[Boost-bugs] [Boost C++ Libraries] #12309: Is "Reference counting" example misleading and wrong?

Subject: [Boost-bugs] [Boost C++ Libraries] #12309: Is "Reference counting" example misleading and wrong?
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2016-07-01 09:29:54

#12309: Is "Reference counting" example misleading and wrong?
 Reporter: Adam Badura <adam.f.badura@…> | Owner:
     Type: Bugs | timblechmann
Milestone: To Be Determined | Status: new
  Version: Boost 1.61.0 | Component: atomic
 Keywords: atomic, shared_ptr, reference | Severity: Cosmetic
  counting, documentation |
 Boost.Atomic in its documentation provides "Reference counting" example:

 The example itself is excellent. After all this is a typical use case so
 providing an example with brief discussion is mostly welcome.

 The problem is however whether the example is correct after all. I mean
 here the part about counter decrement. Example uses `release` order and
 then a fance with `acquire`:
 friend void intrusive_ptr_release(const X * x)
   if (x->refcount_.fetch_sub(1, boost::memory_order_release) == 1) {
     delete x;
 while description states later:
   It would be possible to use `memory_order_acq_rel` for the `fetch_sub`
 operation, but this results in unneeded "acquire" operations when the
 reference counter does not yet reach zero and may impose a performance

 At the same time `boost::shared_ptr` uses `acq_rel` in its `std::atomic`
 implementation in file

 Also Herb Sutter in his "C++ and Beyond 2012: Herb Sutter - atomic<>
 Weapons, 2 of 2" (https://channel9.msdn.com/Shows/Going+Deep/Cpp-and-
 Beyond-2012-Herb-Sutter-atomic-Weapons-2-of-2) lecture mentions reference
 counting topic (starting at 1:19:51) where he also calls for `acq_rel`.
 And it seems that he discourages use fences in this talk.

 So which one is good? Because either Boost.Atomic documentation should be
 updated or `boost::shared_ptr` should be improved.

 And let me note here that this Boost.Atomic documentation part seems to be
 quoted (or refereed to) at various places, including Stack Overflow, quite
 often. So it is important for it to be correct.


 BTW (although beyond this ticket) it seems strange to me that
 `boost::shared_ptr` doesn't use `boost::atomic` and instead provides like
 19 different implementations. It smells like code repetition to me. But
 maybe there are valid reasons for it...

Ticket URL: <https://svn.boost.org/trac/boost/ticket/12309>
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