[osg-submissions] Use atomic reference counts

[Mathias Fröhlich]

Hi Robert,

On Tuesday 03 June 2008 14:46, Robert Osfield wrote:
> I'm just beginning the review of your submission, not far in yet but
> have a question/query.  Looking at src/osg/Referenced.cpp I see that
> it contains two sets of changes - the atomic reference count changes
> that are to be expected, and then changes to the observer management
> which looks to be a knock on change required because the Mutex
> _refMutex disappears along with the introduction of the atomic ref
> counts.
>
> My inclination would be to separate these two sets of changes, for
> instance leave _refMutex in place and leave the old observer code in
> place, then do a second submission that removes the observer code's
> dependence on _refMutex.  The observer code itself that you have
> introduced does leave me a bit perplexed on a first read through, I
> presume the complexity is down to the attempt at avoiding the
> _refMutex, but suspect that as such one could still introduced
> potential threading problems over initialization of the observer set
> structure.   It might be that your code is safe, but it's complex
> enough that I can't say this right away.  Perhaps a static ref mutex
> might be sufficient for this particular support.
>
> Thoughts?

I can seperate that functional changes.

I would suggest to first care for the ground work in openthreads.
That is wee need:
- an atomic integer counter class,
- an atomic pointer assignment.
That also covers the config include header stuff.

Once we have worked out how you like to have that, we can start using that in 
Referenced.

So, the ground work:
We have a configure time check if we have some kind of compiler intrinsics or 
library support for smp atomic integer increment/decrement operations.
Also we will need an atomic 'compare and exchange' operation on pointers.

cmake will then generate that <build>/include/OpenThreads/Config file from the 
<src>/include/OpenThreads/Config.in. Note that this file needs to be 
installed later into the include directory of the installation.

Based on the Config file, the implementation of the atomics are choosen.
And no, the define you mentioned in the other mail should be right as is. That 
is, if there is no atomic operation on that single integer available, make 
sure that we have an atomic integer using a mutex.

So all the defines in this current Config file are defined when we should use. 
That is 'have a mutex in the atomic' if we 'should use a mutex to make that 
atomic' ...
Only one of them should be defined, and if non is defined in the end - which 
might be a yet unimplemented configure option - we will assume a single 
threaded case where an atomic integer is just an integer ...

The open question:
Where do you want to have the Config.in file?

GReetings

MAthias

-- 
Dr. Mathias Fröhlich, science + computing ag, Software Solutions
Hagellocher Weg 71-75, D-72070 Tuebingen, Germany
Phone: +49 7071 9457-268, Fax: +49 7071 9457-511
-- 
Vorstand/Board of Management:
Dr. Bernd Finkbeiner, Dr. Florian Geyer,
Dr. Roland Niemeier, Dr. Arno Steitz, Dr. Ingrid Zech
Vorsitzender des Aufsichtsrats/
Chairman of the Supervisory Board:
Prof. Dr. Hanns Ruder
Sitz/Registered Office: Tuebingen
Registergericht/Registration Court: Stuttgart
Registernummer/Commercial Register No.: HRB 382196