumption that pointer
load/stores are atomic and the unlocking of a mutex executes at least a
release-barrier, and the loading of a shared variable executes at least a
data-dependant load-barrier; very similar to RCU without the explicit
#LoadStore | #StoreStore before storing into a shared pointer location...
Something like:

struct foo {
  int a;

static foo* shared_f = NULL;

// single producer thread {
  foo* local_f = new foo;
  pthread_mutex_t* lock = get_per_thread_mutex();
  local_f->a = 666;
  shared_f = local_f;

// single consumer thread {
  foo* local_f;
  while (! (local_f = shared_f)) {
  assert(local_f->a == 666);
  delete local_f;

If the `pthread_mutex_unlock()' function does not execute at least a
release-barrier in the producer, and if the load of the shared variable does
not execute at least a data-dependant load-barrier in the consumer, the
"most portable" version of vZOOM will NOT work on that platform in any way
shape or form, it will need a platform-dependant version. However, the only
platform I can think of where the intra-node memory visibility requirements
do not hold is the Alpha... For multi-node super-computers, inter-node
communication is adapted to using MPI.

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